Beej's Guide to C Programming

What’s this? You say something’s still not clear about this whole C programming language thing?

Well, to be quite honest, I’m not even sure what the above code does. It’s a snippet from one of the entries in the 2001 International Obfuscated C Code Contest ¹, a wonderful competition wherein the entrants attempt to write the most unreadable C code possible, with often surprising results.

The bad news is that if you’re a beginner in this whole thing, all C code you see probably looks obfuscated! The good news is, it’s not going to be that way for long.

What we’ll try to do over the course of this guide is lead you from complete and utter sheer lost confusion on to the sort of enlightened bliss that can only be obtained through pure C programming. Right on.

In the old days, C was a simpler language. A good number of the features contained in this book and a lot of the features in the Library Reference volume didn’t exist when K&R wrote the famous second edition of their book in 1988. Nevertheless, the language remains small at its core, and I hope I’ve presented it here in a way that starts with that simple core and builds outward.

And that’s my excuse for writing such a hilariously large book for such a small, concise language.

1.1 Audience

This guide assumes that you’ve already got some programming knowledge under your belt from another language, such as Python ², JavaScript ³, Java ⁴, Rust ⁵, Go ⁶, Swift ⁷, etc. (Objective-C ⁸ devs will have a particularly easy time of it!)

We’re going to assume you know what variables are, what loops do, how functions work, and so on.

If that’s not you for whatever reason the best I can hope to provide is some honest entertainment for your reading pleasure. The only thing I can reasonably promise is that this guide won’t end on a cliffhanger… or will it?

1.2 How to Read This Book

The second volume is the library reference ⁹, and it’s far more reference than tutorial.

If you’re new, go through the tutorial part in order, generally. The higher you get in chapters, the less important it is to go in order.

And no matter your skill level, the reference part is there with complete examples of the standard library function calls to help refresh your memory whenever needed. Good for reading over a bowl of cereal or other time.

Finally, glancing at the index (if you’re reading the print version), the reference section entries are italicized.

1.3 Platform and Compiler

I’ll try to stick to Plain Ol’-Fashioned ISO-standard C ¹⁰. Well, for the most part. Here and there I might go crazy and start talking about POSIX ¹¹ or something, but we’ll see.

Unix users (e.g. Linux, BSD, etc.) try running cc or gcc from the command line–you might already have a compiler installed. If you don’t, search your distribution for installing gcc or clang.

Windows users should check out Visual Studio Community ¹². Or, if you’re looking for a more Unix-like experience (recommended!), install WSL ¹³ and gcc.

Mac users will want to install XCode ¹⁴, and in particular the command line tools.

There are a lot of compilers out there, and virtually all of them will work for this book. And a C++ compiler will compile a lot of (but not all!) C code. Best use a proper C compiler if you can.

1.4 Official Homepage

This official location of this document is https://beej.us/guide/bgc/¹⁵. Maybe this’ll change in the future, but it’s more likely that all the other guides are migrated off Chico State computers.

1.5 Email Policy

I’m generally available to help out with email questions so feel free to write in, but I can’t guarantee a response. I lead a pretty busy life and there are times when I just can’t answer a question you have. When that’s the case, I usually just delete the message. It’s nothing personal; I just won’t ever have the time to give the detailed answer you require.

As a rule, the more complex the question, the less likely I am to respond. If you can narrow down your question before mailing it and be sure to include any pertinent information (like platform, compiler, error messages you’re getting, and anything else you think might help me troubleshoot), you’re much more likely to get a response.

If you don’t get a response, hack on it some more, try to find the answer, and if it’s still elusive, then write me again with the information you’ve found and hopefully it will be enough for me to help out.

Now that I’ve badgered you about how to write and not write me, I’d just like to let you know that I fully appreciate all the praise the guide has received over the years. It’s a real morale boost, and it gladdens me to hear that it is being used for good! :-) Thank you!

1.6 Mirroring

You are more than welcome to mirror this site, whether publicly or privately. If you publicly mirror the site and want me to link to it from the main page, drop me a line at beej@beej.us.

1.7 Note for Translators

If you want to translate the guide into another language, write me at beej@beej.us and I’ll link to your translation from the main page. Feel free to add your name and contact info to the translation.

Please note the license restrictions in the Copyright and Distribution section, below.

1.8 Copyright and Distribution

With specific exceptions for source code and translations, below, this work is licensed under the Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License. To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-nd/3.0/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.

One specific exception to the “No Derivative Works” portion of the license is as follows: this guide may be freely translated into any language, provided the translation is accurate, and the guide is reprinted in its entirety. The same license restrictions apply to the translation as to the original guide. The translation may also include the name and contact information for the translator.

The C source code presented in this document is hereby granted to the public domain, and is completely free of any license restriction.

Educators are freely encouraged to recommend or supply copies of this guide to their students.

1.9 Dedication

A lot of people have helped me through this process, and I want to acknowledge those who have made this book possible.

2 Hello, World!

2.1 What to Expect from C

It didn’t use to be. Back in the day when people carved punch cards out of granite, C was an incredible way to be free of the drudgery of lower-level languages like assembly ¹⁹.

But now in these modern times, current-generation languages offer all kinds of features that didn’t exist in 1972 when C was invented. This means C is a pretty basic language with not a lot of features. It can do anything, but it can make you work for it.

If you’re familiar with another language, a lot of things about C are easy. C inspired many other languages, and you’ll see bits of it in Go, Rust, Swift, Python, JavaScript, Java, and all kinds of other languages. Those parts will be familiar.

The one thing about C that hangs people up is pointers. Virtually everything else is familiar, but pointers are the weird one. The concept behind pointers is likely one you already know, but C forces you to be explicit about it, using operators you’ve likely never seen before.

It’s especially insidious because once you grok²⁴ pointers, they’re suddenly easy. But up until that moment, they’re slippery eels.

Everything else in C is just memorizing another way (or sometimes the same way!) of doing something you’ve done already. Pointers are the weird bit. And, arguably, even pointers are variations on a theme you’re probably familiar with.

So get ready for a rollicking adventure as close to the core of the computer as you can get without assembly, in the most influential computer language of all time²⁵. Hang on!

2.2 Hello, World!

This is the canonical example of a C program. Everyone uses it. (Note that the numbers to the left are for reader reference only, and are not part of the source code.)

/* Hello world program */

#include <stdio.h>

int main(void)
{
    printf("Hello, World!\n");  // Actually do the work here
}

We’re going to don our long-sleeved heavy-duty rubber gloves, grab a scalpel, and rip into this thing to see what makes it tick. So, scrub up, because here we go. Cutting very gently…

Let’s get the easy thing out of the way: anything between the digraphs /* and */ is a comment and will be completely ignored by the compiler. Same goes for anything on a line after a //. This allows you to leave messages to yourself and others, so that when you come back and read your code in the distant future, you’ll know what the heck it was you were trying to do. Believe me, you will forget; it happens.

Now, what is this #include? GROSS! Well, it tells the C Preprocessor to pull the contents of another file and insert it into the code right there.

Wait—what’s a C Preprocessor? Good question. There are two stages²⁶ to compilation: the preprocessor and the compiler. Anything that starts with pound sign, or “octothorpe”, (#) is something the preprocessor operates on before the compiler even gets started. Common preprocessor directives, as they’re called, are #include and #define. More on that later.

Before we go on, why would I even begin to bother pointing out that a pound sign is called an octothorpe? The answer is simple: I think the word octothorpe is so excellently funny, I have to gratuitously spread its name around whenever I get the opportunity. Octothorpe. Octothorpe, octothorpe, octothorpe.

So anyway. After the C preprocessor has finished preprocessing everything, the results are ready for the compiler to take them and produce assembly code ²⁷, machine code ²⁸, or whatever it’s about to do. Machine code is the “language” the CPU understands, and it can understand it very rapidly. This is one of the reasons C programs tend to be quick.

Don’t worry about the technical details of compilation for now; just know that your source runs through the preprocessor, then the output of that runs through the compiler, then that produces an executable for you to run.

What about the rest of the line? What’s <stdio.h>? That is what is known as a header file. It’s the dot-h at the end that gives it away. In fact it’s the “Standard I/O” (stdio) header file that you will grow to know and love. It gives us access to a bunch of I/O functionality²⁹. For our demo program, we’re outputting the string “Hello, World!”, so we in particular need access to the printf() function to do this. The <stdio.h> file gives us this access. Basically, if we tried to use printf() without #include <stdio.h>, the compiler would have complained to us about it.

How did I know I needed to #include <stdio.h> for printf()? Answer: it’s in the documentation. If you’re on a Unix system, man 3 printf and it’ll tell you right at the top of the man page what header files are required. Or see the reference section in this book. :-)

Holy moly. That was all to cover the first line! But, let’s face it, it has been completely dissected. No mystery shall remain!

So take a breather…look back over the sample code. Only a couple easy lines to go.

Welcome back from your break! I know you didn’t really take a break; I was just humoring you.

The next line is main(). This is the definition of the function main(); everything between the squirrelly braces ({ and }) is part of the function definition.

(How do you call a different function, anyway? The answer lies in the printf() line, but we’ll get to that in a minute.)

Now, the main function is a special one in many ways, but one way stands above the rest: it is the function that will be called automatically when your program starts executing. Nothing of yours gets called before main(). In the case of our example, this works fine since all we want to do is print a line and exit.

Oh, that’s another thing: once the program executes past the end of main(), down there at the closing squirrelly brace, the program will exit, and you’ll be back at your command prompt.

So now we know that that program has brought in a header file, stdio.h, and declared a main() function that will execute when the program is started. What are the goodies in main()?

I am so happy you asked. Really! We only have the one goodie: a call to the function printf(). You can tell this is a function call and not a function definition in a number of ways, but one indicator is the lack of squirrelly braces after it. And you end the function call with a semicolon so the compiler knows it’s the end of the expression. You’ll be putting semicolons after almost everything, as you’ll see.

You’re passing one argument to the function printf(): a string to be printed when you call it. Oh, yeah—we’re calling a function! We rock! Wait, wait—don’t get cocky. What’s that crazy \n at the end of the string? Well, most characters in the string will print out just like they are stored. But there are certain characters that you can’t print on screen well that are embedded as two-character backslash codes. One of the most popular is \n (read “backslash-N” or simply “newline”) that corresponds to the newline character. This is the character that causes further printing to continue at the beginning of the next line instead of the current. It’s like hitting return at the end of the line.

So copy that code into a file called hello.c and build it. On a Unix-like platform (e.g. Linux, BSD, Mac, or WSL), from the command line you’ll build with a command like so:

gcc -o hello hello.c

After that’s done, you should have a file called hello that you can run with this command:

2.3 Compilation Details

Let’s talk a bit more about how to build C programs, and what happens behind the scenes there.

Like other languages, C has source code. But, depending on what language you’re coming from, you might never have had to compile your source code into an executable.

Compilation is the process of taking your C source code and turning it into a program that your operating system can execute.

JavaScript and Python devs aren’t used to a separate compilation step at all–though behind the scenes it’s happening! Python compiles your source code into something called bytecode that the Python virtual machine can execute. Java devs are used to compilation, but that produces bytecode for the Java Virtual Machine.

When compiling C, machine code is generated. This is the 1s and 0s that can be executed directly and speedily by the CPU.

As we’ve already said, gcc is a compiler that’s installed on a lot of Unix-like operating systems ³⁰. And it’s commonly run from the command line in a terminal, but not always. You can run it from your IDE, as well.

2.4 Building with gcc

If you have a source file called hello.c in the current directory, you can build that into a program called hello with this command typed in a terminal:

gcc -o hello hello.c

The -o means “output to this file”³¹. And there’s hello.c at the end, the name of the file we want to compile.

If your source is broken up into multiple files, you can compile them all together (almost as if they were one file, but the rules are actually more complex than that) by putting all the .c files on the command line:

gcc -o awesomegame ui.c characters.c npc.c items.c

That’s enough to get started—later we’ll talk details about multiple source files, object files, and all kinds of fun stuff.

2.5 Building with clang

On Macs, the stock compiler isn’t gcc—it’s clang. But a wrapper is also installed so you can run gcc and have it still work.

You can also install the gcc compiler proper through Homebrew ³² or some other means.

2.6 Building from IDEs

If you’re using an Integrated Development Environment (IDE), you probably don’t have to build from the command line.

With VS Code, you can hit F5 to run via the debugger. (You’ll have to install the C/C++ Extension.)

With XCode, you can build with COMMAND-B and run with COMMAND-R. To get the command line tools, Google for “XCode command line tools” and you’ll find instructions for installing them.

For getting started, I encourage you to also try to build from the command line—it’s history!

2.7 C Versions

C has come a long way over the years, and it had many named version numbers to describe which dialect of the language you’re using.

The most famous are C89, C99, C11, and C2x. We’ll focus on the latter in this book.

Version	Description
K&R C	1978, the original. Named after Brian Kernighan and Dennis Ritchie. Ritchie designed and coded the language, and Kernighan co-authored the book on it. You rarely see original K&R code today. If you do, it’ll look odd, like Middle English looks odd to modern English readers.
C89, ANSI C, C90	In 1989, the American National Standards Institute (ANSI) produced a C language specification that set the tone for C that persists to this day. A year later, the reins were handed to the International Organization for Standardization (ISO) that produced the identical C90.
C95	A rarely-mentioned addition to C89 that included wide character support.
C99	The first big overhaul with lots of language additions. The thing most people remember is the addition of `//`-style comments. This is the most popular version of C in use as of this writing.
C11	This major version update includes Unicode support and multi-threading. Be advised that if you start using these language features, you might be sacrificing portability with places that are stuck in C99 land. But, honestly, 1999 is getting to be a while back now.
C17, C18	Bugfix update to C11. C17 seems to be the official name, but the publication was delayed until 2018. As far as I can tell, these two are interchangeable, with C17 being preferred.
C2x	What’s coming next! Expected to eventually become C23.

You can force GCC to use one of these standards with the -std= command line argument. If you want it to be picky about the standard, add -pedantic.

gcc -std=c11 -pedantic foo.c

gcc -Wall -Wextra -std=c2x -pedantic foo.c

3 Variables and Statements

And for various reasons, it’ll be easier for all of us if we classify some of the types of things you can find in a program, so we can be clear what we’re talking about.

3.1 Variables

It’s said that “variables hold values”. But another way to think about it is that a variable is a human-readable name that refers to some data in memory.

We’re going to take a second here and take a peek down the rabbit hole that is pointers. Don’t worry about it.

You can think of memory as a big array of bytes³³. Data is stored in this “array”³⁴. If a number is larger than a single byte, it is stored in multiple bytes. Because memory is like an array, each byte of memory can be referred to by its index. This index into memory is also called an address, or a location, or a pointer.

When you have a variable in C, the value of that variable is in memory somewhere, at some address. Of course. After all, where else would it be? But it’s a pain to refer to a value by its numeric address, so we make a name for it instead, and that’s what the variable is.

So a variable is a name for some data that’s stored in memory at some address.

3.1.1 Variable Names

You can use any characters in the range 0-9, A-Z, a-z, and underscore for variable names, with the following rules:

For Unicode, just try it. There are some rules in the spec in §D.2 that talk about which Unicode codepoint ranges are allowed in which parts of identifiers, but that’s too much to write about here and is probably something you’ll never have to think about anyway.

3.1.2 Variable Types

Depending on which languages you already have in your toolkit, you might or might not be familiar with the idea of types. But C’s kinda picky about them, so we should do a refresher.

C makes an effort to convert automatically between most numeric types when you ask it to. But other than that, all conversions are manual, notably between string and numeric.

Type	Example	C Type
Integer	`3490`	`int`
Floating point	`3.14159`	`float`³⁵
Character (single)	`'c'`	`char`
String	`"Hello, world!"`	`char *`³⁶

Before you can use a variable, you have to declare that variable and tell C what type the variable holds. Once declared, the type of variable cannot be changed later at runtime. What you set it to is what it is until it falls out of scope and is reabsorbed into the universe.

Let’s take our previous “Hello, world” code and add a couple variables to it:

#include <stdio.h>

int main(void)
{
    int i;    // Holds signed integers, e.g. -3, -2, 0, 1, 10
    float f;  // Holds signed floating point numbers, e.g. -3.1416

    printf("Hello, World!\n");  // Ah, blessed familiarity
}

There! We’ve declared a couple of variables. We haven’t used them yet, and they’re both uninitialized. One holds an integer number, and the other holds a floating point number (a real number, basically, if you have a math background).

Uninitialized variables have indeterminate value³⁷. They have to be initialized or else you must assume they contain some nonsense number.

int main(void)
{
    int i;

    i = 2; // Assign the value 2 into the variable i

    printf("Hello, World!\n");
}

We’re going to do that by passing two amazing arguments to the printf() function. The first argument is a string that describes what to print and how to print it (called the format string), and the second is the value to print, namely whatever is in the variable i.

printf() hunts through the format string for a variety of special sequences which start with a percent sign (%) that tell it what to print. For example, if it finds a %d, it looks to the next parameter that was passed, and prints it out as an integer. If it finds a %f, it prints the value out as a float. If it finds a %s, it prints a string.

#include <stdio.h>

int main(void)
{
    int i = 2;
    float f = 3.14;
    char *s = "Hello, world!";  // char * ("char pointer") is the string type

    printf("%s  i = %d and f = %f!\n", s, i, f);
}

Hello, world!  i = 2 and f = 3.14!

In this way, printf() might be similar to various types of format strings or parameterized strings in other languages you’re familiar with.

3.1.3 Boolean Types

Historically, C didn’t have a Boolean type, and some might argue it still doesn’t.

int x = 1;

if (x) {
    printf("x is true!\n");
}

If you #include <stdbool.h>, you also get access to some symbolic names that might make things look more familiar, namely a bool type and true and false values:

#include <stdio.h>
#include <stdbool.h>

int main(void) {
    bool x = true;

    if (x) {
        printf("x is true!\n");
    }
}

But these are identical to using integer values for true and false. They’re just a facade to make things look nice.

3.2 Operators and Expressions

C operators should be familiar to you from other languages. Let’s blast through some of them here.

(There are a bunch more details than this, but we’re going to do enough in this section to get started.)

3.2.1 Arithmetic

i = i + 3;  // Addition (+) and assignment (=) operators, add 3 to i
i = i - 8;  // Subtraction, subtract 8 from i
i = i * 9;  // Multiplication
i = i / 2;  // Division
i = i % 5;  // Modulo (division remainder)

There are shorthand variants for all of the above. Each of those lines could more tersely be written as:

i += 3;  // Same as "i = i + 3", add 3 to i
i -= 8;  // Same as "i = i - 8"
i *= 9;  // Same as "i = i * 9"
i /= 2;  // Same as "i = i / 2"
i %= 5;  // Same as "i = i % 5"

There is no exponentiation. You’ll have to use one of the pow() function variants from math.h.

Let’s get into some of the weirder stuff you might not have in your other languages!

3.2.2 Ternary Operator

C also includes the ternary operator. This is an expression whose value depends on the result of a conditional embedded in it.

// If x > 10, add 17 to y. Otherwise add 37 to y.

y += x > 10? 17: 37;

What a mess! You’ll get used to it the more you read it. To help out a bit, I’ll rewrite the above expression using if statements:

// This expression:

y += x > 10? 17: 37;

// is equivalent to this non-expression:

if (x > 10)
    y += 17;
else
    y += 37;

printf("The number %d is %s.\n", x, x % 2 == 0? "even": "odd");

The %s format specifier in printf() means print a string. If the expression x % 2 evaluates to 0, the value of the entire ternary expression evaluates to the string "even". Otherwise it evaluates to the string "odd". Pretty cool!

It’s important to note that the ternary operator isn’t flow control like the if statement is. It’s just an expression that evaluates to a value.

3.2.3 Pre-and-Post Increment-and-Decrement

i++;        // Add one to i (post-increment)
i--;        // Subtract one from i (post-decrement)

i += 1;        // Add one to i
i -= 1;        // Subtract one from i

++i;        // Add one to i (pre-increment)
--i;        // Subtract one from i (pre-decrement)

With pre-increment and pre-decrement, the value of the variable is incremented or decremented before the expression is evaluated. Then the expression is evaluated with the new value.

With post-increment and post-decrement, the value of the expression is first computed with the value as-is, and then the value is incremented or decremented after the value of the expression has been determined.

i = 10;
j = 5 + i++;  // Compute 5 + i, _then_ increment i

printf("%d, %d\n", i, j);  // Prints 11, 15

i = 10;
j = 5 + ++i;  // Increment i, _then_ compute 5 + i

printf("%d, %d\n", i, j);  // Prints 11, 16

This technique is used frequently with array and pointer access and manipulation. It gives you a way to use the value in a variable, and also increment or decrement that value before or after it is used.

for (i = 0; i < 10; i++)
    printf("i is %d\n", i);

3.2.4 The Comma Operator

This is an uncommonly-used way to separate expressions that will run left to right:

x = 10, y = 20;  // First assign 10 to x, then 20 to y

Seems a bit silly, since you could just replace the comma with a semicolon, right?

x = 10; y = 20;  // First assign 10 to x, then 20 to y

But that’s a little different. The latter is two separate expressions, while the former is a single expression!

With the comma operator, the value of the comma expression is the value of the rightmost expression:

x = (1, 2, 3);

printf("x is %d\n", x);  // Prints 3, because 3 is rightmost in the comma list

But even that’s pretty contrived. One common place the comma operator is used is in for loops to do multiple things in each section of the statement:

for (i = 0, j = 10; i < 100; i++, j++)
    printf("%d, %d\n", i, j);

3.2.5 Conditional Operators

a == b;  // True if a is equivalent to b
a != b;  // True if a is not equivalent to b
a < b;   // True if a is less than b
a > b;   // True if a is greater than b
a <= b;  // True if a is less than or equal to b
a >= b;  // True if a is greater than or equal to b

Don’t mix up assignment = with comparison ==! Use two equals to compare, one to assign.

if (a <= 10)
    printf("Success!\n");

3.2.6 Boolean Operators

We can chain together or alter conditional expressions with Boolean operators for and, or, and not.

Operator	Boolean meaning
`&&`	and
`\|\|`	or
`!`	not

// Do something if x less than 10 and y greater than 20:

if (x < 10 && y > 20)
    printf("Doing something!\n");

if (!(x < 12))
    printf("x is not less than 12\n");

! has higher precedence than the other Boolean operators, so we have to use parentheses in that case.

if (x >= 12)
    printf("x is not less than 12\n");

3.2.7 The sizeof Operator

This operator tells you the size (in bytes) that a particular variable or data type uses in memory.

More particularly, it tells you the size (in bytes) that the type of a particular expression (which might be just a single variable) uses in memory.

This can be different on different systems, except for char and its variants (which are always 1 byte).

And this might not seem very useful now, but we’ll be making references to it here and there, so it’s worth covering.

Since this computes the number of bytes needed to store a type, you might think it would return an int. Or… since the size can’t be negative, maybe an unsigned?

But it turns out C has a special type to represent the return value from sizeof. It’s size_t, pronounced “size tee”³⁹. All we know is that it’s an unsigned integer type that can hold the size in bytes of anything you can give to sizeof.

size_t shows up a lot of different places where counts of things are passed or returned. Think of it as a value that represents a count.

int a = 999;

// %zu is the format specifier for type size_t

printf("%zu\n", sizeof a);      // Prints 4 on my system
printf("%zu\n", sizeof(2 + 7)); // Prints 4 on my system
printf("%zu\n", sizeof 3.14);   // Prints 8 on my system

// If you need to print out negative size_t values, use %zd

Remember: it’s the size in bytes of the type of the expression, not the size of the expression itself. That’s why the size of 2+7 is the same as the size of a—they’re both type int. We’ll revisit this number 4 in the very next block of code…

…Where we’ll see you can take the sizeof a type (note the parentheses are required around a type name, unlike an expression):

printf("%zu\n", sizeof(int));   // Prints 4 on my system
printf("%zu\n", sizeof(char));  // Prints 1 on all systems

It’s important to note that sizeof is a compile-time operation⁴⁰. The result of the expression is determined entirely at compile-time, not at runtime.

3.3 Flow Control

Booleans are all good, but of course we’re nowhere if we can’t control program flow. Let’s take a look at a number of constructs: if, for, while, and do-while.

First, a general forward-looking note about statements and blocks of statements brought to you by your local friendly C developer:

After something like an if or while statement, you can either put a single statement to be executed, or a block of statements to all be executed in sequence.

if (x == 10) printf("x is 10\n");

This is also sometimes written on a separate line. (Whitespace is largely irrelevant in C—it’s not like Python.)

if (x == 10)
    printf("x is 10\n");

But what if you want multiple things to happen due to the conditional? You can use squirrelly braces to mark a block or compound statement.

if (x == 10) {
    printf("x is 10\n");
    printf("And also this happens when x is 10\n");
}

It’s a really common style to always use squirrelly braces even if they aren’t necessary:

if (x == 10) {
    printf("x is 10\n");
}

Some devs feel the code is easier to read and avoids errors like this where things visually look like they’re in the if block, but actually they aren’t.

// BAD ERROR EXAMPLE

if (x == 10)
    printf("This happens if x is 10\n");
    printf("This happens ALWAYS\n");  // Surprise!! Unconditional!

while and for and the other looping constructs work the same way as the examples above. If you want to do multiple things in a loop or after an if, wrap them up in squirrelly braces.

In other words, the if is going to run the one thing after the if. And that one thing can be a single statement or a block of statements.

3.3.1 The if-else statement

We’ve already been using if for multiple examples, since it’s likely you’ve seen it in a language before, but here’s another:

int i = 10;

if (i > 10) {
    printf("Yes, i is greater than 10.\n");
    printf("And this will also print if i is greater than 10.\n");
}

if (i <= 10) printf("i is less than or equal to 10.\n");

In the example code, the message will print if i is greater than 10, otherwise execution continues to the next line. Notice the squirrley braces after the if statement; if the condition is true, either the first statement or expression right after the if will be executed, or else the collection of code in the squirlley braces after the if will be executed. This sort of code block behavior is common to all statements.

Of course, because C is fun this way, you can also do something if the condition is false with an else clause on your if:

int i = 99;

if (i == 10)
    printf("i is 10!\n");
else {
    printf("i is decidedly not 10.\n");
    printf("Which irritates me a little, frankly.\n");
}

int i = 99;

if (i == 10)
    printf("i is 10!\n");

else if (i == 20)
    printf("i is 20!\n");

else if (i == 99) {
    printf("i is 99! My favorite\n");
    printf("I can't tell you how happy I am.\n");
    printf("Really.\n");
}
    
else
    printf("i is some crazy number I've never heard of.\n");

Though if you’re going that route, be sure to check out the switch statement for a potentially better solution. The catch is switch only works with equality comparisons with constant numbers. The above if-else cascade could check inequality, ranges, variables, or anything else you can craft in a conditional expression.

3.3.2 The while statement

while is your average run-of-the-mill looping construct. Do a thing while a condition expression is true.

// Print the following output:
//
//   i is now 0!
//   i is now 1!
//   [ more of the same between 2 and 7 ]
//   i is now 8!
//   i is now 9!

int i = 0;

while (i < 10) {
    printf("i is now %d!\n", i);
    i++;
}

printf("All done!\n");

That gets you a basic loop. C also has a for loop which would have been cleaner for that example.

A not-uncommon use of while is for infinite loops where you repeat while true:

while (1) {
    printf("1 is always true, so this repeats forever.\n");
}

3.3.3 The do-while statement

So now that we’ve gotten the while statement under control, let’s take a look at its closely related cousin, do-while.

They are basically the same, except if the loop condition is false on the first pass, do-while will execute once, but while won’t execute at all. In other words, the test to see whether or not to execute the block happens at the end of the block with do-while. It happens at the beginning of the block with while.

// Using a while statement:

i = 10;

// this is not executed because i is not less than 10:
while(i < 10) {
    printf("while: i is %d\n", i);
    i++;
}

// Using a do-while statement:

i = 10;

// this is executed once, because the loop condition is not checked until
// after the body of the loop runs:

do {
    printf("do-while: i is %d\n", i);
    i++;
} while (i < 10);

printf("All done!\n");

Notice that in both cases, the loop condition is false right away. So in the while, the loop fails, and the following block of code is never executed. With the do-while, however, the condition is checked after the block of code executes, so it always executes at least once. In this case, it prints the message, increments i, then fails the condition, and continues to the “All done!” output.

The moral of the story is this: if you want the loop to execute at least once, no matter what the loop condition, use do-while.

All these examples might have been better done with a for loop. Let’s do something less deterministic—repeat until a certain random number comes up!

#include <stdio.h>   // For printf
#include <stdlib.h>  // For rand

int main(void)
{
    int r;

    do {
        r = rand() % 100; // Get a random number between 0 and 99
        printf("%d\n", r);
    } while (r != 37);    // Repeat until 37 comes up
}

Side note: did you run that more than once? If you did, did you notice the same sequence of numbers came up again. And again. And again? This is because rand() is a pseudorandom number generator that must be seeded with a different number in order to generate a different sequence. Look up the srand()⁴¹ function for more details.

3.3.4 The for statement

This is a great loop if you know the number of times you want to loop in advance.

You could do the same thing using just a while loop, but the for loop can help keep the code cleaner.

Here are two pieces of equivalent code—note how the for loop is just a more compact representation:

// Print numbers between 0 and 9, inclusive...

// Using a while statement:

i = 0;
while (i < 10) {
    printf("i is %d\n", i);
    i++;
}

// Do the exact same thing with a for-loop:

for (i = 0; i < 10; i++) {
    printf("i is %d\n", i);
}

That’s right, folks—they do exactly the same thing. But you can see how the for statement is a little more compact and easy on the eyes. (JavaScript users will fully appreciate its C origins at this point.)

It’s split into three parts, separated by semicolons. The first is the initialization, the second is the loop condition, and the third is what should happen at the end of the block if the loop condition is true. All three of these parts are optional.

for (initialize things; loop if this is true; do this after each loop)

Note that the loop will not execute even a single time if the loop condition starts off false.

for(;;) {  // "forever"
    printf("I will print this again and again and again\n" );
    printf("for all eternity until the heat-death of the universe.\n");

    printf("Or until you hit CTRL-C.\n");
}

3.3.5 The switch Statement

Depending on what languages you’re coming from, you might or might not be familiar with switch, or C’s version might even be more restrictive than you’re used to. This is a statement that allows you to take a variety of actions depending on the value of an integer expression.

Basically, it evaluates an expression to an integer value, jumps to the case that corresponds to that value. Execution resumes from that point. If a break statement is encountered, then execution jumps out of the switch.

Here’s an example where, for a given number of goats, we print out a gut-feel of how many goats that is.

#include <stdio.h>

int main(void)
{
    int goat_count = 2;

    switch (goat_count) {
        case 0:
            printf("You have no goats.\n");
            break;

        case 1:
            printf("You have a singular goat.\n");
            break;

        case 2:
            printf("You have a brace of goats.\n");
            break;

        default:
            printf("You have a bona fide plethora of goats!\n");
            break;
    }
}

In that example, the switch will jump to the case 2 and execute from there. When (if) it hits a break, it jumps out of the switch.

Also, you might see that default label there at the bottom. This is what happens when no cases match.

Every case, including default, is optional. And they can occur in any order, but it’s really typical for default, if any, to be listed last.

if (goat_count == 0)
    printf("You have no goats.\n");
else if (goat_count == 1)
    printf("You have a singular goat.\n");
else if (goat_count == 2)
    printf("You have a brace of goats.\n");
else
    printf("You have a bona fide plethora of goats!\n");

There’s one more neat thing about switch that you sometimes see that is quite interesting: fall through.

switch (x) {
    case 1:
        printf("1\n");
        // Fall through!
    case 2:
        printf("2\n");
        break;
    case 3:
        printf("3\n");
        break;
}

If x == 1, this switch will first hit case 1, it’ll print the 1, but then it just continues on to the next line of code… which prints 2!

ProTip: ALWAYS put a comment in the code where you intend to fall through, like I did above. It will save other programmers from wondering if you meant to do that.

In fact, this is one of the common places to introduce bugs in C programs: forgetting to put a break in your case. You gotta do it if you don’t want to just roll into the next case⁴².

Earlier I said that switch works with integer types—keep it that way. Don’t use floating point or string types in there. One loophole-ish thing here is that you can use character types because those are secretly integers themselves. So this is perfectly acceptable:

char c = 'b';

switch (c) {
    case 'a':
        printf("It's 'a'!\n");
        break;

    case 'b':
        printf("It's 'b'!\n");
        break;

    case 'c':
        printf("It's 'c'!\n");
        break;
}

Finally, you can use enums in switch since they are also integer types. But more on that in the enum chapter.

4 Functions

Very much like other languages you’re used to, C has the concept of functions.

Functions can accept a variety of arguments and return a value. One important thing, though: the arguments and return value types are predeclared—because that’s how C likes it!

Let’s take a look at a function. This is a function that takes an int as an argument, and returns an int.

#include <stdio.h>

int plus_one(int n)  // The "definition"
{
    return n + 1;
}

The int n indicates that this function takes one int argument, stored in parameter n. A parameter is a special type of local variable into which the arguments are copied.

I’m going to drive home the point that the arguments are copied into the parameters, here. Lots of things in C are easier to understand if you know that the parameter is a copy of the argument, not the argument itself. More on that in a minute.

Continuing the program down into main(), we can see the call to the function, where we assign the return value into local variable j:

int main(void)
{
    int i = 10, j;
    
    j = plus_one(i);  // The "call"

    printf("i + 1 is %d\n", j);
}

But what’s this void thing? This is a keyword that’s used to indicate that the function accepts no arguments.

#include <stdio.h>

// This function takes no arguments and returns no value:

void hello(void)
{
    printf("Hello, world!\n");
}

int main(void)
{
    hello();  // Prints "Hello, world!"
}

4.1 Passing by Value

I’d mentioned earlier that when you pass an argument to a function, a copy of that argument gets made and stored in the corresponding parameter.

If the argument is a variable, a copy of the value of that variable gets made and stored in the parameter.

More generally, the entire argument expression is evaluated and its value determined. That value is copied to the parameter.

In any case, the value in the parameter is its own thing. It is independent of whatever values or variables you used as arguments when you made the function call.

So let’s look at an example here. Study it and see if you can determine the output before running it:

#include <stdio.h>

void increment(int a)
{
    a++;
}

int main(void)
{
    int i = 10;

    increment(i);

    printf("i == %d\n", i);  // What does this print?
}

At first glance, it looks like i is 10, and we pass it to the function increment(). There the value gets incremented, so when we print it, it must be 11, right?

It’s all about the fact that the expressions you pass to functions get copied onto their corresponding parameters. The parameter is a copy, not the original.

So i is 10 out in main(). And we pass it to increment(). The corresponding parameter is called a in that function.

And the copy happens, as if by assignment. Loosely, a = i. So at that point, a is 10. And out in main(), i is also 10.

Then we increment a to 11. But we’re not touching i at all! It remains 10.

Finally, the function is complete. All its local variables are discarded (bye, a!) and we return to main(), where i is still 10.

This is why in the previous example with the plus_one() function, we returned the locally modified value so that we could see it again in main().

Seems a little bit restrictive, huh? Like you can only get one piece of data back from a function, is what you’re thinking. There is, however, another way to get data back; C folks call it passing by reference and that’s a story we’ll tell another time.

But no fancy-schmancy name will distract you from the fact that EVERYTHING you pass to a function WITHOUT EXCEPTION is copied into its corresponding parameter, and the function operates on that local copy, NO MATTER WHAT. Remember that, even when we’re talking about this so-called passing by reference.

4.2 Function Prototypes

So if you recall back in the ice age a few sections ago, I mentioned that you had to define the function before you used it, otherwise the compiler wouldn’t know about it ahead of time, and would bomb out with an error.

This isn’t quite strictly true. You can notify the compiler in advance that you’ll be using a function of a certain type that has a certain parameter list. That way the function can be defined anywhere (even in a different file), as long as the function prototype has been declared before you call that function.

Fortunately, the function prototype is really quite easy. It’s merely a copy of the first line of the function definition with a semicolon tacked on the end for good measure. For example, this code calls a function that is defined later, because a prototype has been declared first:

#include <stdio.h>

int foo(void);  // This is the prototype!

int main(void)
{
    int i;
    
    // We can call foo() here before it's definition because the
    // prototype has already been declared, above!

    i = foo();
    
    printf("%d\n", i);  // 3490
}

int foo(void)  // This is the definition, just like the prototype!
{
    return 3490;
}

If you don’t declare your function before you use it (either with a prototype or its definition), you’re performing something called an implicit declaration. This was allowed in the first C standard (C89), and that standard has rules about it, but is no longer allowed today. And there is no legitimate reason to rely on it in new code.

You might notice something about the sample code we’ve been using… That is, we’ve been using the good old printf() function without defining it or declaring a prototype! How do we get away with this lawlessness? We don’t, actually. There is a prototype; it’s in that header file stdio.h that we included with #include, remember? So we’re still legit, officer!

4.3 Empty Parameter Lists

You might see these from time to time in older code, but you shouldn’t ever code one up in new code. Always use void to indicate that a function takes no parameters. There’s never⁴³ a reason to skip this in modern code.

If you’re good at just remembering to put void in for empty parameter lists in functions and prototypes, you can skip the rest of this section.

void foo()  // Should really have a `void` in there
{
    printf("Hello, world!\n");
}

While the spec spells out that the behavior in this instance is as-if you’d indicated void (C11 §6.7.6.3¶14), the void type is there for a reason. Use it.

But in the case of a function prototype, there is a significant difference between using void and not:

void foo();
void foo(void);  // Not the same!

Leaving void out of the prototype indicates to the compiler that there is no additional information about the parameters to the function. It effectively turns off all that type checking.

With a prototype definitely use void when you have an empty parameter list.

5 Pointers—Cower In Fear!

Pointers are one of the most feared things in the C language. In fact, they are the one thing that makes this language challenging at all. But why?

Because they, quite honestly, can cause electric shocks to come up through the keyboard and physically weld your arms permanently in place, cursing you to a life at the keyboard in this language from the 70s!

Depending on what language you came from, you might already understand the concept of references, where a variable refers to an object of some type.

This is very much the same, except we have to be more explicit with C about when we’re talking about the reference or the thing it refers to.

5.1 Memory and Variables

Computer memory holds data of all kinds, right? It’ll hold floats, ints, or whatever you have. To make memory easy to cope with, each byte of memory is identified by an integer. These integers increase sequentially as you move up through memory⁴⁴. You can think of it as a bunch of numbered boxes, where each box holds a byte⁴⁵ of data. Or like a big array where each element holds a byte, if you come from a language with arrays. The number that represents each box is called its address.

Now, not all data types use just a byte. For instance, an int is often four bytes, as is a float, but it really depends on the system. You can use the sizeof operator to determine how many bytes of memory a certain type uses.

// %zu is the format specifier for type size_t

printf("an int uses %zu bytes of memory\n", sizeof(int));

// That prints "4" for me, but can vary by system.

So anyway, if we can get on with it and get a drum roll and some foreboding music playing for the definition of a pointer, a pointer is a variable that holds an address. Imagine the classical score from 2001: A Space Odyssey at this point. Ba bum ba bum ba bum BAAAAH!

Ok, so maybe a bit overwrought here, yes? There’s not a lot of mystery about pointers. They are the address of data. Just like an int variable can hold the value 12, a pointer variable can hold the address of data.

This means that all these things mean the same thing, i.e. a number that represents a point in memory:

I’m going to use these interchangeably. And yes, I just threw location in there because you can never have enough words that mean the same thing.

And a pointer variable holds that address number. Just like a float variable might hold 3.14159.

Imagine you have a bunch of Post-it® notes all numbered in sequence with their address. (The first one is at index numbered 0, the next at index 1, and so on.)

In addition to the number representing their positions, you can also write another number of your choice on each. It could be the number of dogs you have. Or the number of moons around Mars…

If you have written the number of dogs you have, that’s just a regular variable. But if you wrote the index of another Post-it in there, that’s a pointer. It points to the other note!

Another analogy might be with house addresses. You can have a house with certain qualities, yard, metal roof, solar, etc. Or you could have the address of that house. The address isn’t the same as the house itself. One’s a full-blown house, and the other is just a few lines of text. But the address of the house is a pointer to that house. It’s not the house itself, but it tells you where to find it.

And we can do the same thing in the computer with data. You can have a data variable that’s holding some value. And that value is in memory at some address. And you could have a different pointer variable hold the address of that data variable.

It’s not the data variable itself, but, like with a house address, it tells us where to find it.

When we have that, we say we have a “pointer to” that data. And we can follow the pointer to access the data itself.

(Though it doesn’t seem particularly useful yet, this all becomes indispensable when used with function calls. Bear with me until we get there.)

So if we have an int, say, and we want a pointer to it, what we want is some way to get the address of that int, right? After all, the pointer just holds the address of the data. What operator do you suppose we’d use to find the address of the int?

Well, by a shocking surprise that must come as something of a shock to you, gentle reader, we use the address-of operator (which happens to be an ampersand: “&”)to find the address of the data. Ampersand.

So for a quick example, we’ll introduce a new format specifier for printf() so you can print a pointer. You know already how %d prints a decimal integer, yes? Well, %p prints a pointer. Now, this pointer is going to look like a garbage number (and it might be printed in hexadecimal⁴⁷ instead of decimal), but it is merely the index into memory the data is stored in. (Or the index into memory that the first byte of data is stored in, if the data is multi-byte.) In virtually all circumstances, including this one, the actual value of the number printed is unimportant to you, and I show it here only for demonstration of the address-of operator.

#include <stdio.h>

int main(void)
{
    int i = 10;

    printf("The value of i is %d\n", i);
    printf("And its address is %p\n", (void *)&i);
}

The value of i is 10
And its address is 0x7ffddf7072a4

If you’re curious, that hexadecimal number is 140,727,326,896,068 in decimal (base 10 just like Grandma used to use). That’s the index into memory where the variable i’s data is stored. It’s the address of i. It’s the location of i. It’s a pointer to i.

It’s a pointer because it lets you know where i is in memory. Like a home address written on a scrap of paper tells you where you can find a particular house, this number indicates to us where in memory we can find the value of i. It points to i.

Again, we don’t really care what the address’s exact number is, generally. We just care that it’s a pointer to i.

5.2 Pointer Types

So… this is all well and good. You can now successfully take the address of a variable and print it on the screen. There’s a little something for the ol’ resume, right? Here’s where you grab me by the scruff of the neck and ask politely what the frick pointers are good for.

Excellent question, and we’ll get to that right after these messages from our sponsor.

Welcome back to another installment of Beej’s Guide. When we met last we were talking about how to make use of pointers. Well, what we’re going to do is store a pointer off in a variable so that we can use it later. You can identify the pointer type because there’s an asterisk (*) before the variable name and after its type:

int main(void)
{
    int i;  // i's type is "int"
    int *p; // p's type is "pointer to an int", or "int-pointer"
}

Hey, so we have here a variable that is a pointer type, and it can point to other ints. That is, it can hold the address of other ints. We know it points to ints, since it’s of type int* (read “int-pointer”).

When you do an assignment into a pointer variable, the type of the right hand side of the assignment has to be the same type as the pointer variable. Fortunately for us, when you take the address-of a variable, the resultant type is a pointer to that variable type, so assignments like the following are perfect:

int i;
int *p;  // p is a pointer, but is uninitialized and points to garbage

p = &i;  // p is assigned the address of i--p now "points to" i

On the left of the assignment, we have a variable of type pointer-to-int (int*), and on the right side, we have expression of type pointer-to-int since i is an int (because address-of int gives you a pointer to int). The address of a thing can be stored in a pointer to that thing.

Get it? I know it still doesn’t quite make much sense since you haven’t seen an actual use for the pointer variable, but we’re taking small steps here so that no one gets lost. So now, let’s introduce you to the anti-address-of operator. It’s kind of like what address-of would be like in Bizarro World.

5.3 Dereferencing

A pointer variable can be thought of as referring to another variable by pointing to it. It’s rare you’ll hear anyone in C land talking about “referring” or “references”, but I bring it up just so that the name of this operator will make a little more sense.

When you have a pointer to a variable (roughly “a reference to a variable”), you can use the original variable through the pointer by dereferencing the pointer. (You can think of this as “de-pointering” the pointer, but no one ever says “de-pointering”.)

Back to our analogy, this is vaguely like looking at a home address and then going to that house.

Now, what do I mean by “get access to the original variable”? Well, if you have a variable called i, and you have a pointer to i called p, you can use the dereferenced pointer p exactly as if it were the original variable i!

You almost have enough knowledge to handle an example. The last tidbit you need to know is actually this: what is the dereference operator? It’s actually called the indirection operator, because you’re accessing values indirectly via the pointer. And it is the asterisk, again: *. Now, don’t get this confused with the asterisk you used in the pointer declaration, earlier. They are the same character, but they have different meanings in different contexts⁴⁹.

#include <stdio.h>

int main(void)
{
    int i;
    int *p;  // this is NOT a dereference--this is a type "int*"

    p = &i;  // p now points to i, p holds address of i

    i = 10;  // i is now 10
    *p = 20; // the thing p points to (namely i!) is now 20!!

    printf("i is %d\n", i);   // prints "20"
    printf("i is %d\n", *p);  // "20"! dereference-p is the same as i!
}

Remember that p holds the address of i, as you can see where we did the assignment to p on line 8. What the indirection operator does is tells the computer to use the object the pointer points to instead of using the pointer itself. In this way, we have turned *p into an alias of sorts for i.

5.4 Passing Pointers as Arguments

Right about now, you’re thinking that you have an awful lot of knowledge about pointers, but absolutely zero application, right? I mean, what use is *p if you could just simply say i instead?

Well, my friend, the real power of pointers comes into play when you start passing them to functions. Why is this a big deal? You might recall from before that you could pass all kinds of arguments to functions and they’d be dutifully copied into parameters, and then you could manipulate local copies of those variables from within the function, and then you could return a single value.

What if you wanted to bring back more than one single piece of data from the function? I mean, you can only return one thing, right? What if I answered that question with another question? …Er, two questions?

What happens when you pass a pointer as an argument to a function? Does a copy of the pointer get put into its corresponding parameter? You bet your sweet peas it does. Remember how earlier I rambled on and on about how EVERY SINGLE ARGUMENT gets copied into parameters and the function uses a copy of the argument? Well, the same is true here. The function will get a copy of the pointer.

But, and this is the clever part: we will have set up the pointer in advance to point at a variable… and then the function can dereference its copy of the pointer to get back to the original variable! The function can’t see the variable itself, but it can certainly dereference a pointer to that variable!

This is analogous to writing a home address on a piece of paper, and then copying that onto another piece of paper. You now have two pointers to that house, and both are equally good at getting you to the house itself.

In the case of a function call. one of the copies is stored in a pointer variable out in the calling scope, and the other is stored in a pointer variable that is the parameter of the function.

Example! Let’s revisit our old increment() function, but this time let’s make it so that it actually increments the value out in the caller.

#include <stdio.h>

void increment(int *p)  // note that it accepts a pointer to an int
{
    *p = *p + 1;        // add one to the thing p points to
}

int main(void)
{
    int i = 10;
    int *j = &i;  // note the address-of; turns it into a pointer to i

    printf("i is %d\n", i);        // prints "10"
    printf("i is also %d\n", *j);  // prints "10"

    increment(j);                  // j is an int*--to i

    printf("i is %d\n", i);        // prints "11"!
}

Ok! There are a couple things to see here… not the least of which is that the increment() function takes an int* as an argument. We pass it an int* in the call by changing the int variable i to an int* using the address-of operator. (Remember, a pointer holds an address, so we make pointers to variables by running them through the address-of operator.)

The increment() function gets a copy of the pointer. Both the original pointer j (in main()) and the copy of that pointer p (the parameter in increment()) point to the same address, namely the one holding the value i. (Again, by analogy, like two pieces of paper with the same home address written on them.) Dereferencing either will allow you to modify the original variable i! The function can modify a variable in another scope! Rock on!

The above example is often more concisely written in the call just by using address-of right in the argument list:

printf("i is %d\n", i);  // prints "10"
increment(&i);
printf("i is %d\n", i);  // prints "11"!

As a general rule, if you want the function to modify the thing that you’re passing in such that you see the result, you’ll have to pass a pointer to that thing.

5.5 The NULL Pointer

Any pointer variable of any pointer type can be set to a special value called NULL. This indicates that this pointer doesn’t point to anything.

Since it doesn’t point to a value, dereferencing it is undefined behavior, and probably will result in a crash:

int *p = NULL;

*p = 12;  // CRASH or SOMETHING PROBABLY BAD. BEST AVOIDED.

(Of course, like other variables, the pointer points to garbage unless you explicitly assign it to point to an address or NULL.)

5.6 A Note on Declaring Pointers

The syntax for declaring a pointer can get a little weird. Let’s look at this example:

int a, b;  // Same thing

The rule is that the * goes in front of any variable that is a pointer type. That is. the * is not part of the int in this example. it’s a part of variable p.

int a, *p;  // Same thing

It’s important to note that the following line does not declare two pointers:

int *p, q;  // p is a pointer to an int; q is just an int.

This can be particularly insidious-looking if the programmer writes this following (valid) line of code which is functionally identical to the one above.

int* p, q;  // p is a pointer to an int; q is just an int.

So take a look at this and determine which variables are pointers and which are not:

int *a, b, c, *d, e, *f, g, h, *i;

5.7 sizeof and Pointers

Just a little bit of syntax here that might be confusing and you might see from time to time.

int *p;

// Prints size of an 'int'
printf("%zu\n", sizeof(int));

// p is type 'int *', so prints size of 'int*'
printf("%zu\n", sizeof p);

// *p is type 'int', so prints size of 'int'
printf("%zu\n", sizeof *p);

You might see code in the wild with that last sizeof in there. Just remember that sizeof is all about the type of the expression, not the variables in the expression themselves.

6 Arrays

Luckily, C has arrays. I mean, I know it’s considered a low-level language⁵³ but it does at least have the concept of arrays built-in. And since a great many languages drew inspiration from C’s syntax, you’re probably already familiar with using [ and ] for declaring and using arrays.

But C only barely has arrays! As we’ll find out later, arrays are just syntactic sugar in C—they’re actually all pointers and stuff deep down. Freak out! But for now, let’s just use them as arrays. Phew.

6.1 Easy Example

#include <stdio.h>

int main(void)
{
    int i;
    float f[4];  // Declare an array of 4 floats

    f[0] = 3.14159;  // Indexing starts at 0, of course.
    f[1] = 1.41421;
    f[2] = 1.61803;
    f[3] = 2.71828;

    // Print them all out:

    for (i = 0; i < 4; i++) {
        printf("%f\n", f[i]);
    }
}

When you declare an array, you have to give it a size. And the size has to be fixed⁵⁴.

In the above example, we made an array of 4 floats. The value in the square brackets in the declaration lets us know that.

Later on in subsequent lines, we access the values in the array, setting them or getting them, again with square brackets.

6.2 Getting the Length of an Array

You can’t…ish. C doesn’t record this information⁵⁵. You have to manage it separately in another variable.

When I say “can’t”, I actually mean there are some circumstances when you can. There is a trick to get the number of elements in an array in the scope in which an array is declared. But, generally speaking, this won’t work the way you want if you pass the array to a function⁵⁶.

Let’s take a look at this trick. The basic idea is that you take the sizeof the array, and then divide that by the size of each element to get the length. For example, if an int is 4 bytes, and the array is 32 bytes long, there must be room for $\frac{32}{4}$ or $8$ ints in there.

int x[12];  // 12 ints

printf("%zu\n", sizeof x);     // 48 total bytes
printf("%zu\n", sizeof(int));  // 4 bytes per int

printf("%zu\n", sizeof x / sizeof(int));  // 48/4 = 12 ints!

If it’s an array of chars, then sizeof the array is the number of elements, since sizeof(char) is defined to be 1. For anything else, you have to divide by the size of each element.

But this trick only works in the scope in which the array was defined. If you pass the array to a function, it doesn’t work. Even if you make it “big” in the function signature:

void foo(int x[12])
{
    printf("%zu\n", sizeof x);     // 8?! What happened to 48?
    printf("%zu\n", sizeof(int));  // 4 bytes per int

    printf("%zu\n", sizeof x / sizeof(int));  // 8/4 = 2 ints?? WRONG.
}

This is because when you “pass” arrays to functions, you’re only passing a pointer to the first element, and that’s what sizeof measures. More on this in the Passing Single Dimensional Arrays to Functions section, below.

One more thing you can do with sizeof and arrays is get the size of an array of a fixed number of elements without declaring the array. This is like how you can get the size of an int with sizeof(int).

For example, to see how many bytes would be needed for an array of 48 doubles, you can do this:

sizeof(double [48]);

6.3 Array Initializers

#include <stdio.h>

int main(void)
{
    int i;
    int a[5] = {22, 37, 3490, 18, 95};  // Initialize with these values

    for (i = 0; i < 5; i++) {
        printf("%d\n", a[i]);
    }
}

You should never have more items in your initializer than there is room for in the array, or the compiler will get cranky:

foo.c: In function ‘main’:
foo.c:6:39: warning: excess elements in array initializer
    6 |     int a[5] = {22, 37, 3490, 18, 95, 999};
      |                                       ^~~
foo.c:6:39: note: (near initialization for ‘a’)

But (fun fact!) you can have fewer items in your initializer than there is room for in the array. The remaining elements in the array will be automatically initialized with zero. This is true in general for all types of array initializers: if you have an initializer, anything not explicitly set to a value will be set to zero.

int a[5] = {22, 37, 3490};

// is the same as:

int a[5] = {22, 37, 3490, 0, 0};

It’s a common shortcut to see this in an initializer when you want to set an entire array to zero:

Which means, “Make the first element zero, and then automatically make the rest zero, as well.”

You can set specific array elements in the initializer, as well, by specifying an index for the value! When you do this, C will happily keep initializing subsequent values for you until the initializer runs out, filling everything else with 0.

To do this, put the index in square brackets with an = after, and then set the value.

int a[10] = {0, 11, 22, [5]=55, 66, 77};

Because we listed index 5 as the start for 55, the resulting data in the array is:

0 11 22 0 0 55 66 77 0 0

#define COUNT 5

int a[COUNT] = {[COUNT-3]=3, 2, 1};

Lastly, you can also have C compute the size of the array from the initializer, just by leaving the size off:

int a[3] = {22, 37, 3490};

// is the same as:

int a[] = {22, 37, 3490};  // Left the size off!

6.4 Out of Bounds!

C doesn’t stop you from accessing arrays out of bounds. It might not even warn you.

Let’s steal the example from above and keep printing off the end of the array. It only has 5 elements, but let’s try to print 10 and see what happens:

#include <stdio.h>

int main(void)
{
    int i;
    int a[5] = {22, 37, 3490, 18, 95};

    for (i = 0; i < 10; i++) {  // BAD NEWS: printing too many elements!
        printf("%d\n", a[i]);
    }
}

Yikes! What’s that? Well, turns out printing off the end of an array results in what C developers call undefined behavior. We’ll talk more about this beast later, but for now it means, “You’ve done something bad, and anything could happen during your program run.”

And by anything, I mean typically things like finding zeroes, finding garbage numbers, or crashing. But really the C spec says in this circumstance the compiler is allowed to emit code that does anything⁵⁷.

6.5 Multidimensional Arrays

int a[10];
int b[2][7];
int c[4][5][6];

These are stored in memory in row-major order ⁵⁹. This means with a 2D array, the first index listed indicates the row, and the second the column.

#include <stdio.h>

int main(void)
{
    int row, col;

    int a[2][5] = {      // Initialize a 2D array
        {0, 1, 2, 3, 4},
        {5, 6, 7, 8, 9}
    };

    for (row = 0; row < 2; row++) {
        for (col = 0; col < 5; col++) {
            printf("(%d,%d) = %d\n", row, col, a[row][col]);
        }
    }
}

(0,0) = 0
(0,1) = 1
(0,2) = 2
(0,3) = 3
(0,4) = 4
(1,0) = 5
(1,1) = 6
(1,2) = 7
(1,3) = 8
(1,4) = 9

// Make a 3x3 identity matrix

int a[3][3] = {[0][0]=1, [1][1]=1, [2][2]=1};

6.6 Arrays and Pointers

[Casually] So… I kinda might have mentioned up there that arrays were pointers, deep down? We should take a shallow dive into that now so that things aren’t completely confusing. Later on, we’ll look at what the real relationship between arrays and pointers is, but for now I just want to look at passing arrays to functions.

6.6.1 Getting a Pointer to an Array

I want to tell you a secret. Generally speaking, when a C programmer talks about a pointer to an array, they’re talking about a pointer to the first element of the array⁶⁰.

#include <stdio.h>

int main(void)
{
    int a[5] = {11, 22, 33, 44, 55};
    int *p;

    p = &a[0];  // p points to the array
                // Well, to the first element, actually

    printf("%d\n", *p);  // Prints "11"
}

p = &a[0];  // p points to the array

// is the same as:

p = a;      // p points to the array, but much nicer-looking!

Just referring to the array name in isolation is the same as getting a pointer to the first element of the array! We’re going to use this extensively in the upcoming examples.

But hold on a second—isn’t p an int*? And *p gives us 11, same as a[0]? Yessss. You’re starting to get a glimpse of how arrays and pointers are related in C.

6.6.2 Passing Single Dimensional Arrays to Functions

Let’s do an example with a single dimensional array. I’m going to write a couple functions that we can pass the array to that do different things.

#include <stdio.h>

// Passing as a pointer to the first element
void times2(int *a, int len)
{
    for (int i = 0; i < len; i++)
        printf("%d\n", a[i] * 2);
}

// Same thing, but using array notation
void times3(int a[], int len)
{
    for (int i = 0; i < len; i++)
        printf("%d\n", a[i] * 3);
}

// Same thing, but using array notation with size
void times4(int a[5], int len)
{
    for (int i = 0; i < len; i++)
        printf("%d\n", a[i] * 4);
}

int main(void)
{
    int x[5] = {11, 22, 33, 44, 55};

    times2(x, 5);
    times3(x, 5);
    times4(x, 5);
}

All those methods of listing the array as a parameter in the function are identical.

void times2(int *a, int len)
void times3(int a[], int len)
void times4(int a[5], int len)

And, in fact, in the latter situation, the compiler doesn’t even care what number you pass in (other than it has to be greater than zero⁶¹). It doesn’t enforce anything at all.

Now that I’ve said that, the size of the array in the function declaration actually does matter when you’re passing multidimensional arrays into functions, but let’s come back to that.

6.6.3 Changing Arrays in Functions

We’ve said that arrays are just pointers in disguise. This means that if you pass an array to a function, you’re likely passing a pointer to the first element in the array.

But if the function has a pointer to the data, it is able to manipulate that data! So changes that a function makes to an array will be visible back out in the caller.

Here’s an example where we pass a pointer to an array to a function, the function manipulates the values in that array, and those changes are visible out in the caller.

#include <stdio.h>

void double_array(int *a, int len)
{
    // Multiply each element by 2
    //
    // This doubles the values in x in main() since x and a both point
    // to the same array in memory!

    for (int i = 0; i < len; i++)
        a[i] *= 2;
}

int main(void)
{
    int x[5] = {1, 2, 3, 4, 5};

    double_array(x, 5);

    for (int i = 0; i < 5; i++)
        printf("%d\n", x[i]);  // 2, 4, 6, 8, 10!
}

Even though we passed the array in as parameter a which is type int*, look at how we access it using array notation with a[i]! Whaaaat. This is totally allowed.

Later when we talk about the equivalence between arrays and pointers, we’ll see how this makes a lot more sense. For now, it’s enough to know that functions can make changes to arrays that are visible out in the caller.

6.6.4 Passing Multidimensional Arrays to Functions

The story changes a little when we’re talking about multidimensional arrays. C needs to know all the dimensions (except the first one) so it has enough information to know where in memory to look to find a value.

#include <stdio.h>

void print_2D_array(int a[2][3])
{
    for (int row = 0; row < 2; row++) {
        for (int col = 0; col < 3; col++)
            printf("%d ", a[row][col]);
        printf("\n");
    }
}

int main(void)
{
    int x[2][3] = {
        {1, 2, 3},
        {4, 5, 6}
    };

    print_2D_array(x);
}

void print_2D_array(int a[2][3])
void print_2D_array(int a[][3])

The compiler really only needs the second dimension so it can figure out how far in memory to skip for each increment of the first dimension. In general, it needs to know all the dimensions except the first one.

Also, remember that the compiler does minimal compile-time bounds checking (if you’re lucky), and C does zero runtime checking of bounds. No seat belts! Don’t crash by accessing array elements out of bounds!

7 Strings

Well, turns out strings aren’t actually strings in C. That’s right! They’re pointers! Of course they are!

7.1 String Literals

Before we start, let’s talk about string literals in C. These are sequences of characters in double quotes ("). (Single quotes enclose characters, and are a different animal entirely.)

"Hello, world!\n"
"This is a test."
"When asked if this string had quotes in it, she replied, \"It does.\""

The last one has quotes embedded within it, but you see each is preceded by (we say “escaped by”) a backslash (\) indicating that a literal quote belongs in the string at this point. This is how the C compiler can tell the difference between printing a double quote and the double quote at the end of the string.

7.2 String Variables

Now that we know how to make a string literal, let’s assign it to a variable so we can do something with it.

char *s = "Hello, world!";

Check out that type: pointer to a char. The string variable s is actually a pointer to the first character in that string, namely the H.

char *s = "Hello, world!";

printf("%s\n", s);  // "Hello, world!"

7.3 String Variables as Arrays

char s[14] = "Hello, world!";

// or, if we were properly lazy and have the compiler
// figure the length for us:

char s[] = "Hello, world!";

This means you can use array notation to access characters in a string. Let’s do exactly that to print all the characters in a string on the same line:

#include <stdio.h>

int main(void)
{
    char s[] = "Hello, world!";

    for (int i = 0; i < 13; i++)
        printf("%c\n", s[i]);
}

Also, check this out. The program will still work fine if we change the definition of s to be a char* type:

#include <stdio.h>

int main(void)
{
    char *s = "Hello, world!";   // char* here

    for (int i = 0; i < 13; i++)
        printf("%c\n", s[i]);    // But still use arrays here...?
}

And we still can use array notation to get the job done when printing it out! This is surprising, but is still only because we haven’t talked about array/pointer equivalence yet. But this is yet another hint that arrays and pointers are the same thing, deep down.

7.4 String Initializers

We’ve already seen some examples with initializing string variables with string literals:

char *s = "Hello, world!";
char t[] = "Hello, again!";

But these two initializations are subtly different. A string literal, similar to an integer literal, has it’s memory automatically managed by the compiler for you! With an integer, i.e. a fixed size piece of data, the compiler can pretty easily manage it. But strings are a variable-byte beast which the compiler tames by tossing into a chunk of memory, and giving you a pointer to it.

This form points to wherever that string was placed. Typically, that place is in a land faraway from the rest of your program’s memory – read-only memory – for reasons related to performance & safety.

char *s = "Hello, world!";

char *s = "Hello, world!";

s[0] = 'z';  // BAD NEWS: tried to mutate a string literal!

The behavior is undefined. Probably, depending on your system, a crash will result.

But declaring it as an array is different. The compiler doesn’t stow those bytes in another part of town, they’re right down the street. This one is a mutable copy of the string – one we can change at will:

char t[] = "Hello, again!";  // t is an array copy of the string 
t[0] = 'z'; //  No problem

printf("%s\n", t);  // "zello, again!"

So remember: if you have a pointer to a string literal, don’t try to change it! And if you use a string in double quotes to initialize an array, that’s not actually a string literal.

7.5 Getting String Length

You can’t, since C doesn’t track it for you. And when I say “can’t”, I actually mean “can”⁶³. There’s a function in <string.h> called strlen() that can be used to compute the length of any string in bytes⁶⁴.

#include <stdio.h>
#include <string.h>

int main(void)
{
    char *s = "Hello, world!";

    printf("The string is %zu bytes long.\n", strlen(s));
}

The strlen() function returns type size_t, which is an integer type so you can use it for integer math. We print size_t with %zu.

The string is 13 bytes long.

But… if C doesn’t track the length of the string anywhere, how does it know how long the string is?

7.6 String Termination

C does strings a little differently than many programming languages, and in fact differently than almost every modern programming language.

When you’re making a new language, you have basically two options for storing a string in memory:

If you want strings longer than 255 characters, option 1 requires at least two bytes to store the length. Whereas option 2 only requires one byte to terminate the string. So a bit of savings there.

Of course, these days it seems ridiculous to worry about saving a byte (or 3—lots of languages will happily let you have strings that are 4 gigabytes in length). But back in the day, it was a bigger deal.

So C took approach #2. In C, a “string” is defined by two basic characteristics:

A NUL character can be written in C code as \0, though you don’t often have to do this.

When you include a string in double quotes in your code, the NUL character is automatically, implicitly included.

char *s = "Hello!";  // Actually "Hello!\0" behind the scenes

So with this in mind, let’s write our own strlen() function that counts chars in a string until it finds a NUL.

The procedure is to look down the string for a single NUL character, counting as we go⁶⁶:

int my_strlen(char *s)
{
    int count = 0;

    while (s[count] != '\0')  // Single quotes for single char
        count++;

    return count;
}

7.7 Copying a String

You can’t copy a string through the assignment operator (=). All that does is make a copy of the pointer to the first character… so you end up with two pointers to the same string:

#include <stdio.h>

int main(void)
{
    char s[] = "Hello, world!";
    char *t;

    // This makes a copy of the pointer, not a copy of the string!
    t = s;

    // We modify t
    t[0] = 'z';

    // But printing s shows the modification!
    // Because t and s point to the same string!

    printf("%s\n", s);  // "zello, world!"
}

If you want to make a copy of a string, you have to copy it a byte at a time—but this is made easier with the strcpy() function⁶⁷.

Before you copy the string, make sure you have room to copy it into, i.e. the destination array that’s going to hold the characters needs to be at least as long as the string you’re copying.

#include <stdio.h>
#include <string.h>

int main(void)
{
    char s[] = "Hello, world!";
    char t[100];  // Each char is one byte, so plenty of room

    // This makes a copy of the string!
    strcpy(t, s);

    // We modify t
    t[0] = 'z';

    // And s remains unaffected because it's a different string
    printf("%s\n", s);  // "Hello, world!"

    // But t has been changed
    printf("%s\n", t);  // "zello, world!"
}

Notice with strcpy(), the destination pointer is the first argument, and the source pointer is the second. A mnemonic I use to remember this is that it’s the order you would have put t and s if an assignment = worked for strings, with the source on the right and the destination on the left.

8 Structs

In C, we have something called a struct, which is a user-definable type that holds multiple pieces of data, potentially of different types.

It’s a convenient way to bundle multiple variables into a single one. This can be beneficial for passing variables to functions (so you just have to pass one instead of many), and useful for organizing data and making code more readable.

If you’ve come from another language, you might be familiar with the idea of classes and objects. These don’t exist in C, natively⁶⁸. You can think of a struct as a class with only data members, and no methods.

8.1 Declaring a Struct

struct car {
    char *name;
    float price;
    int speed;
};

This is often done at the global scope outside any functions so that the struct is globally available.

When you do this, you’re making a new type. The full type name is struct car. (Not just car—that won’t work.)

struct car saturn;  // Variable "saturn" of type "struct car"

We should initialize it! But how do we set the values of those individual fields?

Like in many other languages that stole it from C, we’re going to use the dot operator (.) to access the individual fields.

saturn.name = "Saturn SL/2";
saturn.price = 15999.99;
saturn.speed = 175;

printf("Name:           %s\n", saturn.name);
printf("Price (USD):    %f\n", saturn.price);
printf("Top Speed (km): %d\n", saturn.speed);

There on the first lines, we set the values in the struct car, and then in the next bit, we print those values out.

8.2 Struct Initializers

That example in the previous section was a little unwieldy. There must be a better way to initialize that struct variable!

You can do it with an initializer by putting values in for the fields in the order they appear in the struct when you define the variable. (This won’t work after the variable has been defined—it has to happen in the definition).

struct car {
    char *name;
    float price;
    int speed;
};

// Now with an initializer! Same field order as in the struct declaration:
struct car saturn = {"Saturn SL/2", 16000.99, 175};

printf("Name:      %s\n", saturn.name);
printf("Price:     %f\n", saturn.price);
printf("Top Speed: %d km\n", saturn.speed);

The fact that the fields in the initializer need to be in the same order is a little freaky. If someone changes the order in struct car, it could break all the other code!

struct car saturn = {.speed=175, .name="Saturn SL/2"};

Now it’s independent of the order in the struct declaration. Which is safer code, for sure.

Similar to array initializers, any missing field designators are initialized to zero (in this case, that would be .price, which I’ve omitted).

8.3 Passing Structs to Functions

Recall that when you pass something to a function, a copy of that thing gets made for the function to operate on, whether it’s a copy of a pointer, an int, a struct, or anything.

There are basically two cases when you’d want to pass a pointer to the struct:

For those two reasons, it’s far more common to pass a pointer to a struct to a function, though its by no means illegal to pass the struct itself.

Let’s try passing in a pointer, making a function that will allow you to set the .price field of the struct car:

#include <stdio.h>

struct car {
    char *name;
    float price;
    int speed;
};

int main(void)
{
    struct car saturn = {.speed=175, .name="Saturn SL/2"};

    // Pass a pointer to this struct car, along with a new,
    // more realistic, price:
    set_price(&saturn, 799.99);

    printf("Price: %f\n", saturn.price);
}

You should be able to come up with the function signature for set_price() just by looking at the types of the arguments we have there.

saturn is a struct car, so &saturn must be the address of the struct car, AKA a pointer to a struct car, namely a struct car*.

void set_price(struct car *c, float new_price)

void set_price(struct car *c, float new_price) {
    c.price = new_price;  // ERROR!!
}

That won’t work because the dot operator only works on structs… it doesn’t work on pointers to structs.

Ok, so we can dereference the variable c to de-pointer it to get to the struct itself. Dereferencing a struct car* results in the struct car that the pointer points to, which we should be able to use the dot operator on:

void set_price(struct car *c, float new_price) {
    (*c).price = new_price;  // Works, but is ugly and non-idiomatic :(
}

And that works! But it’s a little clunky to type all those parens and the asterisk. C has some syntactic sugar called the arrow operator that helps with that.

8.4 The Arrow Operator

void set_price(struct car *c, float new_price) {
    // (*c).price = new_price;  // Works, but non-idiomatic :(
    //
    // The line above is 100% equivalent to the one below:

    c->price = new_price;  // That's the one!
}

8.5 Copying and Returning structs

struct car a, b;

b = a;  // Copy the struct

And returning a struct (as opposed to a pointer to one) from a function also makes a similar copy to the receiving variable.

This is not a “deep copy”⁷¹. All fields are copied as-is, including pointers to things.

8.6 Comparing structs

You might think you could use memcmp()⁷², but that doesn’t handle the case of the possible padding bytes that might be in there.

9 File Input/Output

We’ve already seen some examples of I/O with printf() for doing I/O at the console.

9.1 The FILE* Data Type

When we do any kind of I/O in C, we do so through a piece of data that you get in the form of a FILE* type. This FILE* holds all the information needed to communicate with the I/O subsystem about which file you have open, where you are in the file, and so on.

The spec refers to these as streams, i.e. a stream of data from a file or from any source. I’m going to use “files” and “streams” interchangeably, but really you should think of a “file” as a special case of a “stream”. There are other ways to stream data into a program than just reading from a file.

We’ll see in a moment how to go from having a filename to getting an open FILE* for it, but first I want to mention three streams that are already open for you and ready for use.

We’ve actually been using these implicitly already, it turns out. For example, these two calls are the same:

`FILE*` name	Description
`stdin`	Standard Input, generally the keyboard by default
`stdout`	Standard Output, generally the screen by default
`stderr`	Standard Error, generally the screen by default, as well

printf("Hello, world!\n");
fprintf(stdout, "Hello, world!\n");  // printf to a file

Also you’ll notice that both stdout and stderr go to the screen. While this seems at first either like an oversight or redundancy, it actually isn’t. Typical operating systems allow you to redirect the output of either of those into different files, and it can be convenient to be able to separate error messages from regular non-error output.

For example, in a POSIX shell (like sh, ksh, bash, zsh, etc.) on a Unix-like system, we could run a program and send just the non-error (stdout) output to one file, and all the error (stderr) output to another file.

./foo > output.txt 2> errors.txt   # This command is Unix-specific

For this reason, you should send serious error messages to stderr instead of stdout.

9.2 Reading Text Files

Text streams are allowed to do significant translation of the data, most notably translations of newlines to their different representations⁷⁵. Text files are logically a sequence of lines separated by newlines. To be portable, your input data should always end with a newline.

But the general rule is that if you’re able to edit the file in a regular text editor, it’s a text file. Otherwise, it’s binary. More on binary later.

So let’s get to work—how do we open a file for reading, and pull data out of it?

And let’s write a program to open the file, read a character out of it, and then close the file when we’re done. That’s the game plan!

#include <stdio.h>

int main(void)
{
    FILE *fp;                      // Variable to represent open file

    fp = fopen("hello.txt", "r");  // Open file for reading

    int c = fgetc(fp);             // Read a single character
    printf("%c\n", c);             // Print char to stdout

    fclose(fp);                    // Close the file when done
}

See how when we opened the file with fopen(), it returned the FILE* to us so we could use it later.

(I’m leaving it out for brevity, but fopen() will return NULL if something goes wrong, like file-not-found, so you should really error check it!)

Also notice the "r" that we passed in—this means “open a text stream for reading”. (There are various strings we can pass to fopen() with additional meaning, like writing, or appending, and so on.)

After that, we used the fgetc() function to get a character from the stream. You might be wondering why I’ve made c an int instead of a char—hold that thought!

Finally, we close the stream when we’re done with it. All streams are automatically closed when the program exits, but it’s good form and good housekeeping to explicitly close any files yourself when done with them.

The FILE* keeps track of our position in the file. So subsequent calls to fgetc() would get the next character in the file, and then the next, until the end.

9.3 End of File: EOF

There is a special character defined as a macro: EOF. This is what fgetc() will return when the end of the file has been reached and you’ve attempted to read another character.

How about I share that Fun Fact™, now. Turns out EOF is the reason why fgetc() and functions like it return an int instead of a char. EOF isn’t a character proper, and its value likely falls outside the range of char. Since fgetc() needs to be able to return any byte and EOF, it needs to be a wider type that can hold more values. so int it is. But unless you’re comparing the returned value against EOF, you can know, deep down, it’s a char.

#include <stdio.h>

int main(void)
{
    FILE *fp;
    int c;

    fp = fopen("hello.txt", "r");

    while ((c = fgetc(fp)) != EOF)
        printf("%c", c);

    fclose(fp);
}

(If line 10 is too weird, just break it down starting with the innermost-nested parens. The first thing we do is assign the result of fgetc() into c, and then we compare that against EOF. We’ve just crammed it into a single line. This might look hard to read, but study it—it’s idiomatic C.)

But still, we’re operating a character at a time, and lots of text files make more sense at the line level. Let’s switch to that.

9.3.1 Reading a Line at a Time

So how can we get an entire line at once? fgets() to the rescue! For arguments, it takes a pointer to a char buffer to hold bytes, a maximum number of bytes to read, and a FILE* to read from. It returns NULL on end-of-file or error. fgets() is even nice enough to NUL-terminate the string when its done⁷⁶.

Let’s do a similar loop as before, except let’s have a multiline file and read it in a line at a time.

A wise man can learn more from
a foolish question than a fool
can learn from a wise answer.
                  --Bruce Lee

And here’s some code that reads that file a line at a time and prints out a line number before each one:

#include <stdio.h>

int main(void)
{
    FILE *fp;
    char s[1024];  // Big enough for any line this program will encounter
    int linecount = 0;

    fp = fopen("quote.txt", "r");

    while (fgets(s, sizeof s, fp) != NULL) 
        printf("%d: %s", ++linecount, s);

    fclose(fp);
}

1: A wise man can learn more from
2: a foolish question than a fool
3: can learn from a wise answer.
4:                   --Bruce Lee

9.4 Formatted Input

You know how you can get formatted output with printf() (and, thus, fprintf() like we’ll see, below)?

Let’s have a file with a series of data records in it. In this case, whales, with name, length in meters, and weight in tonnes. whales.txt:

blue 29.9 173
right 20.7 135
gray 14.9 41
humpback 16.0 30

Yes, we could read these with fgets() and then parse the string with sscanf() (and in that’s more resilient against corrupted files), but in this case, let’s just use fscanf() and pull it in directly.

The fscanf() function skips leading whitespace when reading, and returns EOF on end-of-file or error.

#include <stdio.h>

int main(void)
{
    FILE *fp;
    char name[1024];  // Big enough for any line this program will encounter
    float length;
    int mass;

    fp = fopen("whales.txt", "r");

    while (fscanf(fp, "%s %f %d", name, &length, &mass) != EOF)
        printf("%s whale, %d tonnes, %.1f meters\n", name, mass, length);

    fclose(fp);
}

blue whale, 173 tonnes, 29.9 meters
right whale, 135 tonnes, 20.7 meters
gray whale, 41 tonnes, 14.9 meters
humpback whale, 30 tonnes, 16.0 meters

9.5 Writing Text Files

In much the same way we can use fgetc(), fgets(), and fscanf() to read text streams, we can use fputc(), fputs(), and fprintf() to write text streams.

To do so, we have to fopen() the file in write mode by passing "w" as the second argument. Opening an existing file in "w" mode will instantly truncate that file to 0 bytes for a full overwrite.

We’ll put together a simple program that outputs a file output.txt using a variety of output functions.

#include <stdio.h>

int main(void)
{
    FILE *fp;
    int x = 32;

    fp = fopen("output.txt", "w");

    fputc('B', fp);
    fputc('\n', fp);   // newline
    fprintf(fp, "x = %d\n", x);
    fputs("Hello, world!\n", fp);

    fclose(fp);
}

B
x = 32
Hello, world!

and the program would have outputted to the console instead of to a file. Try it!

9.6 Binary File I/O

So far we’ve just been talking text files. But there’s that other beast we mentioned early on called binary files, or binary streams.

These work very similarly to text files, except the I/O subsystem doesn’t perform any translations on the data like it might with a text file. With binary files, you get a raw stream of bytes, and that’s all.

The big difference in opening the file is that you have to add a "b" to the mode. That is, to read a binary file, open it in "rb" mode. To write a file, open it in "wb" mode.

Because it’s streams of bytes, and streams of bytes can contain NUL characters, and the NUL character is the end-of-string marker in C, it’s rare that people use the fprintf()-and-friends functions to operate on binary files.

Instead the most common functions are fread() and fwrite(). The functions read and write a specified number of bytes to the stream.

To demo, we’ll write a couple programs. One will write a sequence of byte values to disk all at once. And the second program will read a byte at a time and print them out⁷⁷.

#include <stdio.h>

int main(void)
{
    FILE *fp;
    unsigned char bytes[6] = {5, 37, 0, 88, 255, 12};

    fp = fopen("output.bin", "wb");  // wb mode for "write binary"!

    // In the call to fwrite, the arguments are:
    //
    // * Pointer to data to write
    // * Size of each "piece" of data
    // * Count of each "piece" of data
    // * FILE*

    fwrite(bytes, sizeof(char), 6, fp);

    fclose(fp);
}

Those two middle arguments to fwrite() are pretty odd. But basically what we want to tell the function is, “We have items that are this big, and we want to write that many of them.” This makes it convenient if you have a record of a fixed length, and you have a bunch of them in an array. You can just tell it the size of one record and how many to write.

In the example above, we tell it each record is the size of a char, and we have 6 of them.

Running the program gives us a file output.bin, but opening it in a text editor doesn’t show anything friendly! It’s binary data—not text. And random binary data I just made up, at that!

And those values in hex do match up to the values (in decimal) that we wrote out.

But now let’s try to read them back in with a different program. This one will open the file for binary reading ("rb" mode) and will read the bytes one at a time in a loop.

fread() has the neat feature where it returns the number of bytes read, or 0 on EOF. So we can loop until we see that, printing numbers as we go.

#include <stdio.h>

int main(void)
{
    FILE *fp;
    unsigned char c;

    fp = fopen("output.bin", "rb"); // rb for "read binary"!

    while (fread(&c, sizeof(char), 1, fp) > 0)
        printf("%d\n", c);

    fclose(fp);
}

9.6.1 struct and Number Caveats

As we saw in the structs section, the compiler is free to add padding to a struct as it sees fit. And different compilers might do this differently. And the same compiler on different architectures could do it differently. And the same compiler on the same architectures could do it differently.

What I’m getting at is this: it’s not portable to just fwrite() an entire struct out to a file when you don’t know where the padding will end up.

How do we fix this? Hold that thought—we’ll look at some ways to do this after looking at another related problem.

Let’s look at a simple fwrite() of a 2-byte number. We’ll write it in hex so each byte is clear. The most significant byte will have the value 0x12 and the least significant will have the value 0x34.

unsigned short v = 0x1234;  // Two bytes, 0x12 and 0x34

fwrite(&v, sizeof v, 1, fp);

This has something to do with what’s called the endianess⁷⁹ of the architecture. Some write the most significant bytes first, and some the least significant bytes first.

This means that if you write a multibyte number out straight from memory, you can’t do it in a portable way⁸⁰.

A similar problem exists with floating point. Most systems use the same format for their floating point numbers, but some do not. No guarantees!

So… how can we fix all these problems with numbers and structs to get our data written in a portable way?

The summary is to serialize the data, which is a general term that means to take all the data and write it out in a format that you control, that is well-known, and programmable to work the same way on all platforms.

As you might imagine, this is a solved problem. There are a bunch of serialization libraries you can take advantage of, such as Google’s protocol buffers⁸¹, out there and ready to use. They will take care of all the gritty details for you, and even will allow data from your C programs to interoperate with other languages that support the same serialization methods.

Do yourself and everyone a favor! Serialize your binary data when you write it to a stream! This will keep things nice and portable, even if you transfer data files from one architecture to another.

10 typedef: Making New Types

Well, not so much making new types as getting new names for existing types. Sounds kinda pointless on the surface, but we can really use this to make our code cleaner.

10.1 typedef in Theory

Basically, you take an existing type and you make an alias for it with typedef.

typedef int antelope;  // Make "antelope" an alias for "int"

antelope x = 10;       // Type "antelope" is the same as type "int"

You can take any existing type and do it. You can even make a number of types with a comma list:

typedef int antelope, bagel, mushroom;  // These are all "int"

That’s really useful, right? That you can type mushroom instead of int? You must be super excited about this feature!

OK, Professor Sarcasm—we’ll get to some more common applications of this in a moment.

10.1.1 Scoping

For this reason, it’s quite common to find typedef at file scope (“global”) so that all functions can use the new types at will.

10.2 typedef in Practice

So renaming int to something else isn’t that exciting. Let’s see where typedef commonly makes an appearance.

10.2.1 typedef and structs

Sometimes a struct will be typedef’d to a new name so you don’t have to type the word struct over and over.

struct animal {
    char *name;
    int leg_count, speed;
};

//  original name      new name
//            |         |
//            v         v
//      |-----------| |----|
typedef struct animal animal;

struct animal y;  // This works
animal z;         // This also works because "animal" is an alias

Personally, I don’t care for this practice. I like the clarity the code has when you add the word struct to the type; programmers know what they’re getting. But it’s really common so I’m including it here.

Now I want to run the exact same example in a way that you might commonly see. We’re going to put the struct animal in the typedef. You can mash it all together like this:

//  original name
//            |
//            v
//      |-----------|
typedef struct animal {
    char *name;
    int leg_count, speed;
} animal;                         // <-- new name

struct animal y;  // This works
animal z;         // This also works because "animal" is an alias

But that’s not all! There’s another common shortcut that you might see in code using what are called anonymous structures⁸². It turns out you don’t actually need to name the structure in a variety of places, and with typedef is one of them.

//  Anonymous struct! It has no name!
//         |
//         v
//      |----|
typedef struct {
    char *name;
    int leg_count, speed;
} animal;                         // <-- new name

//struct animal y;  // ERROR: this no longer works--no such struct!
animal z;           // This works because "animal" is an alias

typedef struct {
    int x, y;
} point;

point p = {.x=20, .y=40};

printf("%d, %d\n", p.x, p.y);  // 20, 40

10.2.2 typedef and Other Types

It’s not that using typedef with a simple type like int is completely useless… it helps you abstract the types to make it easier to change them later.

For example, if you have float all over your code in 100 zillion places, it’s going to be painful to change them all to double if you find you have to do that later for some reason.

typedef float app_float;

// and

app_float f1, f2, f3;

Then if later you want to change to another type, like long double, you just need to change the typedef:

//        voila!
//      |---------|
typedef long double app_float;

// and no need to change this line:

app_float f1, f2, f3;  // Now these are all long doubles

10.2.3 typedef and Pointers

typedef int *intptr;

int a = 10;
intptr x = &a;  // "intptr" is type "int*"

I really don’t like this practice. It hides the fact that x is a pointer type because you don’t see a * in the declaration.

IMHO, it’s better to explicitly show that you’re declaring a pointer type so that other devs can clearly see it and don’t mistake x for having a non-pointer type.

10.2.4 typedef and Capitalization

typedef struct {
    int x, y;
} my_point;          // lower snake case

typedef struct {
    int x, y;
} MyPoint;          // CamelCase

typedef struct {
    int x, y;
} Mypoint;          // Leading uppercase

typedef struct {
    int x, y;
} MY_POINT;          // UPPER SNAKE CASE

The C11 specification doesn’t dictate one way or another, and shows examples in all uppercase and all lowercase.

K&R2 uses leading uppercase predominantly, but show some examples in uppercase and snake case (with _t).

If you have a style guide in use, stick with it. If you don’t, grab one and stick with it.

10.3 Arrays and typedef

The syntax is a little weird, and this is rarely seen in my experience, but you can typedef an array of some number of items.

// Make type five_ints an array of 5 ints
typedef int five_ints[5];

five_ints x = {11, 22, 33, 44, 55};

I don’t like it because it hides the array nature of the variable, but it’s possible to do.

11 Pointers II: Arithmetic

11.1 Pointer Arithmetic

In short, if you have a pointer to a type, adding one to the pointer moves to the next item of that type directly after it in memory.

It’s important to remember that as we move pointers around and look at different places in memory, we need to make sure that we’re always pointing to a valid place in memory before we dereference. If we’re off in the weeds and we try to see what’s there, the behavior is undefined and a crash is a common result.

11.1.1 Adding to Pointers

int a[5] = {11, 22, 33, 44, 55};

int a[5] = {11, 22, 33, 44, 55};

int *p = &a[0];  // Or "int *p = a;" works just as well

printf("%d\n", *p);  // Prints 11

Now let’s use pointer arithmetic to print the next element in the array, the one at index 1:

printf("%d\n", *(p + 1));  // Prints 22!!

What happened there? C knows that p is a pointer to an int. So it knows the sizeof an int⁸³ and it knows to skip that many bytes to get to the next int after the first one!

printf("%d\n", *p);        // Prints 11
printf("%d\n", *(p + 0));  // Prints 11

Let’s think of the upshot here. We can iterate over elements of an array this way instead of using an array:

int a[5] = {11, 22, 33, 44, 55};

int *p = &a[0];  // Or "int *p = a;" works just as well

for (int i = 0; i < 5; i++) {
    printf("%d\n", *(p + i));  // Same as p[i]!
}

And that works the same as if we used array notation! Oooo! Getting closer to that array/pointer equivalence thing! More on this later in this chapter.

Remember from early on that memory is like a big array, where a byte is stored at each array index?

For a random example, say that a number 3490 was stored at address (“index”) 23,237,489,202. If we have an int pointer to that 3490, that value of that pointer is 23,237,489,202… because the pointer is the memory address. Different words for the same thing.

And now let’s say we have another number, 4096, stored right after the 3490 at address 23,237,489,210 (8 higher than the 3490 because each int in this example is 8 bytes long).

If we add 1 to that pointer, it actually jumps ahead sizeof(int) bytes to the next int. It knows to jump that far ahead because it’s an int pointer. If it were a float pointer, it’d jump sizeof(float) bytes ahead to get to the next float!

So you can look at the next int, by adding 1 to the pointer, the one after that by adding 2 to the pointer, and so on.

11.1.2 Changing Pointers

We saw how we could add an integer to a pointer in the previous section. This time, let’s modify the pointer, itself.

You can just add (or subtract) integer values directly to (or from) any pointer!

Let’s do that example again, except with a couple changes. First, I’m going to add a 999 to the end of our numbers to act as a sentinel value. This will let us know where the end of the data is.

int a[] = {11, 22, 33, 44, 55, 999};  // Add 999 here as a sentinel

int *p = &a[0];  // p points to the 11

And we also have p pointing to the element at index 0 of a, namely 11, just like before.

Now—let’s start incrementing p so that it points at subsequent elements of the array. We’ll do this until p points to the 999; that is, we’ll do it until *p == 999:

while (*p != 999) {       // While the thing p points to isn't 999
    printf("%d\n", *p);   // Print it
    p++;                  // Move p to point to the next int!
}

When we give it a run, first p points to 11. Then we increment p, and it points to 22, and then again, it points to 33. And so on, until it points to 999 and we quit.

11.1.3 Subtracting Pointers

You can subtract a value from a pointer to get to earlier address, as well, just like we were adding to them before.

But we can also subtract two pointers to find the difference between them, e.g. we can calculate how many ints there are between two int*s. The catch is that this only works within a single array⁸⁴—if the pointers point to anything else, you get undefined behavior.

Remember how strings are char*s in C? Let’s see if we can use this to write another variant of strlen() to compute the length of a string that utilizes pointer subtraction.

The idea is that if we have a pointer to the beginning of the string, we can find a pointer to the end of the string by scanning ahead for the NUL character.

And if we have a pointer to the beginning of the string, and we computed the pointer to the end of the string, we can just subtract the two pointers to come up with the length!

#include <stdio.h>

int my_strlen(char *s)
{
    // Start scanning from the beginning of the string
    char *p = s;

    // Scan until we find the NUL character
    while (*p != '\0')
        p++;

    // Return the difference in pointers
    return p - s;
}

int main(void)
{
    printf("%d\n", my_strlen("Hello, world!"));  // Prints "13"
}

Remember that you can only use pointer subtraction between two pointers that point to the same array!

11.2 Array/Pointer Equivalence

We’re finally ready to talk about this! We’ve seen plenty of examples of places where we’ve intermixed array notation, but let’s give out the fundamental formula of array/pointer equivalence:

I’ve oversimplified a bit, because in my above example a and b can both be expressions, and we might want a few more parentheses to force order of operations in case the expressions are complex.

but that’s a little harder to grok. Just make sure you include parentheses if the expressions are complicated so all your math happens in the right order.

This means we can decide if we’re going to use array or pointer notation for any array or pointer (assuming it points to an element of an array).

#include <stdio.h>

int main(void)
{
    int a[] = {11, 22, 33, 44, 55};

    int *p = a;  // p points to the first element of a, 11

    // Print all elements of the array a variety of ways:

    for (int i = 0; i < 5; i++)
        printf("%d\n", a[i]);      // Array notation with a

    for (int i = 0; i < 5; i++)
        printf("%d\n", p[i]);      // Array notation with p

    for (int i = 0; i < 5; i++)
        printf("%d\n", *(a + i));  // Pointer notation with a

    for (int i = 0; i < 5; i++)
        printf("%d\n", *(p + i));  // Pointer notation with p

    for (int i = 0; i < 5; i++)
        printf("%d\n", *(p++));    // Moving pointer p
        //printf("%d\n", *(a++));    // Moving array variable a--ERROR!
}

So you can see that in general, if you have an array variable, you can use pointer or array notion to access elements. Same with a pointer variable.

The one big difference is that you can modify a pointer to point to a different address, but you can’t do that with an array variable.

11.2.1 Array/Pointer Equivalence in Function Calls

int my_strlen(char *s)

this means you can pass either an array or a pointer to this function and have it work!

char s[] = "Antelopes";
char *t = "Wombats";

printf("%d\n", my_strlen(s));  // Works!
printf("%d\n", my_strlen(t));  // Works, too!

int my_strlen(char *s)    // Works!
int my_strlen(char s[])   // Works, too!

11.3 void Pointers

You’ve already seen the void keyword used with functions, but this is an entirely separate, unrelated animal.

Sometimes it’s useful to have a pointer to a thing that you don’t know the type of.

1. A function is going to operate on something byte-by-byte. For example, memcpy() copies bytes of memory from one pointer to another, but those pointers can point to any type. memcpy() takes advantage of the fact that if you iterate through char*s, you’re iterating through the bytes of an object no matter what type the object is. More on this in the Multibyte Values subsection.

void *memcpy(void *s1, void *s2, size_t n);

This function copies n bytes of memory starting from address s2 into the memory starting at address s1.

But look! s1 and s2 are void*s! Why? What does it mean? Let’s run more examples to see.

For instance, we could copy a string with memcpy() (though strcpy() is more appropriate for strings):

#include <stdio.h>
#include <string.h>

int main(void)
{
    char s[] = "Goats!";
    char t[100];

    memcpy(t, s, 7);  // Copy 7 bytes--including the NUL terminator!

    printf("%s\n", t);  // "Goats!"
}

#include <stdio.h>
#include <string.h>

int main(void)
{
    int a[] = {11, 22, 33};
    int b[3];

    memcpy(b, a, 3 * sizeof(int));  // Copy 3 ints of data

    printf("%d\n", b[1]);  // 22
}

That one’s a little wild—you see what we did there with memcpy()? We copied the data from a to b, but we had to specify how many bytes to copy, and an int is more than one byte.

OK, then—how many bytes does an int take? Answer: depends on the system. But we can tell how many bytes any type takes with the sizeof operator.

And if we have 3 of them in our array, like we did in that example, the entire space used for the 3 ints must be 3 * sizeof(int).

(In the string example, earlier, it would have been more technically accurate to copy 7 * sizeof(char) bytes. But chars are always one byte large, by definition, so that just devolves into 7 * 1.)

We could even copy a float or a struct with memcpy()! (Though this is abusive—we should just use = for that):

struct antelope my_antelope;
struct antelope my_clone_antelope;

// ...

memcpy(&my_clone_antelope, &my_antelope, sizeof my_antelope);

Look at how versatile memcpy() is! If you have a pointer to a source and a pointer to a destination, and you have the number of bytes you want to copy, you can copy any type of data.

Imagine if we didn’t have void*. We’d have to write specialized memcpy() functions for each type:

memcpy_int(int *a, int *b, int count);
memcpy_float(float *a, float *b, int count);
memcpy_double(double *a, double *b, int count);
memcpy_char(char *a, char *b, int count);
memcpy_unsigned_char(unsigned char *a, unsigned char *b, int count);

// etc... blech!

That’s the power of void*. You can write functions that don’t care about the type and is still able to do things with it.

But with great power comes great responsibility. Maybe not that great in this case, but there are some limits.

1. You cannot do pointer arithmetic on a void*. 2. You cannot dereference a void*. 3. You cannot use the arrow operator on a void*, since it’s also a dereference. 4. You cannot use array notation on a void*, since it’s also a dereference, as well⁸⁷.

And if you think about it, these rules make sense. All those operations rely on knowing the sizeof the type of data pointed to, and with void*, we don’t know the size of the data being pointed to—it could be anything!

Like with memcpy(), it helps you write generic functions that can handle multiple types of data. But the secret is that, deep down, you convert the void* to another type before you use it!

And conversion is easy: you can just assign into a variable of the desired type⁸⁸.

char a = 'X';  // A single char

void *p = &a;  // p points to the 'X'
char *q = p;   // q also points to the 'X'

printf("%c\n", *p);  // ERROR--cannot dereference void*!
printf("%c\n", *q);  // Prints "X"

Let’s write our own memcpy() to try this out. We can copy bytes (chars), and we know the number of bytes because it’s passed in.

void *my_memcpy(void *dest, void *src, int byte_count)
{
    // Convert void*s to char*s
    char *s = src, *d = dest;

    // Now that we have char*s, we can dereference and copy them
    while (byte_count--) {
        *d++ = *s++;
    }

    // Most of these functions return the destination, just in case
    // that's useful to the caller.
    return dest;
}

Right there at the beginning, we copy the void*s into char*s so that we can use them as char*s. It’s as easy as that.

Then some fun in a while loop, where we decrement byte_count until it becomes false (0). Remember that with post-decrement, the value of the expression is computed (for while to use) and then the variable is decremented.

And some fun in the copy, where we assign *d = *s to copy the byte, but we do it with post-increment so that both d and s move to the next byte after the assignment is made.

Lastly, most memory and string functions return a copy of a pointer to the destination string just in case the caller wants to use it.

Now that we’ve done that, I just want to quickly point out that we can use this technique to iterate over the bytes of any object in C, floats, structs, or anything!

Let’s run one more real-world example with the built-in qsort() routine that can sort anything thanks to the magic of void*s.

(In the following example, you can ignore the word const, which we haven’t covered yet.)

#include <stdio.h>
#include <stdlib.h>

// The type of structure we're going to sort
struct animal {
    char *name;
    int leg_count;
};

// This is a comparison function called by qsort() to help it determine
// what exactly to sort by. We'll use it to sort an array of struct
// animals by leg_count.
int compar(const void *elem1, const void *elem2)
{
    // We know we're sorting struct animals, so let's make both
    // arguments pointers to struct animals
    const struct animal *animal1 = elem1;
    const struct animal *animal2 = elem2;

    // Return <0 =0 or >0 depending on whatever we want to sort by.

    // Let's sort ascending by leg_count, so we'll return the difference
    // in the leg_counts
    if (animal1->leg_count > animal2->leg_count)
        return 1;
    
    if (animal1->leg_count < animal2->leg_count)
        return -1;

    return 0;
}

int main(void)
{
    // Let's build an array of 4 struct animals with different
    // characteristics. This array is out of order by leg_count, but
    // we'll sort it in a second.
    struct animal a[4] = {
        {.name="Dog", .leg_count=4},
        {.name="Monkey", .leg_count=2},
        {.name="Antelope", .leg_count=4},
        {.name="Snake", .leg_count=0}
    };

    // Call qsort() to sort the array. qsort() needs to be told exactly
    // what to sort this data by, and we'll do that inside the compar()
    // function.
    //
    // This call is saying: qsort array a, which has 4 elements, and
    // each element is sizeof(struct animal) bytes big, and this is the
    // function that will compare any two elements.
    qsort(a, 4, sizeof(struct animal), compar);

    // Print them all out
    for (int i = 0; i < 4; i++) {
        printf("%d: %s\n", a[i].leg_count, a[i].name);
    }
}

As long as you give qsort() a function that can compare two items that you have in your array to be sorted, it can sort anything. And it does this without needing to have the types of the items hardcoded in there anywhere. qsort() just rearranges blocks of bytes based on the results of the compar() function you passed in.

12 Manual Memory Allocation

This is one of the big areas where C likely diverges from languages you already know: manual memory management.

Other languages uses reference counting, garbage collection, or other means to determine when to allocate new memory for some data—and when to deallocate it when no variables refer to it.

And that’s nice. It’s nice to be able to not worry about it, to just drop all the references to an item and trust that at some point the memory associated with it will be freed.

Of course, in C, some variables are automatically allocated and deallocated when they come into scope and leave scope. We call these automatic variables. They’re your average run-of-the-mill block scope “local” variables. No problem.

But what if you want something to persist longer than a particular block? This is where manual memory management comes into play.

You can tell C explicitly to allocate for you a certain number of bytes that you can use as you please. And these bytes will remain allocated until you explicitly free that memory⁸⁹.

It’s important to free the memory you’re done with! If you don’t, we call that a memory leak and your process will continue to reserve that memory until it exits.

If you manually allocated it, you have to manually free it when you’re done with it.

So how do we do this? We’re going to learn a couple new functions, and make use of the sizeof operator to help us learn how many bytes to allocate.

In common C parlance, devs say that automatic local variables are allocated “on the stack”, and manually-allocated memory is “on the heap”. The spec doesn’t talk about either of those things, but all C devs will know what you’re talking about if you bring them up.

All functions we’re going to learn in this chapter can be found in <stdlib.h>.

12.1 Allocating and Deallocating, malloc() and free()

The malloc() function accepts a number of bytes to allocate, and returns a void pointer to that block of newly-allocated memory.

Since it’s a void*, you can assign it into whatever pointer type you want… normally this will correspond in some way to the number of bytes you’re allocating.

So… how many bytes should I allocate? We can use sizeof to help with that. If we want to allocate enough room for a single int, we can use sizeof(int) and pass that to malloc().

After we’re done with some allocated memory, we can call free() to indicate we’re done with that memory and it can be used for something else. As an argument, you pass the same pointer you got from malloc() (or a copy of it). It’s undefined behavior to use a memory region after you free() it.

Let’s try. We’ll allocate enough memory for an int, and then store something there, and the print it.

// Allocate space for a single int (sizeof(int) bytes-worth):

int *p = malloc(sizeof(int));

*p = 12;  // Store something there

printf("%d\n", *p);  // Print it: 12

free(p);  // All done with that memory

//*p = 3490;  // ERROR: undefined behavior! Use after free()!

Now, in that contrived example, there’s really no benefit to it. We could have just used an automatic int and it would have worked. But we’ll see how the ability to allocate memory this way has its advantages, especially with more complex data structures.

One more thing you’ll commonly see takes advantage of the fact that sizeof can give you the size of the result type of any constant expression. So you could put a variable name in there, too, and use that. Here’s an example of that, just like the previous one:

int *p = malloc(sizeof *p);  // *p is an int, so same as sizeof(int)

12.2 Error Checking

All the allocation functions return a pointer to the newly-allocated stretch of memory, or NULL if the memory cannot be allocated for some reason.

Some OSes like Linux can be configured in such a way that malloc() never returns NULL, even if you’re out of memory. But despite this, you should always code it up with protections in mind.

int *x;

x = malloc(sizeof(int) * 10);

if (x == NULL) {
    printf("Error allocating 10 ints\n");
    // do something here to handle it
}

Here’s a common pattern that you’ll see, where we do the assignment and the condition on the same line:

int *x;

if ((x = malloc(sizeof(int) * 10)) == NULL)
    printf("Error allocating 10 ints\n");
    // do something here to handle it
}

12.3 Allocating Space for an Array

We’ve seen how to allocate space for a single thing; now what about for a bunch of them in an array?

In C, an array is a bunch of the same thing back-to-back in a contiguous stretch of memory.

We can allocate a contiguous stretch of memory—we’ve seen how to do that. If we wanted 3490 bytes of memory, we could just ask for it:

char *p = malloc(3490);  // Voila

And—indeed!—that’s an array of 3490 chars (AKA a string!) since each char is 1 byte. In other words, sizeof(char) is 1.

Note: there’s no initialization done on the newly-allocated memory—it’s full of garbage. Clear it with memset() if you want to, or see calloc(), below.

But we can just multiply the size of the thing we want by the number of elements we want, and then access them using either pointer or array notation. Example!

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    // Allocate space for 10 ints
    int *p = malloc(sizeof(int) * 10);

    // Assign them values 0-45:
    for (int i = 0; i < 10; i++)
        p[i] = i * 5;

    // Print all values 0, 5, 10, 15, ..., 40, 45
    for (int i = 0; i < 10; i++)
        printf("%d\n", p[i]);

    // Free the space
    free(p);
}

The key’s in that malloc() line. If we know each int takes sizeof(int) bytes to hold it, and we know we want 10 of them, we can just allocate exactly that many bytes with:

And this trick works for every type. Just pass it to sizeof and multiply by the size of the array.

12.4 An Alternative: calloc()

This is another allocation function that works similarly to malloc(), with two key differences:

// Allocate space for 10 ints with calloc(), initialized to 0:
int *p = calloc(10, sizeof(int));

// Allocate space for 10 ints with malloc(), initialized to 0:
int *q = malloc(10 * sizeof(int));
memset(q, 0, 10 * sizeof(int));   // set to 0

Again, the result is the same for both except malloc() doesn’t zero the memory by default.

12.5 Changing Allocated Size with realloc()

If you’ve already allocated 10 ints, but later you decide you need 20, what can you do?

One option is to allocate some new space, and then memcpy() the memory over… but it turns out that sometimes you don’t need to move anything. And there’s one function that’s just smart enough to do the right thing in all the right circumstances: realloc().

It takes a pointer to some previously-allocted memory (by malloc() or calloc()) and a new size for the memory region to be.

It then grows or shrinks that memory, and returns a pointer to it. Sometimes it might return the same pointer (if the data didn’t have to be copied elsewhere), or it might return a different one (if the data did have to be copied).

Be sure when you call realloc(), you specify the number of bytes to allocate, and not just the number of array elements! That is:

num_floats *= 2;

np = realloc(p, num_floats);  // WRONG: need bytes, not number of elements!

np = realloc(p, num_floats * sizeof(float));  // Better!

Let’s allocate an array of 20 floats, and then change our mind and make it an array of 40.

We’re going to assign the return value of realloc() into another pointer just to make sure it’s not NULL. If it’s not, then we can reassign it into our original pointer. (If we just assigned the return value directly into the original pointer, we’d lose that pointer if the function returned NULL and we’d have no way to get it back.)

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    // Allocate space for 20 floats
    float *p = malloc(sizeof *p * 20);  // sizeof *p same as sizeof(float)

    // Assign them fractional values 0.0-1.0:
    for (int i = 0; i < 20; i++)
        p[i] = i / 20.0;

    // But wait! Let's actually make this an array of 40 elements
    float *new_p = realloc(p, sizeof *p * 40);

    // Check to see if we successfully reallocated
    if (new_p == NULL) {
        printf("Error reallocing\n");
        return 1;
    }

    // If we did, we can just reassign p
    p = new_p;

    // And assign the new elements values in the range 1.0-2.0
    for (int i = 20; i < 40; i++)
        p[i] = 1.0 + (i - 20) / 20.0;

    // Print all values 0.0-2.0 in the 40 elements:
    for (int i = 0; i < 40; i++)
        printf("%f\n", p[i]);

    // Free the space
    free(p);
}

Notice in there how we took the return value from realloc() and reassigned it into the same pointer variable p that we passed in. That’s pretty common to do.

Also if line 7 is looking weird, with that sizeof *p in there, remember that sizeof works on the size of the type of the expression. And the type of *p is float, so that line is equivalent to sizeof(float).

12.5.1 Reading in Lines of Arbitrary Length

What we see here is a loop that calls fgetc() over and over to append to a buffer until we see that the last character is a newline.

Once it finds the newline, it shrinks the buffer to just the right size and returns it.

#include <stdio.h>
#include <stdlib.h>

// Read a line of arbitrary size from a file
//
// Returns a pointer to the line.
// Returns NULL on EOF or error.
//
// It's up to the caller to free() this pointer when done with it.
//
// Note that this strips the newline from the result. If you need
// it in there, probably best to switch this to a do-while.

char *readline(FILE *fp)
{
    int offset = 0;   // Index next char goes in the buffer
    int bufsize = 4;  // Preferably power of 2 initial size
    char *buf;        // The buffer
    int c;            // The character we've read in

    buf = malloc(bufsize);  // Allocate initial buffer

    if (buf == NULL)   // Error check
        return NULL;

    // Main loop--read until newline or EOF
    while (c = fgetc(fp), c != '\n' && c != EOF) {

        // Check if we're out of room in the buffer accounting
        // for the extra byte for the NUL terminator
        if (offset == bufsize - 1) {  // -1 for the NUL terminator
            bufsize *= 2;  // 2x the space

            char *new_buf = realloc(buf, bufsize);

            if (new_buf == NULL) {
                free(buf);   // On error, free and bail
                return NULL;
            }

            buf = new_buf;  // Successful realloc
        }

        buf[offset++] = c;  // Add the byte onto the buffer
    }

    // We hit newline or EOF...

    // If at EOF and we read no bytes, free the buffer and
    // return NULL to indicate we're at EOF:
    if (c == EOF && offset == 0) {
        free(buf);
        return NULL;
    }

    // Shrink to fit
    if (offset < bufsize - 1) {  // If we're short of the end
        char *new_buf = realloc(buf, offset + 1); // +1 for NUL terminator

        // If successful, point buf to new_buf;
        // otherwise we'll just leave buf where it is
        if (new_buf != NULL)
            buf = new_buf;
    }

    // Add the NUL terminator
    buf[offset] = '\0';

    return buf;
}

int main(void)
{
    FILE *fp = fopen("foo.txt", "r");

    char *line;

    while ((line = readline(fp)) != NULL) {
        printf("%s\n", line);
        free(line);
    }

    fclose(fp);
}

When growing memory like this, it’s common (though hardly a law) to double the space needed each step just to minimize the number of realloc()s that occur.

Finally you might note that readline() returns a pointer to a malloc()d buffer. As such, it’s up to the caller to explicitly free() that memory when it’s done with it.

12.5.2 realloc() with NULL

char *p = malloc(3490);
char *p = realloc(NULL, 3490);

That could be convenient if you have some kind of allocation loop and you don’t want to special-case the first malloc().

int *p = NULL;
int length = 0;

while (!done) {
    // Allocate 10 more ints:
    length += 10;
    p = realloc(p, sizeof *p * length);

    // Do amazing things
    // ...
}

In that example, we didn’t need an initial malloc() since p was NULL to start.

12.6 Aligned Allocations

And I don’t want to get too far off in the weeds talking about it right now, but there’s this thing called memory alignment, which has to do with the memory address (pointer value) being a multiple of a certain number.

For example, a system might require that 16-bit values begin on memory addresses that are multiples of 2. Or that 64-bit values begin on memory addresses that are multiples of 2, 4, or 8, for example. It depends on the CPU.

Some systems require this kind of alignment for fast memory access, or some even for memory access at all.

Now, if you use malloc(), calloc(), or realloc(), C will give you a chunk of memory that’s well-aligned for any value at all, even structs. Works in all cases.

But there might be times that you know that some data can be aligned at a smaller boundary, or must be aligned at a larger one for some reason. I imagine this is more common with embedded systems programming.

The alignment is an integer power of two greater than zero, so 2, 4, 8, 16, etc. and you give that to aligned_alloc() before the number of bytes you’re interested in.

The other restriction is that the number of bytes you allocate needs to be a multiple of the alignment. But this might be changing. See C Defect Report 460 ⁹⁰

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    // Allocate 256 bytes aligned on a 64-byte boundary
    char *p = aligned_alloc(64, 256);  // 256 == 64 * 4

    // Copy a string in there and print it
    strcpy(p, "Hello, world!");
    printf("%s\n", p);

    // Free the space
    free(p);
}

I want to throw a note here about realloc() and aligned_alloc(). realloc() doesn’t have any alignment guarantees, so if you need to get some aligned reallocated space, you’ll have to do it the hard way with memcpy().

void *aligned_realloc(void *ptr, size_t old_size, size_t alignment, size_t size)
{
    char *new_ptr = aligned_alloc(alignment, size);

    if (new_ptr == NULL)
        return NULL;

    size_t copy_size = old_size < size? old_size: size;  // get min

    if (ptr != NULL)
        memcpy(new_ptr, ptr, copy_size);

    free(ptr);

    return new_ptr;
}

Note that it always copies data, taking time, while real realloc() will avoid that if it can. So this is hardly efficient. Avoid needing to reallocate custom-aligned data.

13 Scope

13.1 Block Scope

This is the scope of almost all the variables devs define. It includes what other languages might call “function scope”, i.e. variables that are declared inside functions.

The basic rule is that if you’ve declared a variable in a block delimited by squirrelly braces, the scope of that variable is that block.

If there’s a block inside a block, then variables declared in the inner block are local to that block, and cannot be seen in the outer scope.

Once a variable’s scope ends, that variable can no longer be referenced, and you can consider its value to be gone into the great bit bucket ⁹¹ in the sky.

#include <stdio.h>

int main(void)
{
    int a = 12;         // Local to outer block, but visible in inner block

    if  (a == 12) {
        int b = 99;     // Local to inner block, not visible in outer block

        printf("%d %d\n", a, b);  // OK: "12 99"
    }

    printf("%d\n", a);  // OK, we're still in a's scope

    printf("%d\n", b);  // ILLEGAL, out of b's scope
}

13.1.1 Where To Define Variables

Another fun fact is that you can define variables anywhere in the block, within reason—they have the scope of that block, but cannot be used before they are defined.

#include <stdio.h>

int main(void)
{
    int i = 0;

    printf("%d\n", i);     // OK: "0"

    //printf("%d\n", j);   // ILLEGAL--can't use j before it's defined

    int j = 5;

    printf("%d %d\n", i, j);   // OK: "0 5"
}

Historically, C required all the variables be defined before any code in the block, but this is no longer the case in the C99 standard.

13.1.2 Variable Hiding

If you have a variable named the same thing at an inner scope as one at an outer scope, the one at the inner scope takes precedence as long as you’re running in the inner scope. That is, it hides the one at outer scope for the duration of its lifetime.

#include <stdio.h>

int main(void)
{
    int i = 10;

    {
        int i = 20;

        printf("%d\n", i);  // Inner scope i, 20 (outer i is hidden)
    }

    printf("%d\n", i);  // Outer scope i, 10
}

You might have noticed in that example that I just threw a block in there at line 7, not so much as a for or if statement to kick it off! This is perfectly legal. Sometimes a dev will want to group a bunch of local variables together for a quick computation and will do this, but it’s rare to see.

13.2 File Scope

If you define a variable outside of a block, that variable has file scope. It’s visible in all functions in the file that come after it, and shared between them. (An exception is if a block defines a variable of the same name, it would hide the one at file scope.)

This is closest to what you would consider to be “global” scope in another language.

#include <stdio.h>

int shared = 10;    // File scope! Visible to the whole file after this!

void func1(void)
{
    shared += 100;  // Now shared holds 110
}

void func2(void)
{
    printf("%d\n", shared);  // Prints "110"
}

int main(void)
{
    func1();
    func2();
}

Note that if shared were declared at the bottom of the file, it wouldn’t compile. It has to be declared before any functions use it.

There are ways to further modify items at file scope, namely with static and extern, but we’ll talk more about those later.

13.3 for-loop Scope

I really don’t know what to call this, as C11 §6.8.5.3¶1 doesn’t give it a proper name. We’ve done it already a few times in this guide, as well. It’s when you declare a variable inside the first clause of a for-loop:

for (int i = 0; i < 10; i++)
    printf("%d\n", i);

printf("%d\n", i);  // ILLEGAL--i is only in scope for the for-loop

In that example, i’s lifetime begins the moment it is defined, and continues for the duration of the loop.

If the loop body is enclosed in a block, the variables defined in the for-loop are visible from that inner scope.

Unless, of course, that inner scope hides them. This crazy example prints 999 five times:

#include <stdio.h>

int main(void)
{
    for (int i = 0; i < 5; i++) {
        int i = 999;  // Hides the i in the for-loop scope
        printf("%d\n", i);
    }
}

13.4 A Note on Function Scope

The C spec does refer to function scope, but it’s used exclusively with labels, something we haven’t discussed yet. More on that another day.

14 Types II: Way More Types!

We’re used to char, int, and float types, but it’s now time to take that stuff to the next level and see what else we have out there in the types department!

14.1 Signed and Unsigned Integers

So far we’ve used int as a signed type, that is, a value that can be either negative or positive. But C also has specific unsigned integer types that can only hold positive numbers.

int a;           // signed
signed int a;    // signed
signed a;        // signed, "shorthand" for "int" or "signed int", rare
unsigned int b;  // unsigned
unsigned c;      // unsigned, shorthand for "unsigned int"

Answer: you can get larger numbers in an unsigned variable than you can in a signed ones.

You can think of integers being represented by a certain number of bits⁹². On my computer, an int is represented by 64 bits.

And each permutation of bits that are either 1 or 0 represents a number. We can decide how to divvy up these numbers.

With signed numbers, we use (roughly) half the permutations to represent negative numbers, and the other half to represent positive numbers.

On my computer with 64-bit ints using two’s complement ⁹³ to represent unsigned numbers, I have the following limits on integer range:

Type	Minimum	Maximum
`int`	`-9,223,372,036,854,775,808`	`9,223,372,036,854,775,807`
`unsigned int`	`0`	`18,446,744,073,709,551,615`

Notice that the largest positive unsigned int is approximately twice as large as the largest positive int. So you can get some flexibility there.

14.2 Character Types

char c = 'B';

printf("%c\n", c);  // "B"

char c = 'B';

// Change this from %c to %d:
printf("%d\n", c);  // 66 (!!)

Deep down, char is just a small int, namely an integer that uses just a single byte of space, limiting its range to…

Here the C spec gets just a little funky. It assures us that a char is a single byte, i.e. sizeof(char) == 1. But then in C11 §3.6¶3 it goes out of its way to say:

Wait—what? Some of you might be used to the notion that a byte is 8 bits, right? I mean, that’s what it is, right? And the answer is, “Almost certainly.”⁹⁴ But C is an old language, and machines back in the day had, shall we say, a more relaxed opinion over how many bits were in a byte. And through the years, C has retained this flexibility.

But assuming your bytes in C are 8 bits, like they are for virtually all machines in the world that you’ll ever see, the range of a char is…

—So before I can tell you, it turns out that chars might be signed or unsigned depending on your compiler. Unless you explicitly specify.

In many cases, just having char is fine because you don’t care about the sign of the data. But if you need signed or unsigned chars, you must be specific:

char a;           // Could be signed or unsigned
signed char b;    // Definitely signed
unsigned char c;  // Definitely unsigned

OK, now, finally, we can figure out the range of numbers if we assume that a char is 8 bits and your system uses the virtually universal two’s complement representation for signed and unsigned⁹⁵.

`char` type	Minimum	Maximum
`signed char`	`-128`	`127`
`unsigned char`	`0`	`255`

#include <stdio.h>

int main(void)
{
    char a = 10, b = 20;

    printf("%d\n", a + b);  // 30!
}

What about those constant characters in single quotes, like 'B'? How does that have a numeric value?

The spec is also hand-wavey here, since C isn’t designed to run on a single type of underlying system.

But let’s just assume for the moment that your character set is based on ASCII ⁹⁶ for at least the first 128 characters. In that case, the character constant will be converted to a char whose value is the same as the ASCII value of the character.

#include <stdio.h>

int main(void)
{
    char a = 10;
    char b = 'B';  // ASCII value 66

    printf("%d\n", a + b);  // 76!
}

This depends on your execution environment and the character set used ⁹⁷. One of the most popular character sets today is Unicode ⁹⁸ (which is a superset of ASCII), so for your basic 0-9, A-Z, a-z and punctuation, you’ll almost certainly get the ASCII values out of them.

14.3 More Integer Types: short, long, long long

and we recently learned about the unsigned variants of the integer types. And we learned that char was secretly a small int in disguise. So we know the ints can come in multiple bit sizes.

But there are a couple more integer types we should look at, and the minimum minimum and maximum values they can hold.

Yes, I said “minimum” twice. The spec says that these types will hold numbers of at least these sizes, so your implementation might be different. The header file <limits.h> defines macros that hold the minimum and maximum integer values; rely on that to be sure, and never hardcode or assume these values.

These additional types are short int, long int, and long long int. Commonly, when using these types, C developers leave the int part off (e.g. long long), and the compiler is perfectly happy.

// These two lines are equivalent:
long long int x;
long long x;

// And so are these:
short int x;
short x;

Let’s take a look at the integer data types and sizes in ascending order, grouped by signedness.

Type	Minimum Bytes	Minimum Value	Maximum Value
`char`	1	-127 or 0	127 or 255⁹⁹
`signed char`	1	-127	127
`short`	2	-32767	32767
`int`	2	-32767	32767
`long`	4	-2147483647	2147483647
`long long`	8	-9223372036854775807	9223372036854775807
`unsigned char`	1	0	255
`unsigned short`	2	0	65535
`unsigned int`	2	0	65535
`unsigned long`	4	0	4294967295
`unsigned long long`	8	0	18446744073709551615

There is no long long long type. You can’t just keep adding longs like that. Don’t be silly.

Let’s run the same table on my 64-bit, two’s complement system and see what comes out:

That’s a little more sensible, but we can see how my system has larger limits than the minimums in the specification.

Type	My Bytes	Minimum Value	Maximum Value
`char`	1	-128	127¹⁰¹
`signed char`	1	-128	127
`short`	2	-32768	32767
`int`	4	-2147483648	2147483647
`long`	8	-9223372036854775808	9223372036854775807
`long long`	8	-9223372036854775808	9223372036854775807
`unsigned char`	1	0	255
`unsigned short`	2	0	65535
`unsigned int`	4	0	4294967295
`unsigned long`	8	0	18446744073709551615
`unsigned long long`	8	0	18446744073709551615

Type	Min Macro	Max Macro
`char`	`CHAR_MIN`	`CHAR_MAX`
`signed char`	`SCHAR_MIN`	`SCHAR_MAX`
`short`	`SHRT_MIN`	`SHRT_MAX`
`int`	`INT_MIN`	`INT_MAX`
`long`	`LONG_MIN`	`LONG_MAX`
`long long`	`LLONG_MIN`	`LLONG_MAX`
`unsigned char`	`0`	`UCHAR_MAX`
`unsigned short`	`0`	`USHRT_MAX`
`unsigned int`	`0`	`UINT_MAX`
`unsigned long`	`0`	`ULONG_MAX`
`unsigned long long`	`0`	`ULLONG_MAX`

Notice there’s a way hidden in there to determine if a system uses signed or unsigned chars. If CHAR_MAX == UCHAR_MAX, it must be unsigned.

Also notice there’s no minimum macro for the unsigned variants—they’re just 0.

14.4 More Float: double and long double

Let’s see what the spec has to say about floating point numbers in §5.2.4.2.2¶1-2:

Parameter	Definition
\(s\)	sign (\(\pm1\))
\(b\)	base or radix of exponent representation (an integer \(> 1\))
\(e\)	exponent (an integer between a minimum \(e_{min}\) and a maximum \(e_{max}\))
\(p\)	precision (the number of base-\(b\) digits in the significand)
\(f_k\)	nonnegative integers less than \(b\) (the significand digits)

Note: we refer to a bunch of macros in this section. They can be found in the header <float.h>.

Floating point number are encoded in a specific sequence of bits (IEEE-754 format ¹⁰² is tremendously popular) in bytes.

Diving in a bit more, the number is basically represented as the significand (which is the number part—the significant digits themselves, also sometimes referred to as the mantissa) and the exponent, which is what power to raise the digits to. Recall that a negative exponent can make a number smaller.

Imagine we’re using $10$ as a number to raise by an exponent. We could represent the following numbers by using a significand of $12345$, and exponents of $-3$, $4$, and $0$ to encode the following floating point values:

For all those numbers, the significand stays the same. The only difference is the exponent.

On your machine, the base for the exponent is probably $2$, not $10$, since computers like binary. You can check it by printing the FLT_RADIX macro.

So we have a number that’s represented by a number of bytes, encoded in some way. Because there are a limited number of bit patterns, a limited number of floating point numbers can be represented.

But more particularly, only a certain number of significant decimal digits can be represented accurately.

And we have a couple of them. We know about float already, but for more precision we have double. And for even more precision, we have long double (unrelated to long int except by name).

The spec doesn’t go into how many bytes of storage each type should take, but on my system, we can see the relative size increases:

So each of the types (on my system) uses those additional bits for more precision.

Type	`sizeof`
`float`	4
`double`	8
`long double`	16

But how much precision are we talking, here? How many decimal numbers can be represented by these values?

Well, C provides us with a bunch of macros in <float.h> to help us figure that out.

It gets a little wonky if you are using a base-2 (binary) system for storing the numbers (which is virtually everyone on the planet, probably including you), but bear with me while we figure it out.

14.4.1 How Many Decimal Digits?

The million dollar question is, “How many significant decimal digits can I store in a given floating point type so that I get out the same decimal number when I print it?”

The number of decimal digits you can store in a floating point type and surely get the same number back out when you print it is given by these macros:

Type	Decimal Digits You Can Store	Minimum
`float`	`FLT_DIG`	6
`double`	`DBL_DIG`	10
`long double`	`LDBL_DIG`	10

On my system, FLT_DIG is 6, so I can be sure that if I print out a 6 digit float, I’ll get the same thing back. (It could be more digits—some numbers will come back correctly with more digits. But 6 is definitely coming back.)

For example, printing out floats following this pattern of increasing digits, we apparently make it to 8 digits before something goes wrong, but after that we’re back to 7 correct digits.

0.12345
0.123456
0.1234567
0.12345678
0.123456791  <-- Things start going wrong
0.1234567910

Let’s do another demo. In this code we’ll have two floats that both hold numbers that have FLT_DIG significant decimal digits¹⁰³. Then we add those together, for what should be 12 significant decimal digits. But that’s more than we can store in a float and correctly recover as a string—so we see when we print it out, things start going wrong after the 7th significant digit.

#include <stdio.h>
#include <float.h>

int main(void)
{
    // Both these numbers have 6 significant digits, so they can be
    // stored accurately in a float:

    float f = 3.14159f;
    float g = 0.00000265358f;

    printf("%.5f\n", f);   // 3.14159       -- correct!
    printf("%.11f\n", g);  // 0.00000265358 -- correct!

    // Now add them up
    f += g;                // 3.14159265358 is what f _should_ be

    printf("%.11f\n", f);  // 3.14159274101 -- wrong!
}

(The above code has an f after the numeric constants—this indicates that the constant is type float, as opposed to the default of double. More on this later.)

Remember that FLT_DIG is the safe number of digits you can store in a float and retrieve correctly.

Sometimes you might get one or two more out of it. But sometimes you’ll only get FLT_DIG digits back. The sure thing: if you store any number of digits up to and including FLT_DIG in a float, you’re sure to get them back correctly.

14.4.2 Converting to Decimal and Back

But storing a base 10 number in a floating point number and getting it back out is only half the story.

Turns out floating point numbers can encode numbers that require more decimal places to print out completely. It’s just that your big decimal number might not map to one of those numbers.

That is, when you look at floating point numbers from one to the next, there’s a gap. If you try to encode a decimal number in that gap, it’ll use the closest floating point number. That’s why you can only encode FLT_DIG for a float.

But what about those floating point numbers that aren’t in the gap? How many places do you need to print those out accurately?

Another way to phrase this question is for any given floating point number, how many decimal digits do I have to preserve if I want to convert the decimal number back into an identical floating point number? That is, how many digits do I have to print in base 10 to recover all the digits in base 2 in the original number?

Sometimes it might only be a few. But to be sure, you’ll want to convert to decimal with a certain safe number of decimal places. That number is encoded in the following macros:

Macro	Description
`FLT_DECIMAL_DIG`	Number of decimal digits encoded in a `float`.
`DBL_DECIMAL_DIG`	Number of decimal digits encoded in a `double`.
`LDBL_DECIMAL_DIG`	Number of decimal digits encoded in a `long double`.
`DECIMAL_DIG`	Same as the widest encoding, `LDBL_DECIMAL_DIG`.

Let’s see an example where DBL_DIG is 15 (so that’s all we can have in a constant), but DBL_DECIMAL_DIG is 17 (so we have to convert to 17 decimal numbers to preserve all the bits of the original double).

Let’s assign the 15 significant digit number 0.123456789012345 to x, and let’s assign the 1 significant digit number 0.0000000000000006 to y.

x is exact: 0.12345678901234500    Printed to 17 decimal places
y is exact: 0.00000000000000060

But let’s add them together. This should give 0.1234567890123456, but that’s more than DBL_DIG, so strange things might happen… let’s look:

x + y not quite right: 0.12345678901234559    Should end in 4560!

That’s what we get for printing more than DBL_DIG, right? But check this out… that number, above, is exactly representable as it is!

z is exact: 0.12345678901234559   17 digits correct! More than DBL_DIG!

If we’d truncated z down to 15 digits, it wouldn’t have been the same number. That’s why to preserve all the bits of a double, we need DBL_DECIMAL_DIG and not just the lesser DBL_DIG.

All that being said, it’s clear that when we’re messing with decimal numbers in general, it’s not safe to print more than FLT_DIG, DBL_DIG, or LDBL_DIG digits to be sensible in relation to the original base 10 numbers and any subsequent math.

But when converting from float to a decimal representation and back to float, definitely use FLT_DECIMAL_DIG to do that so that all the bits are preserved exactly.

14.5 Constant Numeric Types

When you write down a constant number, like 1234, it has a type. But what type is it? Let’s look at how C decides what type the constant is, and how to force it to choose a specific type.

14.5.1 Hexadecimal and Octal

In addition to good ol’ decimal like Grandma used to bake, C also supports constants of different bases.

int a = 0x1A2B;   // Hexadecimal
int b = 0x1a2b;   // Case doesn't matter for hex digits

printf("%x", a);  // Print a hex number, "1a2b"

int a = 012;

printf("%o\n", a);  // Print an octal number, "12"

This is particularly problematic for beginner programmers who try to pad decimal numbers on the left with 0 to line things up nice and pretty, inadvertently changing the base of the number:

int x = 11111;  // Decimal 11111
int y = 00111;  // Decimal 73 (Octal 111)
int z = 01111;  // Decimal 585 (Octal 1111)

14.5.1.1 A Note on Binary

An unofficial extension¹⁰⁴ in many C compilers allows you to represent a binary number with a 0b prefix:

int x = 0b101010;    // Binary 101010

printf("%d\n", x);   // Prints 42 decimal

There’s no printf() format specifier for printing a binary number. You have to do it a character at a time with bitwise operators.

14.5.2 Integer Constants

You can force a constant integer to be a certain type by appending a suffix to it that indicates the type.

We’ll do some assignments to demo, but most often devs leave off the suffixes unless needed to be precise. The compiler is pretty good at making sure the types are compatible.

int           x = 1234;
long int      x = 1234L;
long long int x = 1234LL

unsigned int           x = 1234U;
unsigned long int      x = 1234UL;
unsigned long long int x = 1234ULL;

The suffix can be uppercase or lowercase. And the U and L or LL can appear either one first.

Type	Suffix
`int`	None
`long int`	`L`
`long long int`	`LL`
`unsigned int`	`U`
`unsigned long int`	`UL`
`unsigned long long int`	`ULL`

I mentioned in the table that “no suffix” means int… but it’s actually more complex than that.

What C will generally do is choose the smallest type from int up that can hold the value.

But specifically, that depends on the number’s base (decimal, hex, or octal), as well.

The spec has a great table indicating which type gets used for what unsuffixed value. In fact, I’m just going to copy it wholesale right here.

C11 §6.4.4.1¶5 reads, “The type of an integer constant is the first of the first of the corresponding list in which its value can be represented.”

Suffix	Decimal Constant	Octal or Hexadecimal Constant
none	`int` `long int`	`int` `unsigned int` `long int` `unsigned long int` `long long int` `unsigned long long int`
`u` or `U`	`unsigned int` `unsigned long int` `unsigned long long int`	`unsigned int` `unsigned long int` `unsigned long long int`
`l` or `L`	`long int` `long long int`	`long int` `unsigned long int` `long long int` `unsigned long long int`
Both `u` or `U` and `l` or `L`	`unsigned long int` `unsigned long long int`	`unsigned long int` `unsigned long long int`
`ll` or `LL`	`long long int`	`long long int` `unsigned long long int`
Both `u` or `U` and `ll` or `LL`	`unsigned long long int`	`unsigned long long int`

What that’s saying is that, for example, if you specify a number like 123456789U, first C will see if it can be unsigned int. If it doesn’t fit there, it’ll try unsigned long int. And then unsigned long long int. It’ll use the smallest type that can hold the number.

14.5.3 Floating Point Constants

You’d think that a floating point constant like 1.23 would have a default type of float, right?

Surprise! Turns out unsuffiexed floating point numbers are type double! Happy belated birthday!

You can force it to be of type float by appending an f (or F—it’s case-insensitive). You can force it to be of type long double by appending l (or L).

Type	Suffix
`float`	`F`
`double`	None
`long double`	`L`

float x       = 3.14f;
double x      = 3.14;
long double x = 3.14L;

Isn’t the left a float and the right a double? Yes! But C’s pretty good with automatic numeric conversions, so it’s more common to have an unsuffixed floating point constant than not. More on that later.

14.5.3.1 Scientific Notation

Remember earlier when we talked about how a floating point number can be represented by a significand, base, and exponent?

Well, there’s a common way of writing such a number, shown here followed by it’s more recognizable equivalent which is what you get when you actually run the math:

Writing numbers in the form $s\times b^e$ is called scientific notation¹⁰⁵. In C, these are written using “E notation”, so these are equivalent:

Scientific Notation	E notation
\(1.2345\times10^{-3}=0.0012345\)	`1.2345e-3`
\(1.2345\times10^8=123450000\)	`1.2345e+8`

printf("%e\n", 123456.0);  // Prints 1.234560e+05

14.5.3.2 Hexadecimal Floating Point Constants

These work similar to decimal floating point numbers, but they begin with a 0x just like integer numbers.

The catch is that you must specify an exponent, and this exponent produces a power of 2. That is: $2^x$.

When using floating point hex constants, We can print hex scientific notation with %a:

double x = 0xa.1p3;

printf("%a\n", x);  // 0x1.42p+6
printf("%f\n", x);  // 80.500000

15 Types III: Conversions

In this chapter, we want to talk all about converting from one type to another. C has a variety of ways of doing this, and some might be a little different that you’re used to in other languages.

Before we talk about how to make conversions happen, let’s talk about how they work when they do happen.

15.1 String Conversions

Unlike many languages, C doesn’t do string-to-number (and vice-versa) conversions in quite as streamlined a manner as it does numeric conversions.

15.1.1 Numeric Value to String

When we want to convert a number to a string, we can use either sprintf() (pronounced SPRINT-f) or snprintf() (s-n-print-f)¹⁰⁶

These basically work like printf(), except they output to a string instead, and you can print that string later, or whatever.

#include <stdio.h>

int main(void)
{
    char s[10];
    float f = 3.14159;

    // Convert "f" to string, storing in "s", writing at most 10 characters
    // including the NUL terminator

    snprintf(s, 10, "%f", f);

    printf("String value: %s\n", s);  // String value: 3.141590
}

15.1.2 String to Numeric Value

There are a couple families of functions to do this in C. We’ll call these the atoi (pronounced a-to-i) family and the strtol (stir-to-long) family.

For basic conversion from a string to a number, try the atoi functions from <stdlib.h>. These have bad error-handling characteristics (including undefined behavior if you pass in a bad string), so use them carefully.

Function	Description
`atoi`	String to `int`
`atof`	String to `float`
`atol`	String to `long int`
`atoll`	String to `long long int`

Though the spec doesn’t cop to it, the a at the beginning of the function stands for ASCII ¹⁰⁷, so really atoi() is “ASCII-to-integer”, but saying so today is a bit ASCII-centric.

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    char *pi = "3.14159";
    float f;

    f = atof(pi);

    printf("%f\n", f);
}

int x = atoi("what");  // "What" ain't no number I ever heard of

(When I run that, I get 0 back, but you really shouldn’t count on that in any way. You could get something completely different.)

For better error handling characteristics, let’s check out all those strtol functions, also in <stdlib.h>. Not only that, but they convert to more types and more bases, too!

These functions all follow a similar pattern of use, and are a lot of people’s first experience with pointers to pointers! But never fret—it’s easier than it looks.

Function	Description
`strtol`	String to `long int`
`strtoll`	String to `long long int`
`strtoul`	String to `unsigned long int`
`strtoull`	String to `unsigned long long int`
`strtof`	String to `float`
`strtod`	String to `double`
`strtold`	String to `long double`

Let’s do an example where we convert a string to an unsigned long, discarding error information (i.e. information about bad characters in the input string):

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    char *s = "3490";

    // Convert string s, a number in base 10, to an unsigned long int.
    // NULL means we don't care to learn about any error information.

    unsigned long int x = strtoul(s, NULL, 10);

    printf("%lu\n", x);  // 3490
}

Notice a couple things there. Even though we didn’t deign to capture any information about error characters in the string, strtoul() won’t give us undefined behavior; it will just return 0.

Does this mean we can convert numbers of different bases? Sure! Let’s do binary!

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    char *s = "101010";  // What's the meaning of this number?

    // Convert string s, a number in base 2, to an unsigned long int.

    unsigned long int x = strtoul(s, NULL, 2);

    printf("%lu\n", x);  // 42
}

OK, that’s all fun and games, but what’s with that NULL in there? What’s that for?

That helps us figure out if an error occurred in the processing of the string. It’s a pointer to a pointer to a char, which sounds scary, but isn’t once you wrap your head around it.

Let’s do an example where we feed in a deliberately bad number, and we’ll see how strtol() lets us know where the first invalid digit is.

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    char *s = "34x90";  // "x" is not a valid digit in base 10!
    char *badchar;

    // Convert string s, a number in base 10, to an unsigned long int.

    unsigned long int x = strtoul(s, &badchar, 10);

    // It tries to convert as much as possible, so gets this far:

    printf("%lu\n", x);  // 34

    // But we can see the offending bad character because badchar
    // points to it!

    printf("Invalid character: %c\n", *badchar);  // "x"
}

So there we have strtoul() modifying what badchar points to in order to show us where things went wrong¹⁰⁸.

But what if nothing goes wrong? In that case, badchar will point to the NUL terminator at the end of the string. So we can test for it:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    char *s = "3490";  // "x" is not a valid digit in base 10!
    char *badchar;

    // Convert string s, a number in base 10, to an unsigned long int.

    unsigned long int x = strtoul(s, &badchar, 10);

    // Check if things went well

    if (*badchar == '\0') {
        printf("Success! %lu\n", x);
    } else  {
        printf("Partial conversion: %lu\n", x);
        printf("Invalid character: %c\n", *badchar);
    }
}

So there you have it. The atoi()-style functions are good in a controlled pinch, but the strtol()-style functions give you far more control over error handling and the base of the input.

15.2 char Conversions

What if you have a single character with a digit in it, like '5'… Is that the same as the value 5?

printf("%d %d\n", 5, '5');

So… no. And 53? What is that? That’s the UTF-8 (and ASCII) code point for the character symbol '5'¹⁰⁹

So how do we convert the character '5' (which apparently has value 53) into the value 5?

The C Standard guarantees that these character will have code points that are in sequence and in this order:

0  1  2  3  4  5  6  7  8  9

0  1  2  3  4  5  6  7  8  9
48 49 50 51 52 53 54 55 56 57

char c = '6';

int x = c;  // x has value 54, the code point for '6'

int y = c - '0'; // y has value 6, just like we want

int x = 6;

char c = x + '0';  // c has value 54

printf("%d\n", c);  // prints 54
printf("%c\n", c);  // prints 6 with %c

You might think this is a weird way to do this conversion, and by today’s standards, it certainly is. But back in the olden days when computers were made literally out of wood, this was the method for doing this conversion. And it wasn’t broke, so C never fixed it.

15.3 Numeric Conversions

15.3.1 Boolean

15.3.2 Integer to Integer Conversions

If an integer type is converted to unsigned and doesn’t fit in it, the unsigned result wraps around odometer-style until it fits in the unsigned¹¹⁰.

If an integer type is converted to a signed number and doesn’t fit, the result is implementation-defined! Something documented will happen, but you’ll have to look it up¹¹¹

15.3.3 Integer and Floating Point Conversions

If a floating point type is converted to an integer type, the fractional part is discarded with prejudice¹¹².

But—and here’s the catch—if the number is too large to fit in the integer, you get undefined behavior. So don’t do that.

Going From integer or floating point to floating point, C makes a best effort to find the closest floating point number to the integer that it can.

Again, though, if the original value can’t be represented, it’s undefined behavior.

15.4 Implicit Conversions

These are conversions the compiler does automatically for you when you mix and match types.

15.4.1 The Integer Promotions

In a number of places, if an int can be used to represent a value from char or short (signed or unsigned), that value is promoted up to int. If it doesn’t fit in an int, it’s promoted to unsigned int.

char x = 10, y = 20;
int i = x + y;

In that case, x and y get promoted to int by C before the math takes place.

The integer promotions take place during The Usual Arithmetic Conversions, with variadic functions¹¹³, unary + and - operators, or when passing values to functions without prototypes¹¹⁴.

15.4.2 The Usual Arithmetic Conversions

These are automatic conversions that C does around numeric operations that you ask for. (That’s actually what they’re called, by the way, by C11 §6.3.1.8.) Note that for this section, we’re just talking about numeric types—strings will come later.

These conversions answer questions about what happens when you mix types, like this:

int x = 3 + 1.2;   // Mixing int and double
                   // 4.2 is converted to int
                   // 4 is stored in x

float y = 12 * 2;  // Mixing float and int
                   // 24 is converted to float
                   // 24.0 is stored in y

If you want to know the gritty details, check out C11 §6.3.1.8. But you probably don’t.

Just generally remember that int types become float types if there’s a floating point type anywhere in there, and the compiler makes an effort to make sure mixed integer types don’t overflow.

Finally, if you convert from one floating point type to another, the compiler will try to make an exact conversion. If it can’t, it’ll do the best approximation it can. If the number is too large to fit in the type you’re converting into, boom: undefined behavior!

15.4.3 void*

The void* type is interesting because it can be converted from or to any pointer type.

int x = 10;

void *p = &x;  // &x is type int*, but we store it in a void*

int *q = p;    // p is void*, but we store it in an int*

15.5 Explicit Conversions

These are conversions from type to type that you have to ask for; the compiler won’t do it for you.

You can convert from one type to another by assigning one type to another with an =.

15.5.1 Casting

You can explicitly change the type of an expression by putting a new type in parentheses in front of it. Some C devs frown on the practice unless absolutely necessary, but it’s likely you’ll come across some C code with these in it.

Let’s do an example where we want to convert an int into a long so that we can store it in a long.

Note: this example is contrived and the cast in this case is completely unnecessary because the x + 12 expression would automatically be changed to long int to match the wider type of y.

int x = 10;
long int y = (long int)x + 12;

In that example, even those x was type int before, the expression (long int)x has type long int. We say, “We cast x to long int.”

More commonly, you might see a cast being used to convert a void* into a specific pointer type so it can be dereferenced.

A callback from the built-in qsort() function might display this behavior since it has void*s passed into it:

int compar(const void *elem1, const void *elem2)
{
    if (*((const int*)elem2) > *((const int*)elem1)) return 1;
    if (*((const int*)elem2) < *((const int*)elem1)) return -1;
    return 0;
}

int compar(const void *elem1, const void *elem2)
{
    const int *e1 = elem1;
    const int *e2 = elem2;

    return *e2 - *e1;
}

One place you’ll see casts more commonly is to avoid a warning when printing pointer values with the rarely-used %p which gets picky with anything other than a void*:

int x = 3490;
int *p = &x;

printf("%p\n", p);

warning: format ‘%p’ expects argument of type ‘void *’, but argument
         2 has type ‘int *’

printf("%p\n", (void *)p);

Another place is with explicit pointer changes, if you don’t want to use an intervening void*, but these are also pretty uncommon:

long x = 3490;
long *p = &x;
unsigned char *c = (unsigned char *)p;

A third place it’s often required is with the character conversion functions in <ctype.h>¹¹⁵ where you should cast questionably-signed values to unsigned char to avoid undefined behavior.

Again, casting is rarely needed in practice. If you find yourself casting, there might be another way to do the same thing, or maybe you’re casting unnecessarily.

Or maybe it is necessary. Personally, I try to avoid it, but am not afraid to use it if I have to.

16 Types IV: Qualifiers and Specifiers

Now that we have some more types under our belts, turns out we can give these types some additional attributes that control their behavior. These are the type qualifiers and storage-class specifiers.

16.1 Type Qualifiers

These are going to allow you to declare constant values, and also to give the compiler optimization hints that it can use.

16.1.1 const

This is the most common type qualifier you’ll see. It means the variable is constant, and any attempt to modify it will result in a very angry compiler.

const int x = 2;

x = 4;  // COMPILER PUKING SOUNDS, can't assign to a constant

void foo(const int x)
{
    printf("%d\n", x + 30);  // OK, doesn't modify "x"
}

16.1.1.1 const and Pointers

This one gets a little funky, because there are two usages that have two meanings when it comes to pointers.

For one thing, we can make it so you can’t change the thing the pointer points to. You do this by putting the const up front with the type name (before the asterisk) in the type declaration.

int x[] = {10, 20};
const int *p = x; 

p++;  // We can modify p, no problem

*p = 30; // Compiler error! Can't change what it points to

const int *p;  // Can't modify what p points to
int const *p;  // Can't modify what p points to, just like the previous line

Great, so we can’t change the thing the pointer points to, but we can change the pointer itself. What if we want the other way around? We want to be able to change what the pointer points to, but not the pointer itself?

int *const p;   // We can't modify "p" with pointer arithmetic

p++;  // Compiler error!

int x = 10;
int *const p = &x;

*p = 20;   // Set "x" to 20, no problem

const int *const p;  // Can't modify p or *p!

Finally, if you have multiple levels of indirection, you should const the appropriate levels. Just because a pointer is const, doesn’t mean the pointer it points to must also be. You can explicitly set them like in the following examples:

char **p;
p++;     // OK!
(*p)++;  // OK!

char **const p;
p++;     // Error!
(*p)++;  // OK!

char *const *p;
p++;     // OK!
(*p)++;  // Error!

char *const *const p;
p++;     // Error!
(*p)++;  // Error!

16.1.1.2 const Correctness

One more thing I have to mention is that the compiler will warn on something like this:

const int x = 20;
int *p = &x;

initialization discards 'const' qualifier from pointer type target

    const int x = 20;
    int *p = &x;
//    ^       ^
//    |       |
//  int*    const int*

The compiler is warning us that the value on the right side of the assignment is const, but the one of the left is not. And the compiler is letting us know that it is discarding the “const-ness” of the expression on the right.

That is, we can still try to do the following, but it’s just wrong. The compiler will warn, and it’s undefined behavior:

const int x = 20;
int *p = &x;

*p = 40;  // Undefined behavior--maybe it modifies "x", maybe not!

printf("%d\n", x);  // 40, if you're lucky

16.1.2 restrict

TLDR: you never have to use this and you can ignore it every time you see it. If you use it correctly, you will likely realize some performance gain. If you use it incorrectly, you will realize undefined behavior.

restrict is a hint to the compiler that a particular piece of memory will only be accessed by one pointer and never another. (That is, there will be no aliasing of the particular object the restrict pointer points to.) If a developer declares a pointer to be restrict and then accesses the object it points to in another way (e.g. via another pointer), the behavior is undefined.

Basically you’re telling C, “Hey—I guarantee that this one single pointer is the only way I access this memory, and if I’m lying, you can pull undefined behavior on me.”

And C uses that information to perform certain optimizations. For instance, if you’re dereferencing the restrict pointer repeatedly in a loop, C might decide to cache the result in a register and only store the final result once the loop completes. If any other pointer referred to that same memory and accessed it in the loop, the results would not be accurate.

(Note that restrict has no effect if the object pointed to is never written to. It’s all about optimizations surrounding writes to memory.)

Let’s write a function to swap two variables, and we’ll use the restrict keyword to assure C that we’ll never pass in pointers to the same thing. And then let’s blow it and try passing in pointers to the same thing.

void swap(int *restrict a, int *restrict b)
{
    int t;

    t = *a;
    *a = *b;
    *b = t;
}

int main(void)
{
    int x = 10, y = 20;

    swap(&x, &y);  // OK! "a" and "b", above, point to different things

    swap(&x, &x);  // Undefined behavior! "a" and "b" point to the same thing
}

If we were to take out the restrict keywords, above, that would allow both calls to work safely. But then the compiler might not be able to optimize.

restrict has block scope, that is, the restriction only lasts for the scope it’s used. If it’s in a parameter list for a function, it’s in the block scope of that function.

If the restricted pointer points to an array, it only applies to the individual objects in the array. Other pointers could read and write from the array as long as they didn’t read or write any of the same elements as the restricted one.

If it’s outside any function in file scope, the restriction covers the entire program.

int printf(const char * restrict format, ...);

Again, that’s just telling the compiler that inside the printf() function, there will be only one pointer that refers to any part of that format string.

One last note: if you’re using array notation in your function parameter for some reason instead of pointer notation, you can use restrict like so:

void foo(int p[restrict])     // With no size

void foo(int p[restrict 10])  // Or with a size

16.1.3 volatile

You’re unlikely to see or need this unless you’re dealing with hardware directly.

volatile tells the compiler that a value might change behind its back and should be looked up every time.

An example might be where the compiler is looking in memory at an address that continuously updates behind the scenes, e.g. some kind of hardware timer.

If the compiler decides to optimize that and store the value in a register for a protracted time, the value in memory will update and won’t be reflected in the register.

By declaring something volatile, you’re telling the compiler, “Hey, the thing this points at might change at any time for reasons outside this program code.”

16.1.4 _Atomic

16.2 Storage-Class Specifiers

Storage-class specifiers are similar to type quantifiers. They give the compiler more information about the type of a variable.

16.2.1 auto

You barely ever see this keyword, since auto is the default for block scope variables. It’s implied.

{
    int a;         // auto is the default...
    auto int a;    // So this is redundant
}

The auto keyword indicates that this object has automatic storage duration. That is, it exists in the scope in which it is defined, and is automatically deallocated when the scope is exited.

One gotcha about automatic variables is that their value is indeterminate until you explicitly initialize them. We say they’re full of “random” or “garbage” data, though neither of those really makes me happy. In any case, you won’t know what’s in it unless you initialize it.

16.2.2 static

This keyword has two meanings, depending on if the variable is file scope or block scope.

16.2.2.1 static in Block Scope

In this case, we’re basically saying, “I just want a single instance of this variable to exist, shared between calls.”

static in block scope with an initializer will only be initialized one time on program startup, not each time the function is called.

#include <stdio.h>

void counter(void)
{
    static int count = 1;  // This is initialized one time

    printf("This has been called %d time(s)\n", count);

    count++;
}

int main(void)
{
    counter();  // "This has been called 1 time(s)"
    counter();  // "This has been called 2 time(s)"
    counter();  // "This has been called 3 time(s)"
    counter();  // "This has been called 4 time(s)"
}

One thing of note is that static block scope variables are initialized to 0 by default.

static int foo;      // Default starting value is `0`...
static int foo = 0;  // So the `0` assignment is redundant

Finally, be advised that if you’re writing multithreaded programs, you have to be sure you don’t let multiple threads trample the same variable.

16.2.2.2 static in File Scope

Variables at file scope already persist between function calls, so that behavior is already there.

Instead what static means in this context is that this variable isn’t visible outside of this particular source file. Kinda like “global”, but only in this file.

16.2.3 extern

The extern storage-class specifier gives us a way to refer to objects in other source files.

// bar.c

int a = 37;

But what if we had another source file, foo.c, and we wanted to refer to the a that’s in bar.c?

// foo.c

extern int a;

int main(void)
{
    printf("%d\n", a);  // 37, from bar.c!

    a = 99;

    printf("%d\n", a);  // Same "a" from bar.c, but it's now 99
}

We could have also made the extern int a in block scope, and it still would have referred to the a in bar.c:

// foo.c

int main(void)
{
    extern int a;

    printf("%d\n", a);  // 37, from bar.c!

    a = 99;

    printf("%d\n", a);  // Same "a" from bar.c, but it's now 99
}

Now, if a in bar.c had been marked static. this wouldn’t have worked. static variables at file scope are not visible outside that file.

A final note about extern on functions. For functions, extern is the default, so it’s redundant. You can declare a function static if you only want it visible in a single source file.

16.2.4 register

This is a keyword to hint to the compiler that this variable is frequently-used, and should be made as fast as possible to access. The compiler is under no obligation to agree to it.

Now, modern C compiler optimizers are pretty effective at figuring this out themselves, so it’s rare to see these days.

#include <stdio.h>

int main(void)
{
    register int a;   // Make "a" as fast to use as possible.

    for (a = 0; a < 10; a++)
        printf("%d\n", a);
}

register int a;
int *p = &a;    // COMPILER ERROR! Can't take address of a register

register int a[] = {11, 22, 33, 44, 55};
int *p = a;  // COMPILER ERROR! Can't take address of a[0]

register int a[] = {11, 22, 33, 44, 55};

int a = *(a + 2);  // COMPILER ERROR! Address of a[0] taken

register int a[] = {11, 22, 33, 44, 55};

int a = a[2];  // COMPILER WARNING!

warning: ISO C forbids subscripting ‘register’ array

The fact that you can’t take the address of a register variable frees the compiler up to make optimizations around that assumption if it hasn’t figured them out already. Also adding register to a const variable prevents one from accidentally passing its pointer to another function that willfully ignore its constness¹¹⁶.

A bit of historic backstory, here: deep inside the CPU are little dedicated “variables” called registers¹¹⁷. They are super fast to access compared to RAM, so using them gets you a speed boost. But they’re not in RAM, so they don’t have an associated memory address (which is why you can’t take the address-of or get a pointer to them).

But, like I said, modern compilers are really good at producing optimal code, using registers whenever possible regardless of whether or not you specified the register keyword. Not only that, but the spec allows them to just treat it as if you’d typed auto, if they want. So no guarantees.

16.2.5 _Thread_local

When you’re using multiple threads and you have some variables in either global or static block scope, this is a way to make sure that each thread gets its own copy of the variable. This’ll help you avoid race conditions and threads stepping on each other’s toes.

If you’re in block scope, you have to use this along with either extern or static.

Also, if you include <threads.h>, you can use the rather more palatable thread_local as an alias for the uglier _Thread_local.

17 Multifile Projects

So far we’ve been looking at toy programs that for the most part fit in a single file. But complex C programs are made up of many files that are all compiled and linked together into a single executable.

In this chapter we’ll check out some of the common patterns and practices for putting together larger projects.

17.1 Includes and Function Prototypes

A really common situation is that some of your functions are defined in one file, and you want to call them from another.

This actually works out of the box with a warning… let’s first try it and then look at the right way to fix the warning.

For these examples, we’ll put the filename as the first comment in the source.

# output file   source files
#     v            v
#   |----| |---------|
gcc -o foo foo.c bar.c

In that examples, foo.c and bar.c get built into the executable named foo.

// File bar.c

int add(int x, int y)
{
    return x + y;
}

// File foo.c

#include <stdio.h>

int main(void)
{
    printf("%d\n", add(2, 3));  // 5!
}

See how from main() we call add()—but add() is in a completely different source file! It’s in bar.c, while the call to it is in foo.c!

gcc -o foo foo.c bar.c

error: implicit declaration of function 'add' is invalid in C99

(Or you might get a warning. Which you should not ignore. Never ignore warnings in C; address them all.)

If you recall from the section on prototypes, implicit declarations are banned in modern C and there’s no legitimate reason to introduce them into new code. We should fix it.

What implicit declaration means is that we’re using a function, namely add() in this case, without letting C know anything about it ahead of time. C wants to know what it returns, what types it takes as arguments, and things such as that.

We saw how to fix that earlier with a function prototype. Indeed, if we add one of those to foo.c before we make the call, everything works well:

// File foo.c

#include <stdio.h>

int add(int, int);  // Add the prototype

int main(void)
{
    printf("%d\n", add(2, 3));  // 5!
}

But that’s a pain—needing to type in the prototype every time you want to use a function. I mean, we used printf() right there and didn’t need to type in a prototype; what gives?

If you remember from what back with hello.c at the beginning of the book, we actually did include the prototype for printf()! It’s in the file stdio.h! And we included that with #include!

Can we do the same with our add() function? Make a prototype for it and put it in a header file?

Header files in C have a .h extension by default. And they often, but not always, have the same name as their corresponding .c file. So let’s make a bar.h file for our bar.c file, and we’ll stick the prototype in it:

// File bar.h

int add(int, int);

And now let’s modify foo.c to include that file. Assuming it’s in the same directory, we include it inside double quotes (as opposed to angle brackets):

// File foo.c

#include <stdio.h>

#include "bar.h"  // Include from current directory

int main(void)
{
    printf("%d\n", add(2, 3));  // 5!
}

Notice how we don’t have the prototype in foo.c anymore—we included it from bar.h. Now any file that wants that add() functionality can just #include "bar.h" to get it, and you don’t need to worry about typing in the function prototype.

As you might have guessed, #include literally includes the named file right there in your source code, just as if you’d typed it in.

But don’t crack open your drink of choice quite yet. We’re almost there! There’s just one more piece of boilerplate we have to add.

17.2 Dealing with Repeated Includes

It’s not uncommon that a header file will itself #include other headers needed for the functionality of its corresponding C files. I mean, why not?

And it could be that you have a header #included multiple times from different places. Maybe that’s no problem, but maybe it would cause compiler errors. And we can’t control how many places #include it!

Even, worse we might get into a crazy situation where header a.h includes header b.h, and b.h includes a.h! It’s an #include infinite cycle!

error: #include nested depth 200 exceeds maximum of 200

What we need to do is make it so that if a file gets included once, subsequent #includes for that file are ignored.

The stuff that we’re about to do is so common that you should just automatically do it every time you make a header file!

And the common way to do this is with a preprocessor variable that we set the first time we #include the file. And then for subsequent #includes, we first check to make sure that the variable isn’t defined.

For that variable name, it’s super common to take the name of the header file, like bar.h, make it uppercase, and replace the period with an underscore: BAR_H.

So put a check at the very, very top of the file where you see if it’s already been included, and effectively comment the whole thing out if it has.

(Don’t put a leading underscore (because a leading underscore followed by a capital letter is reserved) or a double leading underscore (because that’s also reserved.))

#ifndef BAR_H   // If BAR_H isn't defined...
#define BAR_H   // Define it (with no particular value)

// File bar.h

int add(int, int);

#endif          // End of the #ifndef BAR_H

This will effectively cause the header file to be included only a single time, no matter how many places try to #include it.

17.3 static and extern

When it comes to multifile projects, you can make sure file-scope variables and functions are not visible from other source files with the static keyword.

For more info, check out the sections in the book on the static and extern storage-class specifiers.

17.4 Compiling with Object Files

You can compile C files into an intermediate representation called object files. These are compiled machine code that hasn’t been put into an executable yet.

gcc -c foo.c     # produces foo.o
gcc -c bar.c     # produces bar.o

gcc -o foo foo.o bar.o

gcc -o foo foo.c bar.c

But for larger programs, we can take advantage of the fact that compiling from source to object files is relatively slow, and linking together a bunch of object files is relatively fast.

This really shows with the make utility that only rebuilds sources that are newer than their outputs.

Let’s say you had a thousand C files. You could compile them all to object files to start (slowly) and then combine all those object files into an executable (fast).

Now say you modified just one of those C source files—here’s the magic: you only have to rebuild that one object file for that source file! And then you rebuild the executable (fast). All the other C files don’t have to be touched.

In other words, by only rebuilding the object files we need to, we cut down on compilation times radically. (Unless of course you’re doing a “clean” build, in which case all the object files have to be created.)

18 The Outside Environment

When you run a program, it’s actually you talking to the shell, saying, “Hey, please run this thing.” And the shell says, “Sure,” and then tells the operating system, “Hey, could you please make a new process and run this thing?” And if all goes well, the OS complies and your program runs.

But there’s a whole world outside your program in the shell that can be interacted with from within C. We’ll look at a few of those in this chapter.

18.1 Command Line Arguments

Many command line utilities accept command line arguments. For example, if we want to see all files that end in .txt, we can type something like this on a Unix-like system:

In this case, the command is ls, but it arguments are all all files that end with .txt¹¹⁹.

Say we have a program called add that adds all numbers passed on the command line and prints the result:

But seriously, this is a great tool for seeing how to get those arguments from the command line and break them down.

First, let’s see how to get them at all. For this, we’re going to need a new main()!

Here’s a program that prints out all the command line arguments. For example, if we name the executable foo, we can run it like this:

./foo i like turtles

arg 0: ./foo
arg 1: i
arg 2: like
arg 3: turtles

It’s a little weird, because the zeroth argument is the name of the executable, itself. But that’s just something to get used to. The arguments themselves follow directly.

#include <stdio.h>

int main(int argc, char *argv[])
{
    for (int i = 0; i < argc; i++) {
        printf("arg %d: %s\n", i, argv[i]);
    }
}

Whoa! What’s going on with the main() function signature? What’s argc and argv¹²⁰ (pronounced arg-cee and arg-vee)?

Let’s start with the easy one first: argc. This is the argument count, including the program name, itself. If you think of all the arguments as an array of strings, which is exactly what they are, then you can think of argc as the length of that array, which is exactly what it is.

And so what we’re doing in that loop is going through all the argvs and printing them out one at a time, so for a given input:

./foo i like turtles

arg 0: ./foo
arg 1: i
arg 2: like
arg 3: turtles

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
    int total = 0;

    for (int i = 1; i < argc; i++) {  // Start at 1, the first argument
        int value = atoi(argv[i]);    // Use strtol() for better error handling

        total += value;
    }

    printf("%d\n", total);
}

$ ./add
0
$ ./add 1
1
$ ./add 1 2
3
$ ./add 1 2 3
6
$ ./add 1 2 3 4
10

Of course, it might puke if you pass in a non-integer, but hardening against that is left as an exercise to the reader.

18.1.1 The Last argv is NULL

One bit of fun trivia about argv is that after the last string is a pointer to NULL.

argv[argc] == NULL

This might seem pointless, but it turns out to be useful in a couple places; we’ll take a look at one of those right now.

18.1.2 The Alternate: char **argv

Remember that when you call a function, C doesn’t differentiate between array notation and pointer notation in the function signature.

void foo(char a[])
void foo(char *a)

Now, it’s been convenient to think of argv as an array of strings, i.e. an array of char*s, so this made sense:

int main(int argc, char *argv[])

int main(int argc, char **argv)

Yeah, that’s a pointer to a pointer, all right! If it makes it easier, think of it as a pointer to a string. But really, it’s a pointer to a value that points to a char.

argv[i]
*(argv + i)

So an alternate way to consume the command line arguments might be to just walk along the argv array by bumping up a pointer until we hit that NULL at the end.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
    int total = 0;
    
    // Cute trick to get the compiler to stop warning about the
    // unused variable argc:
    (void)argc;

    for (char **p = argv + 1; *p != NULL; p++) {
        int value = atoi(*p);  // Use strtol() for better error handling

        total += value;
    }

    printf("%d\n", total);
}

Personally, I use array notation to access argv, but have seen this style floating around, as well.

18.1.3 Fun Facts

18.2 Exit Status

Did you notice that the function signatures for main() have it returning type int? What’s that all about? It has to do with a thing called the exit status, which is an integer that can be returned to the program that launched yours to let it know how things went.

Now, there are a number of ways a program can exit in C, including returning from main(), or calling one of the exit() variants.

Side note: did you see that in basically all my examples, even though main() is supposed to return an int, I don’t actually return anything? In any other function, this would be illegal, but there’s a special case in C: if execution reaches the end of main() without finding a return, it automatically does a return 0.

But what does the 0 mean? What other numbers can we put there? And how are they used?

The spec is both clear and vague on the matter, as is common. Clear because it spells out what you can do, but vague in that it doesn’t particularly limit it, either.

Let’s get Inception ¹²² for a second: turns out that when you run your program, you’re running it from another program.

Usually this other program is some kind of shell ¹²³ that doesn’t do much on its own except launch other programs.

Now, there’s a little piece of communication that takes place between steps 4 and 5: the program can return a status value that the shell can interrogate. Typically, this value is used to indicate the success or failure of your program, and, if a failure, what type of failure.

Now, the C spec allows for two different status values, which have macro names defined in <stdlib.h>:

Let’s write a short program that multiplies two numbers from the command line. We’ll require that you specify exactly two values. If you don’t, we’ll print an error message, and exit with an error status.

Status	Description
`EXIT_SUCCESS` or `0`	Program terminated successfully.
`EXIT_FAILURE`	Program terminated with an error.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
    if (argc != 3) {
        printf("usage: mult x y\n");
        return EXIT_FAILURE;   // Indicate to shell that it didn't work
    }

    printf("%d\n", atoi(argv[1]) * atoi(argv[2]));

    return 0;  // same as EXIT_SUCCESS, everything was good.
}

Now if we try to run this, we get the expected effect until we specify exactly the right number of command-line arguments:

$ ./mult
usage: mult x y

$ ./mult 3 4 5
usage: mult x y

$ ./mult 3 4
12

But that doesn’t really show the exit status that we returned, does it? We can get the shell to print it out, though. Assuming you’re running Bash or another POSIX shell, you can use echo $? to see it¹²⁴.

$ ./mult
usage: mult x y
$ echo $?
1

$ ./mult 3 4 5
usage: mult x y
$ echo $?
1

$ ./mult 3 4
12
$ echo $?
0

Interesting! We see that on my system, EXIT_FAILURE is 1. The spec doesn’t spell this out, so it could be any number. But try it; it’s probably 1 on your system, too.

18.2.1 Other Exit Status Values

The status 0 most definitely means success, but what about all the other integers, even negative ones?

Here we’re going off the C spec and into Unix land. In general, while 0 means success, a positive non-zero number means failure. So you can only have one type of success, and multiple types of failure. Bash says the exit code should be between 0 and 255, though a number of codes are reserved.

In short, if you want to indicate different error exit statuses in a Unix environment, you can start with 1 and work your way up.

On Linux, if you try any code outside the range 0-255, it will bitwise AND the code with 0xff, effectively clamping it to that range.

You can script the shell to later use these status codes to make decisions about what to do next.

18.3 Environment Variables

Before I get into this, I need to warn you that C doesn’t specify what an environment variable is. So I’m going to describe the environment variable system that works on every major platform I’m aware of.

Basically, the environment is the program that’s going to run your program, e.g. the bash shell. And it might have some bash variables defined. In case you didn’t know, the shell can make its own variables. Each shell is different, but in bash you can just type set and it’ll show you all of them.

HISTFILE=/home/beej/.bash_history
HISTFILESIZE=500
HISTSIZE=500
HOME=/home/beej
HOSTNAME=FBILAPTOP
HOSTTYPE=x86_64
IFS=$' \t\n'

Notice they are in the form of key/value pairs. For example, one key is HOSTTYPE and its value is x86_64. From a C perspective, all values are strings, even if they’re numbers¹²⁵.

So, anyway! Long story short, it’s possible to get these values from inside your C program.

Let’s write a program that uses the standard getenv() function to look up a value that you set in the shell.

getenv() will return a pointer to the value string, or else NULL if the environment variable doesn’t exist.

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    char *val = getenv("FROTZ");  // Try to get the value

    // Check to make sure it exists
    if (val == NULL) {
        printf("Cannot find the FROTZ environment variable\n");
        return EXIT_FAILURE;
    }

    printf("Value: %s\n", val);
}

$ ./foo
Cannot find the FROTZ environment variable

$ export FROTZ="C is awesome!"

$ ./foo
Value: C is awesome!

In this way, you can set up data in environment variables, and you can get it in your C code and modify your behavior accordingly.

18.3.1 Setting Environment Variables

This isn’t standard, but a lot of systems provide ways to set environment variables.

If on a Unix-like, look up the documentation for putenv(), setenv(), and unsetenv(). On Windows, see _putenv().

18.3.2 Unix-like Alternative Environment Variables

If you’re on a Unix-like system, odds are you have another couple ways of getting access to environment variables. Note that although the spec points this out as a common extension, it’s not truly part of the C standard. It is, however, part of the POSIX standard.

extern char **environ;

You should declare it yourself before you use it, or you might find it in the non-standard <unistd.h> header file.

Each string is in the form "key=value" so you’ll have to split it and parse it yourself if you want to get the keys and values out.

Here’s an example of looping through and printing out the environment variables a couple different ways:

#include <stdio.h>

extern char **environ;  // MUST be extern AND named "environ"

int main(void)
{
    for (char **p = environ; *p != NULL; p++) {
        printf("%s\n", *p);
    }

    // Or you could do this:
    for (int i = 0; environ[i] != NULL; i++) {
        printf("%s\n", environ[i]);
    }
}

SHELL=/bin/bash
COLORTERM=truecolor
TERM_PROGRAM_VERSION=1.53.2
LOGNAME=beej
HOME=/home/beej
... etc ...

Use getenv() if at all possible because it’s more portable. But if you have to iterate over environment variables, using environ might be the way to go.

Another non-standard way to get the environment variables is as a parameter to main(). It works much the same way, but you avoid needing to add your extern environ variable. Not even the POSIX spec supports this ¹²⁷ as far as I can tell, but it’s common in Unix land.

#include <stdio.h>

int main(int argc, char **argv, char **env)  // <-- env!
{
    (void)argc; (void)argv;  // Suppress unused warnings

    for (char **p = env; *p != NULL; p++) {
        printf("%s\n", *p);
    }

    // Or you could do this:
    for (int i = 0; env[i] != NULL; i++) {
        printf("%s\n", env[i]);
    }
}

19 The C Preprocessor

Before your program gets compiled, it actually runs through a phase called preprocessing. It’s almost like there’s a language on top of the C language that runs first. And it outputs the C code, which then gets compiled.

We’ve already seen this to an extent with #include! That’s the C Preprocessor! Where it sees that directive, it includes the named file right there, just as if you’d typed it in there. And then the compiler builds the whole thing.

But it turns out it’s a lot more powerful than just being able to include things. You can define macros that are substituted… and even macros that take arguments!

19.1 #include

Let’s start with the one we’ve already seen a bunch. This is, of course, a way to include other sources in your source. Very commonly used with header files.

While the spec allows for all kinds of behavior with #include, we’re going to take a more pragmatic approach and talk about the way it works on every system I’ve ever seen.

We can split header files into two categories: system and local. Things that are built-in, like stdio.h, stdlib.h, math.h, and so on, you can include with angle brackets:

#include <stdio.h>
#include <stdlib.h>

The angle brackets tell C, “Hey, don’t look in the current directory for this header file—look in the system-wide include directory instead.”

Which, of course, implies that there must be a way to include local files from the current directory. And there is: with double quotes:

#include "myheader.h"

Or you can very probably look in relative directories using forward slashes and dots, like this:

#include "mydir/myheader.h"
#include "../someheader.py"

Don’t use a backslash (\) for your path separators in your #include! It’s undefined behavior! Use forward slash (/) only, even on Windows.

In summary, used angle brackets (< and >) for the system includes, and use double quotes (") for your personal includes.

19.2 Simple Macros

A macro is an identifier that gets expanded to another piece of code before the compiler even sees it. Think of it like a placeholder—when the preprocessor sees one of those identifiers, it replaces it with another value that you’ve defined.

#include <stdio.h>

#define HELLO "Hello, world"
#define PI 3.14159

int main(void)
{
    printf("%s, %f\n", HELLO, PI);
}

On lines 3 and 4 we defined a couple macros. Wherever these appear elsewhere in the code (line 8), they’ll be substituted with the defined values.

From the C compiler’s perspective, it’s exactly as if we’d written this, instead:

#include <stdio.h>

int main(void)
{
    printf("%s, %f\n", "Hello, world", 3.14159);
}

See how HELLO was replaced with "Hello, world" and PI was replaced with 3.14159? From the compiler’s perspective, it’s just like those values had appeared right there in the code.

Note that the macros don’t have a specific type, per se. Really all that happens is they get replaced wholesale with whatever they’re #defined as. If the resulting C code is invalid, the compiler will puke.

#define EXTRA_HAPPY

in that case, the macro exists and is defined, but is defined to be nothing. So anyplace it occurs in the text will just be replaced with nothing. We’ll see a use for this later.

It’s conventional to write macro names in ALL_CAPS even though that’s not technically required.

Overall, this gives you a way to define constant values that are effectively global and can be used any place. Even in those places where a const variable won’t work, e.g. in switch cases and fixed array lengths.

That said, the debate rages online whether a typed const variable is better than #define macro in the general case.

It can also be used to replace or modify keywords, a concept completely foreign to const, though this practice should be used sparingly.

19.3 Conditional Compilation

It’s possible to get the preprocessor to decide whether or not to present certain blocks of code to the compiler, or just remove them entirely before compilation.

We do that by basically wrapping up the code in conditional blocks, similar to if-else statements.

19.3.1 If Defined, #ifdef and #endif

First of all, let’s try to compile specific code depending on whether or not a macro is even defined.

#include <stdio.h>

#define EXTRA_HAPPY

int main(void)
{

#ifdef EXTRA_HAPPY
    printf("I'm extra happy!\n");
#endif

    printf("OK!\n");
}

In that example, we define EXTRA_HAPPY (to be nothing, but it is defined), then on line 8 we check to see if it is defined with an #ifdef directive. If it is defined, the subsequent code will be included up until the #endif.

So because it is defined, the code will be included for compilation and the output will be:

I'm extra happy!
OK!

//#define EXTRA_HAPPY

then it wouldn’t be defined, and the code wouldn’t be included in compilation. And the output would just be:

It’s important to remember that these decisions happen at compile time! The code actually gets compiled or removed depending on the condition. This is in contrast to a standard if statement that gets evaluated while the program is running.

19.3.2 If Not Defined, #ifndef

There’s also the negative sense of “if defined”: “if not defined”, or #ifndef. We could change the previous example to output different things based on whether or not something was defined:

#ifdef EXTRA_HAPPY
    printf("I'm extra happy!\n");
#endif

#ifndef EXTRA_HAPPY
    printf("I'm just regular\n");
#endif

Tying it all back in to header files, we’ve seen how we can cause header files to only be included one time by wrapping them in preprocessor directives like this:

#ifndef MYHEADER_H  // First line of myheader.h
#define MYHEADER_H

int x = 12;

#endif  // Last line of myheader.h

This demonstrates how a macro persists across files and multiple #includes. If it’s not yet defined, let’s define it and compile the whole header file.

But the next time it’s included, we see that MYHEADER_H is defined, so we don’t send the header file to the compiler—it gets effectively removed.

19.3.3 #else

But that’s not all we can do! There’s also an #else that we can throw in the mix.

#ifdef EXTRA_HAPPY
    printf("I'm extra happy!\n");
#else
    printf("I'm just regular\n");
#endif

19.3.4 Else-If: #elifdef, #elifndef

What if you want something more complex, though? Perhaps you need an if-else cascade structure to get your code built right?

Luckily we have these directives at our disposal. We can use #elifdef for “else if defined”:

#ifdef MODE_1
    printf("This is mode 1\n");
#elifdef MODE_2
    printf("This is mode 2\n");
#elifdef MODE_3
    printf("This is mode 3\n");
#else
    printf("This is some other mode\n");
#endif

19.3.5 General Conditional: #if, #elif

This works very much like the #ifdef and #ifndef directives in that you can also have an #else and the whole thing wraps up with #endif.

The only difference is that the constant expression after the #if must evaluate to true (non-zero) for the code in the #if to be compiled. So instead of whether or not something is defined, we want an expression that evaluates to true.

#include <stdio.h>

#define HAPPY_FACTOR 1

int main(void)
{

#if HAPPY_FACTOR == 0
    printf("I'm not happy!\n");
#elif HAPPY_FACTOR == 1
    printf("I'm just regular\n");
#else
    printf("I'm extra happy!\n");
#endif

    printf("OK!\n");
}

Again, for the unmatched #if clauses, the compiler won’t even see those lines. For the above code, after the preprocessor gets finished with it, all the compiler sees is:

#include <stdio.h>

int main(void)
{

    printf("I'm just regular\n");

    printf("OK!\n");
}

One hackish thing this is used for is to comment out large numbers of lines quickly¹²⁸.

If you put an #if 0 (“if false”) at the front of the block to be commented out and an #endif at the end, you can get this effect:

#if 0
    printf("All this code"); /* is effectively */
    printf("commented out"); // by the #if 0
#endif

What if you’re on a pre-C23 compiler and you don’t have #elifdef or #elifndef directive support? How can we get the same effect with #if? That is, what if I wanted this:

#ifdef FOO
    x = 2;
#elifdef BAR  // POTENTIAL ERROR: Not supported before C23
    x = 3;
#endif

Turns out there’s a preprocessor operator called defined that we can use with an #if statement.

#ifdef FOO
#if defined FOO
#if defined(FOO)   // Parentheses optional

#ifndef FOO
#if !defined FOO
#if !defined(FOO)   // Parentheses optional

Notice how we can use the standard logical NOT operator (!) for “not defined”.

#ifdef FOO
    x = 2;
#elifdef BAR  // POTENTIAL ERROR: Not supported before C23
    x = 3;
#endif

#if defined FOO
    x = 2;
#elif defined BAR
    x = 3;
#endif

19.3.6 Losing a Macro: #undef

If you’ve defined something but you don’t need it any longer, you can undefine it with #undef.

#include <stdio.h>

int main(void)
{
#define GOATS

#ifdef GOATS
    printf("Goats detected!\n");  // prints
#endif

#undef GOATS  // Make GOATS no longer defined

#ifdef GOATS
    printf("Goats detected, again!\n"); // doesn't print
#endif
}

19.4 Built-in Macros

The standard defines a lot of built-in macros that you can test and use for conditional compilation. Let’s look at those here.

19.4.1 Mandatory Macros

Macro	Description
`__DATE__`	The date of compilation—like when you’re compiling this file—in `Mmm dd yyyy` format
`__TIME__`	The time of compilation in `hh:mm:ss` format
`__FILE__`	A string containing this file’s name
`__LINE__`	The line number of the file this macro appears on
`__func__`	The name of the function this appears in, as a string¹²⁹
`__STDC__`	Defined with `1` if this is a standard C compiler
`__STDC_HOSTED__`	This will be `1` if the compiler is a hosted implementation¹³⁰, otherwise `0`
`__STDC_VERSION__`	This version of C, a constant `long int` in the form `yyyymmL`, e.g. `201710L`

#include <stdio.h>

int main(void)
{
    printf("This function: %s\n", __func__);
    printf("This file: %s\n", __FILE__);
    printf("This line: %d\n", __LINE__);
    printf("Compiled on: %s %s\n", __DATE__, __TIME__);
    printf("C Version: %ld\n", __STDC_VERSION__);
}

This function: main
This file: foo.c
This line: 7
Compiled on: Nov 23 2020 17:16:27
C Version: 201710

__FILE__, __func__ and __LINE__ are particularly useful to report error conditions in messages to developers. The assert() macro in <assert.h> uses these to call out where in the code the assertion failed.

19.4.1.1 __STDC_VERSION__s

In case you’re wondering, here are the version numbers for different major releases of the C Language Spec:

Also note that the plan is that the version numbers will strictly increase, so you could always check for, say, “at least C99” with:

Release	ISO/IEC version	`__STDC_VERSION__`
C89	ISO/IEC 9899:1990	undefined
C89	ISO/IEC 9899:1990/Amd.1:1995	`199409L`
C99	ISO/IEC 9899:1999	`199901L`
C11	ISO/IEC 9899:2011/Amd.1:2012	`201112L`

#if __STDC_VERSION__ >= 1999901L

19.4.2 Optional Macros

Macro	Description
`__STDC_ISO_10646__`	If defined, `wchar_t` holds Unicode values, otherwise something else
`__STDC_MB_MIGHT_NEQ_WC__`	A `1` indicates that the values in multibyte characters might not map equally to values in wide characters
`__STDC_UTF_16__`	A `1` indicates that the system uses UTF-16 encoding in type `char16_t`
`__STDC_UTF_32__`	A `1` indicates that the system uses UTF-32 encoding in type `char32_t`
`__STDC_ANALYZABLE__`	A `1` indicates the code is analyzable¹³¹
`__STDC_IEC_559__`	`1` if IEEE-754 (aka IEC 60559) floating point is supported
`__STDC_IEC_559_COMPLEX__`	`1` if IEC 60559 complex floating point is supported
`__STDC_LIB_EXT1__`	`1` if this implementation supports a variety of “safe” alternate standard library functions (they have `_s` suffixes on the name)
`__STDC_NO_ATOMICS__`	`1` if this implementation does not support `_Atomic` or `<stdatomic.h>`
`__STDC_NO_COMPLEX__`	`1` if this implementation does not support complex types or `<complex.h>`
`__STDC_NO_THREADS__`	`1` if this implementation does not support `<threads.h>`
`__STDC_NO_VLA__`	`1` if this implementation does not support variable-length arrays

19.5 Macros with Arguments

Macros are more powerful than simple substitution, though. You can set them up to take arguments that are substituted in, as well.

A question often arises for when to use parameterized macros versus functions. Short answer: use functions. But you’ll see lots of macros in the wild and in the standard library. People tend to use them for short, mathy things, and also for features that might change from platform to platform. You can define different keywords for one platform or another.

19.5.1 Macros with One Argument

#include <stdio.h>

#define SQR(x) x * x  // Not quite right, but bear with me

int main(void)
{
    printf("%d\n", SQR(12));  // 144
}

What that’s saying is “everywhere you see SQR with some value, replace it with that value times itself”.

    printf("%d\n", 12 * 12);  // 144

Let’s check it out. What if we wanted to compute SQR(3 + 4)? Well, $3+4=7$, so we must want to compute $7^2=49$. That’s it; 49—final answer.

    printf("%d\n", SQR(3 + 4));  // 19!!??

    printf("%d\n", 3 + 4 * 3 + 4);  // 19!

Oops! Since multiplication takes precedence, we do the $4\times3=12$ first, and get $3+12+4=19$. Not what we were after.

This is so common that you should automatically do it every time you make a parameterized math macro!

#define SQR(x) (x) * (x)   // Better... but still not quite good enough!

    printf("%d\n", (3 + 4) * (3 + 4));  // 49! Woo hoo!

But we actually still have the same problem which might manifest if we have a higher-precedence operator than multiply (*) nearby.

So the safe, proper way to put the macro together is to wrap the whole thing in additional parentheses, like so:

#define SQR(x) ((x) * (x))   // Good!

Just make it a habit to do that when you make a math macro and you can’t go wrong.

19.5.2 Macros with More than One Argument

#define TRIANGLE_AREA(w, h) (0.5 * (w) * (h))

Let’s do some macros that solve for $x$ using the quadratic formula. Just in case you don’t have it on the top of your head, it says for equations of the form:

Which is crazy. Also notice the plus-or-minus ($\pm$) in there, indicating that there are actually two solutions.

#define QUADP(a, b, c) ((-(b) + sqrt((b) * (b) - 4 * (a) * (c))) / (2 * (a)))
#define QUADM(a, b, c) ((-(b) - sqrt((b) * (b) - 4 * (a) * (c))) / (2 * (a)))

So that gets us some math. But let’s define one more that we can use as arguments to printf() to print both answers.

//          macro              replacement
//      |-----------| |----------------------------|
#define QUAD(a, b, c) QUADP(a, b, c), QUADM(a, b, c)

That’s just a couple values separated by a comma—and we can use that as a “combined” argument of sorts to printf() like this:

printf("x = %f or x = %f\n", QUAD(2, 10, 5));

#include <stdio.h>
#include <math.h>  // For sqrt()

#define QUADP(a, b, c) ((-(b) + sqrt((b) * (b) - 4 * (a) * (c))) / (2 * (a)))
#define QUADM(a, b, c) ((-(b) - sqrt((b) * (b) - 4 * (a) * (c))) / (2 * (a)))
#define QUAD(a, b, c) QUADP(a, b, c), QUADM(a, b, c)

int main(void)
{
    printf("2*x^2 + 10*x + 5 = 0\n");
    printf("x = %f or x = %f\n", QUAD(2, 10, 5));
}

2*x^2 + 10*x + 5 = 0
x = -0.563508 or x = -4.436492

Plugging in either of those values gives us roughly zero (a bit off because the numbers aren’t exact):

19.5.3 Macros with Variable Arguments

There’s also a way to have a variable number of arguments passed to a macro, using ellipses (...) after the known, named arguments. When the macro is expanded, all of the extra arguments will be in a comma-separated list in the __VA_ARGS__ macro, and can be replaced from there:

#include <stdio.h>

// Combine the first two arguments to a single number,
// then have a commalist of the rest of them:

#define X(a, b, ...) (10*(a) + 20*(b)), __VA_ARGS__

int main(void)
{
    printf("%d %f %s %d\n", X(5, 4, 3.14, "Hi!", 12));
}

    printf("%d %f %s %d\n", (10*(5) + 20*(4)), 3.14, "Hi!", 12);

130 3.140000 Hi! 12

#define X(...) #__VA_ARGS__

printf("%s\n", X(1,2,3));  // Prints "1, 2, 3"

19.5.4 Stringification

Already mentioned, just above, you can turn any argument into a string by preceding it with a # in the replacement text.

For example, we could print anything as a string with this macro and printf():

#define STR(x) #x

printf("%s\n", STR(3.14159));

printf("%s\n", "3.14159");

Let’s see if we can use this to greater effect so that we can pass any int variable name into a macro, and have it print out it’s name and value.

#include <stdio.h>

#define PRINT_INT_VAL(x) printf("%s = %d\n", #x, x)

int main(void)
{
    int a = 5;

    PRINT_INT_VAL(a);  // prints "a = 5"
}

    printf("%s = %d\n", "a", 5);

19.5.5 Concatenation

#define CAT(a, b) a ## b

printf("%f\n", CAT(3.14, 1592));   // 3.141592

19.6 Multiline Macros

It’s possible to continue a macro to multiple lines if you escape the newline with a backslash (\).

Let’s write a multiline macro that prints numbers from 0 to the product of the two arguments passed in.

#include <stdio.h>

#define PRINT_NUMS_TO_PRODUCT(a, b) do { \
    int product = (a) * (b); \
    for (int i = 0; i < product; i++) { \
        printf("%d\n", i); \
    } \
} while(0)

int main(void)
{
    PRINT_NUMS_TO_PRODUCT(2, 4);  // Outputs numbers from 0 to 7
}

The latter point might be a little weird, but it’s all about absorbing the trailing ; the coder drops after the macro.

At first I thought that just using squirrely braces would be enough, but there’s a case where it fails if the coder puts a semicolon after the macro. Here’s that case:

#include <stdio.h>

#define FOO(x) { (x)++; }

int main(void)
{
    int i = 0;

    if (i == 0)
        FOO(i);
    else
        printf(":-(\n");

    printf("%d\n", i);
}

foo.c:11:5: error: ‘else’ without a previous ‘if’


    if (i == 0) {
        (i)++;
    };             // <-- Trouble with a capital-T!

    else
        printf(":-(\n");

The ; puts an end to the if statement, so the else is just floating out there illegally¹³².

19.7 Example: An Assert Macro

Adding asserts to your code is a good way to catch conditions that you think shouldn’t happen. C provides assert() functionality. It checks a condition, and if it’s false, the program bombs out telling you the file and line number on which the assertion failed.

ASSERT(x < 20, "x must be under 20");

I want something like this to happen (assuming the ASSERT() is on line 220 of foo.c):

if (!(x < 20)) {
    fprintf(stderr, "foo.c:220: assertion x < 20 failed: ");
    fprintf(stderr, "x must be under 20\n");
    exit(1);
}

We can get the filename out of the __FILE__ macro, and the line number from __LINE__. The message is already a string, but x < 20 is not, so we’ll have to stringify it with #. We can make a multiline macro by using backslash escapes at the end of the line.

#define ASSERT(c, m) \
do { \
    if (!(c)) { \
        fprintf(stderr, __FILE__ ":%d: assertion %s failed: %s\n", \
                        __LINE__, #c, m); \
        exit(1); \
    } \
} while(0)

(It looks a little weird with __FILE__ out front like that, but remember it is a string literal, and string literals next to each other are automagically concatenated. __LINE__ on the other hand, it’s just an int.)

int x = 30;

ASSERT(x < 20, "x must be under 20");

The only thing left is a way to turn it on and off, and we could do that with conditional compilation.

#include <stdio.h>
#include <stdlib.h>

#define ASSERT_ENABLED 1

#if ASSERT_ENABLED
#define ASSERT(c, m) \
do { \
    if (!(c)) { \
        fprintf(stderr, __FILE__ ":%d: assertion %s failed: %s\n", \
                        __LINE__, #c, m); \
        exit(1); \
    } \
} while(0)
#else
#define ASSERT(c, m)  // Empty macro if not enabled
#endif

int main(void)
{
    int x = 30;

    ASSERT(x < 20, "x must be under 20");
}

foo.c:23: assertion x < 20 failed: x must be under 20

19.8 The #error Directive

Commonly, this is used inside a conditional to prevent compilation unless some prerequisites are met:

#ifndef __STDC_IEC_559__
    #error I really need IEEE-754 floating point to compile. Sorry!
#endif

Some compilers have a non-standard complementary #warning directive that will output a warning but not stop compilation, but this is not in the C11 spec.

19.9 The #embed Directive

And currently not yet working with any of my compilers, so take this section with a grain of salt!

The gist of this is that you can include bytes of a file as integer constants as if you’d typed them in.

For example, if you have a binary file named foo.bin that contains four bytes with decimal values 11, 22, 33, and 44, and you do this:

int a[] = {
#embed "foo.bin"
};

int a[] = {11,22,33,44};

This is a really powerful way to initialize an array with binary data without needing to convert it all to code first—the preprocessor does it for you!

A more typical use case might be a file containing a small image to be displayed that you don’t want to load at runtime.

int a[] = {
#embed <foo.bin>
};

If you use angle brackets, the preprocessor looks in a series of implementation-defined places to locate the file, just like #include would do. If you use double quotes and the resource is not found, the compiler will try it as if you’d used angle brackets in a last desperate attempt to find the file.

#embed works like #include in that it effectively pastes values in before the compiler sees them. This means you can use it in all kinds of places:

Now—are these always bytes? Meaning they’ll have values from 0 to 255, inclusive? The answer is definitely by default “yes”, except when it is “no”.

Technically, the elements will be CHAR_BIT bits wide. And this is very likely 8 on your system, so you’d get that 0-255 range in your values. (They’ll always be non-negative.)

Also, it’s possible that an implementation might allow this to be overridden in some way, e.g. on the command line or with parameters.

The size of the file in bits must be a multiple of the element size. That is, if each element is 8 bits, the file size (in bits) must be a multiple of 8. In regular everyday usage, this is a confusing way of saying that each file needs to be an integer number of bytes… which of course it is. Honestly, I’m not even sure why I bothered with this paragraph. Read the spec if you’re really that curious.

19.9.1 #embed Parameters

There are all kinds of parameters you can specify to the #embed directive. Here’s an example with the yet-unintroduced limit() parameter:

int a[] = {
#embed "/dev/random" limit(5)
};

But what if you already have limit defined somewhere else? Luckily you can put __ around the keyword and it will work the same way:

int a[] = {
#embed "/dev/random" __limit__(5)
};

19.9.2 The limit() Parameter

This is a maximum value, not an absolute value. If the file that’s embedded is shorter than the specified limit, only that many bytes will be imported.

The /dev/random example above is an example of the motivation for this—in Unix, that’s a character device file that will return an infinite stream of pretty-random numbers.

Embedding an infinite number of bytes is hard on your RAM, so the limit parameter gives you a way to stop after a certain number.

Finally, you are allowed to use #define macros in your limit, in case you were curious.

19.9.3 The if_empty Parameter

This parameter defines what the embed result should be if the file exists but contains no data. Let’s say that the file foo.dat contains a single byte with the value 123. If we do this:

int x = 
#embed "foo.dat" if_empty(999)
;

int x = 123;   // When foo.dat contains a 123 byte

But what if the file foo.dat is zero bytes long (i.e. contains no data and is empty)? If that’s the case, it would expand to:

int x = 999;   // When foo.dat is empty

Notably if the limit is set to 0, then the if_empty will always be substituted. That is, a zero limit effectively means the file is empty.

int x = 
#embed "foo.dat" limit(0) if_empty(999)
;

19.9.4 The prefix() and suffix() Parameters

Note that these only affect non-empty data! If the file is empty, neither prefix nor suffix has any effect.

Here’s an example where we embed three random numbers, but prefix the numbers with 11, and suffix them with ,99:

int x[] = {
#embed "/dev/urandom" limit(3) prefix(11,) suffix(,99)
};

int x[] = {11,135,116,220,99};

There’s no requirement that you use both prefix and suffix. You can use both, one, the other, or neither.

We can make use of the characteristic that these are only applied to non-empty files to neat effect, as shown in the following example shamelessly stolen from the spec.

Let’s say we have a file foo.dat that has some data it in. And we want to use this to initialize an array, and then we want a suffix on the array that is a zero element.

int x[] = {
#embed "foo.dat" suffix(,0)
};

int x[] = {11,22,33,0};

int x[] = {
#embed "foo.dat" suffix(,)
    0
};

Since the suffix parameter is omitted if the file is empty, this would just turn into:

19.9.5 The __has_embed() Identifier

This is a great way to test to see if a particular file is available to be embedded, and also whether or not it’s empty.

Here’s a chunk of code that will get 5 random numbers from the random number generator character device. If that doesn’t exist, it tries to get them from a file myrandoms.dat. If that doesn’t exist, it uses some hard-coded values:

    int random_nums[] = {
#if __has_embed("/dev/urandom")
    #embed "/dev/urandom" limit(5)
#elif __has_embed("myrandoms.dat")
    #embed "myrandoms.dat" limit(5)
#else
    140,178,92,167,120
#endif
    };

`__has_embed()` Result	Description
`__STDC_EMBED_NOT_FOUND__`	If the file isn’t found.
`__STDC_EMBED_FOUND__`	If the file is found and is not empty.
`__STDC_EMBED_EMPTY`	If the file is found and is empty.

I have good reason to believe that __STDC_EMBED_NOT_FOUND__ is 0 and the others aren’t zero (because it’s implied in the proposal and it makes logical sense), but I’m having trouble finding that in this version of the draft spec.

19.9.6 Other Parameters

A compiler implementation can define other embed parameters all it wants—look for these non-standard parameters in your compiler’s documentation.

#embed "foo.bin" limit(12) frotz(lamp)

#embed "foo.bin" limit(12) fmc::frotz(lamp)

It might be sensible to try to detect if these are available before you use them, and luckily we can use __has_embed to help us here.

Normally, __has_embed() will just tell us if the file is there or not. But—and here’s the fun bit—it will also return false if any additional parameters are also not supported!

So if we give it a file that we know exists as well as a parameter that we want to test for the existence of, it will effectively tell us if that parameter is supported.

What file always exists, though? Turns out we can use the __FILE__ macro, which expands to the name of the source file that references it! That file must exist, or something is seriously wrong in the chicken-and-egg department.

#if __has_embed(__FILE__ fmc::frotz(lamp))
    puts("fmc::frotz(lamp) is supported!");
#else
    puts("fmc::frotz(lamp) is NOT supported!");
#endif

19.9.7 Embedding Multi-Byte Values

What about getting some ints in there instead of individual bytes? What about multi-byte values in the embedded file?

This is not something supported by the C23 standard, but there could be implementation extensions defined for it in the future.

19.10 The #pragma Directive

This is one funky directive, short for “pragmatic”. You can use it to do… well, anything your compiler supports you doing with it.

Basically the only time you’re going to add this to your code is if some documentation tells you to do so.

19.10.1 Non-Standard Pragmas

Here’s one non-standard example of using #pragma to cause the compiler to execute a for loop in parallel with multiple threads (if the compiler supports the OpenMP ¹³³ extension):

#pragma omp parallel for
for (int i = 0; i < 10; i++) { ... }

There are all kinds of #pragma directives documented across all four corners of the globe.

19.10.2 Standard Pragmas

There are also a few standard ones, and these start with STDC, and follow the same form:

#pragma STDC pragma_name on-off

Pragma Name	Description
`FP_CONTRACT`	Allow floating point expressions to be contracted into a single operation to avoid rounding errors that might occur from multiple operations.
`FENV_ACCESS`	Set to `ON` if you plan to access the floating point status flags. If `OFF`, the compiler might perform optimizations that cause the values in the flags to be inconsistent or invalid.
`CX_LIMITED_RANGE`	Set to `ON` to allow the compiler to skip overflow checks when performing complex arithmetic. Defaults to `OFF`.

#pragma STDC FP_CONTRACT OFF
#pragma STDC CX_LIMITED_RANGE ON

19.10.3 _Pragma Operator

#pragma "Unnecessary" quotes
_Pragma("\"Unnecessary\" quotes")

#define PRAGMA(x) _Pragma(#x)

19.11 The #line Directive

This allows you to override the values for __LINE__ and __FILE__. If you want.

#line 300 "newfilename"

19.12 The Null Directive

A # on a line by itself is ignored by the preprocessor. Now, to be entirely honest, I don’t know what the use case is for this.

#ifdef FOO
    #
#else
    printf("Something");
#endif

which is just cosmetic; the line with the solitary # can be deleted with no ill effect.

#
#ifdef FOO
    x = 2;
#endif
#
#if BAR == 17
    x = 12;
#endif
#

Another post mentions elimination of comments—that in GCC, a comment after a # will not be seen by the compiler. Which I don’t doubt, but the specification doesn’t seem to say this is standard behavior.

My searches for rationale aren’t bearing much fruit. So I’m going to just say this is some good ol’ fashioned C esoterica.

20 structs II: More Fun with structs

Turns out there’s a lot more you can do with structs than we’ve talked about, but it’s just a big pile of miscellaneous things. So we’ll throw them in this chapter.

20.1 Initializers of Nested structs and Arrays

struct foo x = {.a=12, .b=3.14};

Turns out we have more power in these initializers than we’d originally shared. Exciting!

For one thing, if you have a nested substructure like the following, you can initialize members of that substructure by following the variable names down the line:

struct foo x = {.a.b.c=12};

#include <stdio.h>

struct cabin_information {
    int window_count;
    int o2level;
};

struct spaceship {
    char *manufacturer;
    struct cabin_information ci;
};

int main(void)
{
    struct spaceship s = {
        .manufacturer="General Products",
        .ci.window_count = 8,   // <-- NESTED INITIALIZER!
        .ci.o2level = 21
    };

    printf("%s: %d seats, %d%% oxygen\n",
        s.manufacturer, s.ci.window_count, s.ci.o2level);
}

Check out lines 16-17! That’s where we’re initializing members of the struct cabin_information in the definition of s, our struct spaceship.

And here is another option for that same initializer—this time we’ll do something more standard-looking, but either approach works:

    struct spaceship s = {
        .manufacturer="General Products",
        .ci={
            .window_count = 8,
            .o2level = 21
        }
    };

Now, as if the above information isn’t spectacular enough, we can also mix in array initializers in there, too.

Let’s change this up to get an array of passenger information in there, and we can check out how the initializers work in there, too.

#include <stdio.h>

struct passenger {
    char *name;
    int covid_vaccinated; // Boolean
};

#define MAX_PASSENGERS 8

struct spaceship {
    char *manufacturer;
    struct passenger passenger[MAX_PASSENGERS];
};

int main(void)
{
    struct spaceship s = {
        .manufacturer="General Products",
        .passenger = {
            // Initialize a field at a time
            [0].name = "Gridley, Lewis",
            [0].covid_vaccinated = 0,

            // Or all at once
            [7] = {.name="Brown, Teela", .covid_vaccinated=1},
        }
    };

    printf("Passengers for %s ship:\n", s.manufacturer);

    for (int i = 0; i < MAX_PASSENGERS; i++)
        if (s.passenger[i].name != NULL)
            printf("    %s (%svaccinated)\n",
                s.passenger[i].name,
                s.passenger[i].covid_vaccinated? "": "not ");
}

20.2 Anonymous structs

These are “the struct with no name”. We also mention these in the typedef section, but we’ll refresh here.

struct animal {
    char *name;
    int leg_count, speed;
};

struct {              // <-- No name!
    char *name;
    int leg_count, speed;
};

Okaaaaay. So we have a struct, but it has no name, so we have no way of using it later? Seems pretty pointless.

Admittedly, in that example, it is. But we can still make use of it a couple ways.

One is rare, but since the anonymous struct represents a type, we can just put some variable names after it and use them.

struct {                   // <-- No name!
    char *name;
    int leg_count, speed;
} a, b, c;                 // 3 variables of this struct type

a.name = "antelope";
c.leg_count = 4;           // for example

Far more common is use of anonymous structs with a typedef so that we can use it later (e.g. to pass variables to functions).

typedef struct {                   // <-- No name!
    char *name;
    int leg_count, speed;
} animal;                          // New type: animal

animal a, b, c;

a.name = "antelope";
c.leg_count = 4;           // for example

Personally, I don’t use many anonymous structs. I think it’s more pleasant to see the entire struct animal before the variable name in a declaration.

20.3 Self-Referential structs

For any graph-like data structure, it’s useful to be able to have pointers to the connected nodes/vertices. But this means that in the definition of a node, you need to have a pointer to a node. It’s chicken and eggy!

struct node {
    int data;
    struct node *next;
};

It’s important to note that next is a pointer. This is what allows the whole thing to even build. Even though the compiler doesn’t know what the entire struct node looks like yet, all pointers are the same size.

#include <stdio.h>
#include <stdlib.h>

struct node {
    int data;
    struct node *next;
};

int main(void)
{
    struct node *head;

    // Hackishly set up a linked list (11)->(22)->(33)
    head = malloc(sizeof(struct node));
    head->data = 11;
    head->next = malloc(sizeof(struct node));
    head->next->data = 22;
    head->next->next = malloc(sizeof(struct node));
    head->next->next->data = 33;
    head->next->next->next = NULL;

    // Traverse it
    for (struct node *cur = head; cur != NULL; cur = cur->next) {
        printf("%d\n", cur->data);
    }
}

20.4 Flexible Array Members

Back in the good old days, when people carved C code out of wood, some folks thought would be neat if they could allocate structs that had variable length arrays at the end of them.

I want to be clear that the first part of the section is the old way of doing things, and we’re going to do things the new way after that.

For example, maybe you could define a struct for holding strings and the length of that string. It would have a length and an array to hold the data. Maybe something like this:

struct len_string {
    int length;
    char data[8];
};

But that has 8 hardcoded as the maximum length of a string, and that’s not much. What if we did something clever and just malloc()d some extra space at the end after the struct, and then let the data overflow into that space?

struct len_string *s = malloc(sizeof *s + 40);

Because data is the last field of the struct, if we overflow that field, it runs out into space that we already allocated! For this reason, this trick only works if the short array is the last field in the struct.

// Copy more than 8 bytes!

strcpy(s->data, "Hello, world!");  // Won't crash. Probably.

In fact, there was a common compiler workaround for doing this, where you’d allocate a zero length array at the end:

struct len_string {
    int length;
    char data[0];
};

Because data is the last field of the struct, if we overflow that field, it runs out into space that we already allocated!

// Copy more than 8 bytes!

strcpy(s->data, "Hello, world!");  // Won't crash. Probably.

But, of course, actually accessing the data beyond the end of that array is undefined behavior! In these modern times, we no longer deign to resort to such savagery.

Luckily for us, we can still get the same effect with C99 and later, but now it’s legal.

struct len_string {
    int length;
    char data[];
};

Again, this only works if the flexible array member is the last field in the struct.

And then we can allocate all the space we want for those strings by malloc()ing larger than the struct len_string, as we do in this example that makes a new struct len_string from a C string:

struct len_string *len_string_from_c_string(char *s)
{
    int len = strlen(s);

    // Allocate "len" more bytes than we'd normally need
    struct len_string *ls = malloc(sizeof *ls + len);

    ls->length = len;

    // Copy the string into those extra bytes
    memcpy(ls->data, s, len);

    return ls;
}

20.5 Padding Bytes

Beware that C is allowed to add padding bytes within or after a struct as it sees fit. You can’t trust that they will be directly adjacent in memory¹³⁵.

Let’s take a look at this program. We output two numbers. One is the sum of the sizeofs the individual field types. The other is the sizeof the entire struct.

One would expect them to be the same. The size of the total is the size of the sum of its parts, right?

#include <stdio.h>

struct foo {
    int a;
    char b;
    int c;
    char d;
};

int main(void)
{
    printf("%zu\n", sizeof(int) + sizeof(char) + sizeof(int) + sizeof(char));
    printf("%zu\n", sizeof(struct foo));
}

They’re not the same! The compiler has added 6 bytes of padding to help it be more performant. Maybe you got different output with your compiler, but unless you’re forcing it, you can’t be sure there’s no padding.

20.6 offsetof

In the previous section, we saw that the compiler could inject padding bytes at will inside a structure.

What if we needed to know where those were? We can measure it with offsetof, defined in <stddef.h>.

Let’s modify the code from above to print the offsets of the individual fields in the struct:

#include <stdio.h>
#include <stddef.h>

struct foo {
    int a;
    char b;
    int c;
    char d;
};

int main(void)
{
    printf("%zu\n", offsetof(struct foo, a));
    printf("%zu\n", offsetof(struct foo, b));
    printf("%zu\n", offsetof(struct foo, c));
    printf("%zu\n", offsetof(struct foo, d));
}

indicating that we’re using 4 bytes for each of the fields. It’s a little weird, because char is only 1 byte, right? The compiler is putting 3 padding bytes after each char so that all the fields are 4 bytes long. Presumably this will run faster on my CPU.

20.7 Fake OOP

There’s a slightly abusive thing that’s sort of OOP-like that you can do with structs.

Since the pointer to the struct is the same as a pointer to the first element of the struct, you can freely cast a pointer to the struct to a pointer to the first element.

struct parent {
    int a, b;
};

struct child {
    struct parent super;  // MUST be first
    int c, d;
};

Then we are able to pass a pointer to a struct child to a function that expects either that or a pointer to a struct parent!

Because struct parent super is the first item in the struct child, a pointer to any struct child is the same as a pointer to that super field¹³⁶.

Let’s set up an example here. We’ll make structs as above, but then we’ll pass a pointer to a struct child to a function that needs a pointer to a struct parent… and it’ll still work.

#include <stdio.h>

struct parent {
    int a, b;
};

struct child {
    struct parent super;  // MUST be first
    int c, d;
};

// Making the argument `void*` so we can pass any type into it
// (namely a struct parent or struct child)
void print_parent(void *p)
{
    // Expects a struct parent--but a struct child will also work
    // because the pointer points to the struct parent in the first
    // field:
    struct parent *self = p;

    printf("Parent: %d, %d\n", self->a, self->b);
}

void print_child(struct child *self)
{
    printf("Child: %d, %d\n", self->c, self->d);
}

int main(void)
{
    struct child c = {.super.a=1, .super.b=2, .c=3, .d=4};

    print_child(&c);
    print_parent(&c);  // Also works even though it's a struct child!
}

See what we did on the last line of main()? We called print_parent() but passed a struct child* as the argument! Even though print_parent() needs the argument to point to a struct parent, we’re getting away with it because the first field in the struct child is a struct parent.

Again, this works because a pointer to a struct has the same value as a pointer to the first field in that struct.

and my assumption that “suitably converted” means “cast to the effective type of the initial member”.

20.8 Bit-Fields

In my experience, these are rarely used, but you might see them out there from time to time, especially in lower-level applications that pack bits together into larger spaces.

#include <stdio.h>

struct foo {
    unsigned int a;
    unsigned int b;
    unsigned int c;
    unsigned int d;
};

int main(void)
{
    printf("%zu\n", sizeof(struct foo));
}

For me, this prints 16. Which makes sense, since unsigneds are 4 bytes on my system.

But what if we knew that all the values that were going to be stored in a and b could be stored in 5 bits, and the values in c, and d could be stored in 3 bits? That’s only a total 16 bits. Why have 128 bits reserved for them if we’re only going to use 16?

Well, we can tell C to pretty-please try to pack these values in. We can specify the maximum number of bits that values can take (from 1 up the size of the containing type).

We do this by putting a colon after the field name, followed by the field width in bits.

struct foo {
    unsigned int a:5;
    unsigned int b:5;
    unsigned int c:3;
    unsigned int d:3;
};

Now when I ask C how big my struct foo is, it tells me 4! It was 16 bytes, but now it’s only 4. It has “packed” those 4 values down into 4 bytes, which is a four-fold memory savings.

The tradeoff is, of course, that the 5-bit fields can only hold values from 0-31 and the 3-bit fields can only hold values from 0-7. But life’s all about compromise, after all.

20.8.1 Non-Adjacent Bit-Fields

A gotcha: C will only combine adjacent bit-fields. If they’re interrupted by non-bit-fields, you get no savings:

struct foo {            // sizeof(struct foo) == 16 (for me)
    unsigned int a:1;   // since a is not adjacent to c.
    unsigned int b;
    unsigned int c:1;
    unsigned int d;
};

In that example, since a is not adjacent to c, they are both “packed” in their own ints.

So we have one int each for a, b, c, and d. Since my ints are 4 bytes, that’s a grand total of 16 bytes.

A quick rearrangement yields some space savings from 16 bytes down to 12 bytes (on my system):

struct foo {            // sizeof(struct foo) == 12 (for me)
    unsigned int a:1;
    unsigned int c:1;
    unsigned int b;
    unsigned int d;
};

And now, since a is next to c, the compiler puts them together into a single int.

So we have one int for a combined a and c, and one int each for b and d. For a grand total of 3 ints, or 12 bytes.

20.8.2 Signed or Unsigned ints

If you just declare a bit-field to be int, the different compilers will treat it as signed or unsigned. Just like the situation with char.

20.8.3 Unnamed Bit-Fields

In some specific circumstances, you might need to reserve some bits for hardware reasons, but not need to use them in code.

For example, let’s say you have a byte where the top 2 bits have a meaning, the bottom 1 bit has a meaning, but the middle 5 bits do not get used by you¹³⁷.

struct foo {
    unsigned char a:2;
    unsigned char dummy:5;
    unsigned char b:1;
};

And that works—in our code we use a and b, but never dummy. It’s just there to eat up 5 bits to make sure a and b are in the “required” (by this contrived example) positions within the byte.

C allows us a way to clean this up: unnamed bit-fields. You can just leave the name (dummy) out in this case, and C is perfectly happy for the same effect:

struct foo {
    unsigned char a:2;
    unsigned char :5;   // <-- unnamed bit-field!
    unsigned char b:1;
};

20.8.4 Zero-Width Unnamed Bit-Fields

Some more esoterica out here… Let’s say you were packing bits into an unsigned int, and you needed some adjacent bit-fields to pack into the next unsigned int.

struct foo {
    unsigned int a:1;
    unsigned int b:2;
    unsigned int c:3;
    unsigned int d:4;
};

the compiler packs all those into a single unsigned int. But what if you needed a and b in one int, and c and d in a different one?

There’s a solution for that: put an unnamed bit-field of width 0 where you want the compiler to start anew with packing bits in a different int:

struct foo {
    unsigned int a:1;
    unsigned int b:2;
    unsigned int :0;   // <--Zero-width unnamed bit-field
    unsigned int c:3;
    unsigned int d:4;
};

It’s analogous to an explicit page break in a word processor. You’re telling the compiler, “Stop packing bits in this unsigned, and start packing them in the next one.”

By adding the zero-width unnamed bit field in that spot, the compiler puts a and b in one unsigned int, and c and d in another unsigned int. Two total, for a size of 8 bytes on my system (unsigned ints are 4 bytes each).

20.9 Unions

These are basically just like structs, except the fields overlap in memory. The union will be only large enough for the largest field, and you can only use one field at a time.

You declare them just like structs, except it’s union. Take a look at this:

union foo {
    int a, b, c, d, e, f;
    float g, h;
    char i, j, k, l;
};

Now, that’s a lot of fields. If this were a struct, my system would tell me it took 36 bytes to hold it all.

But it’s a union, so all those fields overlap in the same stretch of memory. The biggest one is int (or float), taking up 4 bytes on my system. And, indeed, if I ask for the sizeof the union foo, it tells me 4!

The tradeoff is that you can only portably use one of those fields at a time. However…

20.9.1 Unions and Type Punning

Doing so is called type punning ¹³⁸, and you’d use it if you really knew what you were doing, typically with some kind of low-level programming.

Since the members of a union share the same memory, writing to one member necessarily affects the others. And if you read from one what was written to another, you get some weird effects.

#include <stdio.h>

union foo {
    float b;
    short a;
};

int main(void)
{
    union foo x;

    x.b = 3.14159;

    printf("%f\n", x.b);  // 3.14159, fair enough

    printf("%d\n", x.a);  // But what about this?
}

because under the hood, the object representation for the float 3.14159 was the same as the object representation for the short 4048. On my system. Your results may vary.

20.9.2 Pointers to unions

If you have a pointer to a union, you can cast that pointer to any of the types of the fields in that union and get the values out that way.

In this example, we see that the union has ints and floats in it. And we get pointers to the union, but we cast them to int* and float* types (the cast silences compiler warnings). And then if we dereference those, we see that they have the values we stored directly in the union.

#include <stdio.h>

union foo {
    int a, b, c, d, e, f;
    float g, h;
    char i, j, k, l;
};

int main(void)
{
    union foo x;

    int *foo_int_p = (int *)&x;
    float *foo_float_p = (float *)&x;

    x.a = 12;
    printf("%d\n", x.a);           // 12
    printf("%d\n", *foo_int_p);    // 12, again

    x.g = 3.141592;
    printf("%f\n", x.g);           // 3.141592
    printf("%f\n", *foo_float_p);  // 3.141592, again
}

The reverse is also true. If we have a pointer to a type inside the union, we can cast that to a pointer to the union and access its members.

union foo x;
int *foo_int_p = (int *)&x;             // Pointer to int field
union foo *p = (union foo *)foo_int_p;  // Back to pointer to union

p->a = 12;  // This line the same as...
x.a = 12;   // this one.

All this just lets you know that, under the hood, all these values in a union start at the same place in memory, and that’s the same as where the entire union is.

20.9.3 Common Initial Sequences in Unions

If you have a union of structs, and all those structs begin with a common initial sequence, it’s valid to access members of that sequence from any of the union members.

struct a {
    int x;     //
    float y;   // Common initial sequence

    char *p;
};

struct b {
    int x;     //
    float y;   // Common initial sequence

    double *p;
    short z;
};

Do you see it? It’s that they start with int followed by float—that’s the common initial sequence. The members in the sequence of the structs have to be compatible types. And we see that with x and y, which are int and float respectively.

union foo {
    struct a sa;
    struct b sb;
};

What this rule tells us is that we’re guaranteed that the members of the common initial sequences are interchangeable in code. That is:

Also, the names of the members in the common initial sequence don’t matter—all that matters is that the types are the same.

All together, this allows us a way to safely add some shared information between structs in the union. The best example of this is probably using a field to determine the type of struct out of all the structs in the union that is currently “in use”.

That is, if we weren’t allowed this and we passed the union to some function, how would that function know which member of the union was the one it should look at?

#include <stdio.h>

struct common {
    int type;   // common initial sequence
};

struct antelope {
    int type;   // common initial sequence

    int loudness;
};

struct octopus {
    int type;   // common initial sequence

    int sea_creature;
    float intelligence;
};

union animal {
    struct common common;
    struct antelope antelope;
    struct octopus octopus;
};

#define ANTELOPE 1
#define OCTOPUS  2

So far, nothing special has happened here. It seems like the type field is completely useless.

But now let’s make a generic function that prints a union animal. It has to somehow be able to tell if it’s looking at a struct antelope or a struct octopus.

Because of the magic of common initial sequences, it can look up the animal type in any of these places for a particular union animal x:

int type = x.common.type;    \\ or...
int type = x.antelope.type;  \\ or...
int type = x.octopus.type;

And, as you might have guessed, the struct common is there so code can agnostically look at the type without mentioning a particular animal.

void print_animal(union animal *x)
{
    switch (x->common.type) {
        case ANTELOPE:
            printf("Antelope: loudness=%d\n", x->antelope.loudness);
            break;

        case OCTOPUS:
            printf("Octopus : sea_creature=%d\n", x->octopus.sea_creature);
            printf("          intelligence=%f\n", x->octopus.intelligence);
            break;
        
        default:
            printf("Unknown animal type\n");
    }

}

int main(void)
{
    union animal a = {.antelope.type=ANTELOPE, .antelope.loudness=12};
    union animal b = {.octopus.type=OCTOPUS, .octopus.sea_creature=1,
                                       .octopus.intelligence=12.8};

    print_animal(&a);
    print_animal(&b);
}

See how on line 29 we’re just passing in the union—we have no idea what type of animal struct is in use within it.

But that’s OK! Because on line 31 we check the type to see if it’s an antelope or an octopus. And then we can look at the proper struct to get the members.

It’s definitely possible to get this same effect using just structs, but you can do it this way if you want the memory-saving effects of a union.

20.10 Unions and Unnamed Structs

struct {
    int x, y;
} s;

That defines a variable s that is of anonymous struct type (because the struct has no name tag), with members x and y.

s.x = 34;
s.y = 90;

printf("%d %d\n", s.x, s.y);

Turns out you can drop those unnamed structs in unions just like you might expect:

union foo {
    struct {       // unnamed!
        int x, y;
    } a;

    struct {       // unnamed!
        int z, w;
    } b;
};

union foo f;

f.a.x = 1;
f.a.y = 2;
f.b.z = 3;
f.b.w = 4;

20.11 Passing and Returning structs and unions

You can pass a struct or union to a function by value (as opposed to a pointer to it)—a copy of that object to the parameter will be made as if by assignment as per usual.

You can also return a struct or union from a function and it is also passed by value back.

#include <stdio.h>

struct foo {
    int x, y;
};

struct foo f(void)
{
    return (struct foo){.x=34, .y=90};
}

int main(void)
{
    struct foo a = f();  // Copy is made

    printf("%d %d\n", a.x, a.y);
}

Fun fact: if you do this, you can use the . operator right off the function call:

    printf("%d %d\n", f().x, f().y);

And the same holds true for returning pointers to structs and unions—just be sure to use the -> arrow operator in that case.

21 Characters and Strings II

We’ve talked about how char types are actually just small integer types… but it’s the same for a character in single quotes.

Turns out there are few more types of strings and characters, and it leads down one of the most infamous rabbit holes in the language: the whole multibyte/wide/Unicode/localization thingy.

21.1 Escape Sequences

We’re used to strings and characters with regular letters, punctuation, and numbers:

char *s = "Hello!";
char t = 'c';

But what if we want some special characters in there that we can’t type on the keyboard because they don’t exist (e.g. “€”), or even if we want a character that’s a single quote? We clearly can’t do this:

To do these things, we use something called escape sequences. These are the backslash character (\) followed by another character. The two (or more) characters together have special meaning.

For our single quote character example, we can put an escape (that is, \) in front of the central single quote to solve it:

Now C knows that \' means just a regular quote we want to print, not the end of the character sequence.

You can say either “backslash” or “escape” in this context (“escape that quote”) and C devs will know what you’re talking about. Also, “escape” in this context is different than your Esc key or the ASCII ESC code.

21.1.1 Frequently-used Escapes

In my humble opinion, these escape characters make up 99.2%¹³⁹ of all escapes.

Code	Description
`\n`	Newline character—when printing, continue subsequent output on the next line
`\'`	Single quote—used for a single quote character constant
`\"`	Double quote—used for a double quote in a string literal
`\\`	Backslash—used for a literal `\` in a string or character

printf("Use \\n for newline\n");  // Use \n for newline
printf("Say \"hello\"!\n");       // Say "hello"!
printf("%c\n", '\'');             // '

21.1.2 Rarely-used Escapes

Code	Description
`\a`	Alert. This makes the terminal make a sound or flash, or both!
`\b`	Backspace. Moves the cursor back a character. Doesn’t delete the character.
`\f`	Formfeed. This moves to the next “page”, but that doesn’t have much modern meaning. On my system, this behaves like `\v`.
`\r`	Return. Move to the beginning of the same line.
`\t`	Horizontal tab. Moves to the next horizontal tab stop. On my machine, this lines up on columns that are multiples of 8, but YMMV.
`\v`	Vertical tab. Moves to the next vertical tab stop. On my machine, this moves to the same column on the next line.
`\?`	Literal question mark. Sometimes you need this to avoid trigraphs, as shown below.

21.1.2.1 Single Line Status Updates

A use case for \b or \r is to show status updates that appear on the same line on the screen and don’t cause the display to scroll. Here’s an example that does a countdown from 10. (If your compiler doesn’t support threading, you can use the non-standard POSIX function sleep() from <unistd.h>—if you’re not on a Unix-like, search for your platform and sleep for the equivalent.)

#include <stdio.h>
#include <threads.h>

int main(void)
{
    for (int i = 10; i >= 0; i--) {
        printf("\rT minus %d second%s... \b", i, i != 1? "s": "");

        fflush(stdout);  // Force output to update

        // Sleep for 1 second
        thrd_sleep(&(struct timespec){.tv_sec=1}, NULL);
    }

    printf("\rLiftoff!             \n");
}

Quite a few things are happening on line 7. First of all, we lead with a \r to get us to the beginning of the current line, then we overwrite whatever’s there with the current countdown. (There’s ternary operator out there to make sure we print 1 second instead of 1 seconds.)

Also, there’s a space after the ... That’s so that we properly overwrite the last . when i drops from 10 to 9 and we get a column narrower. Try it without the space to see what I mean.

And we wrap it up with a \b to back up over that space so the cursor sits at the exact end of the line in an aesthetically-pleasing way.

Note that line 14 also has a lot of spaces at the end to overwrite the characters that were already there from the countdown.

Finally, we have a weird fflush(stdout) in there, whatever that means. Short answer is that most terminals are line buffered by default, meaning they don’t actually display anything until a newline character is encountered. Since we don’t have a newline (we just have \r), without this line, the program would just sit there until Liftoff! and then print everything all in one instant. fflush() overrides this behavior and forces output to happen right now.

21.1.2.2 The Question Mark Escape

printf("Doesn't it?\n");

printf("Doesn't it\?\n");   // Note \?

printf("Doesn't it??!\n");

foo.c: In function ‘main’:
foo.c:5:23: warning: trigraph ??! converted to | [-Wtrigraphs]
    5 |     printf("Doesn't it??!\n");
      |

I’m sure we’ll revisit this dusty corner of the language later, but the short of it is the compiler looks for certain triplets of characters starting with ?? and it substitutes other characters in their place. So if you’re on some ancient terminal without a pipe symbol (|) on the keyboard, you can type ??! instead.

printf("Doesn't it?\?!\n");

These days, of course, no one ever uses trigraphs. But that whole ??! does sometimes appear if you decide to use it in a string for emphasis.

21.1.3 Numeric Escapes

In addition, there are ways to specify numeric constants or other character values inside strings or character constants.

If you know an octal or hexadecimal representation of a byte, you can include that in a string or character constant.

The following table has example numbers, but any hex or octal numbers may be used. Pad with leading zeros if necessary to read the proper digit count.

Code	Description
`\123`	Embed the byte with octal value `123`, 3 digits exactly.
`\x4D`	Embed the byte with hex value `4D`, 2 digits.
`\u2620`	Embed the Unicode character at code point with hex value `2620`, 4 digits.
`\U0001243F`	Embed the Unicode character at code point with hex value `1243F`, 8 digits.

Here’s an example of the less-commonly used octal notation to represent the letter B in between A and C. Normally this would be used for some kind of special unprintable character, but we have other ways to do that, below, and this is just an octal demo:

printf("A\102C\n");  // 102 is `B` in ASCII/UTF-8

Note there’s no leading zero on the octal number when you include it this way. But it does need to be three characters, so pad with leading zeros if you need to.

But far more common is to use hex constants these days. Here’s a demo that you shouldn’t use, but it demos embedding the UTF-8 bytes 0xE2, 0x80, and 0xA2 in a string, which corresponds to the Unicode “bullet” character (•).

printf("\xE2\x80\xA2 Bullet 1\n");
printf("\xE2\x80\xA2 Bullet 2\n");
printf("\xE2\x80\xA2 Bullet 3\n");

Produces the following output if you’re on a UTF-8 console (or probably garbage if you’re not):

• Bullet 1
• Bullet 2
• Bullet 3

But that’s a crummy way to do Unicode. You can use the escapes \u (16-bit) or \U (32-bit) to just refer to Unicode by code point number. The bullet is 2022 (hex) in Unicode, so you can do this and get more portable results:

printf("\u2022 Bullet 1\n");
printf("\u2022 Bullet 2\n");
printf("\u2022 Bullet 3\n");

Be sure to pad \u with enough leading zeros to get to four characters, and \U with enough to get to eight.

printf("\U00002022 Bullet 1\n");

22 Enumerated Types: enum

enum {
  ONE=1,
  TWO=2
};

printf("%d %d", ONE, TWO);  // 1 2

In some ways, it can be better—or different—than using a #define. Key differences:

Since they’re integer types, they can be used any place integers can be used, including in array dimensions and case statements.

22.1 Behavior of enum

22.1.1 Numbering

enum {
    SHEEP,  // Value is 0
    WHEAT,  // Value is 1
    WOOD,   // Value is 2
    BRICK,  // Value is 3
    ORE     // Value is 4
};

printf("%d %d\n", SHEEP, BRICK);  // 0 3

enum {
  X=2,
  Y=18,
  Z=-2
};

enum {
  X=2,
  Y=2,
  Z=2
};

if values are omitted, numbering continues counting in the positive direction from whichever value was last specified. For example:

enum {
  A,    // 0, default starting value
  B,    // 1
  C=4,  // 4, manually set
  D,    // 5
  E,    // 6
  F=3   // 3, manually set
  G,    // 4
  H     // 5
}

22.1.2 Trailing Commas

enum {
  X=2,
  Y=18,
  Z=-2,   // <-- Trailing comma
};

It’s gotten more popular in languages of the recent decades so you might be pleased to see it.

22.1.3 Scope

enums scope as you’d expect. If at file scope, the whole file can see it. If in a block, it’s local to that block.

It’s really common for enums to be defined in header files so they can be #included at file scope.

22.1.4 Style

As you’ve noticed, it’s common to declare the enum symbols in uppercase (with underscores).

22.2 Your enum is a Type

This is an important thing to know about enum: they’re a type, analogous to how a struct is a type.

You can give them a tag name so you can refer to the type later and declare variables of that type.

In C, the best reason for this is code clarity–it’s a nice, typed way to describe your thinking in code. C (unlike C++) doesn’t actually enforce any values being in range for a particular enum.

Let’s do an example where we declare a variable r of type enum resource that can hold those values:

// Named enum, type is "enum resource"

enum resource {
    SHEEP,
    WHEAT,
    WOOD,
    BRICK,
    ORE
};

// Declare a variable "r" of type "enum resource"

enum resource r = BRICK;

if (r == BRICK) {
    printf("I'll trade you a brick for two sheep.\n");
}

typedef enum {
    SHEEP,
    WHEAT,
    WOOD,
    BRICK,
    ORE
} RESOURCE;

RESOURCE r = BRICK;

Another shortcut that’s legal but rare is to declare variables when you declare the enum:

// Declare an enum and some initialized variables of that type:

enum {
    SHEEP,
    WHEAT,
    WOOD,
    BRICK,
    ORE
} r = BRICK, s = WOOD;

You can also give the enum a name so you can use it later, which is probably what you want to do in most cases:

// Declare an enum and some initialized variables of that type:

enum resource {   // <-- type is now "enum resource"
    SHEEP,
    WHEAT,
    WOOD,
    BRICK,
    ORE
} r = BRICK, s = WOOD;

23 Pointers III: Pointers to Pointers and More

Here’s where we cover some intermediate and advanced pointer usage. If you don’t have pointers down well, review the previous chapters on pointers and pointer arithmetic before starting on this stuff.

23.1 Pointers to Pointers

If you can have a pointer to a variable, and a variable can be a pointer, can you have a pointer to a variable that it itself a pointer?

Yes! This is a pointer to a pointer, and it’s held in variable of type pointer-pointer.

Before we tear into that, I want to try for a gut feel for how pointers to pointers work.

Remember that a pointer is just a number. It’s a number that represents an index in computer memory, typically one that holds a value we’re interested in for some reason.

That pointer, which is a number, has to be stored somewhere. And that place is memory, just like everything else¹⁴⁰.

But because it’s stored in memory, it must have an index it’s stored at, right? The pointer must have an index in memory where it is stored. And that index is a number. It’s the address of the pointer. It’s a pointer to the pointer.

Let’s start with a regular pointer to an int, back from the earlier chapters:

#include <stdio.h>

int main(void)
{
    int x = 3490;  // Type: int
    int *p = &x;   // Type: pointer to an int

    printf("%d\n", *p);  // 3490
}

Straightforward enough, right? We have two types represented: int and int*, and we set up p to point to x. Then we can dereference p on line 8 and print out the value 3490.

But, like we said, we can have a pointer to any variable… so does that mean we can have a pointer to p?

int x = 3490;  // Type: int
int *p = &x;   // Type: pointer to an int

&p  // <-- What type is the address of p? AKA a pointer to p?

If x is an int, then &x is a pointer to an int that we’ve stored in p which is type int*. Follow? (Repeat this paragraph until you do!)

And therefore &p is a pointer to an int*, AKA a “pointer to a pointer to an int”. AKA “int-pointer-pointer”.

#include <stdio.h>

int main(void)
{
    int x = 3490;  // Type: int
    int *p = &x;   // Type: pointer to an int
    int **q = &p;  // Type: pointer to pointer to int

    printf("%d %d\n", *p, **q);  // 3490 3490
}

Let’s make up some pretend addresses for the above values as examples and see what these three variables might look like in memory. The address values, below are just made up by me for example purposes:

Variable	Stored at Address	Value Stored There
`x`	`28350`	`3490`—the value from the code
`p`	`29122`	`28350`—the address of `x`!
`q`	`30840`	`29122`—the address of `p`!

Indeed, let’s try it for real on my computer¹⁴¹ and print out the pointer values with %p and I’ll do the same table again with actual references (printed in hex).

You can see those addresses are the same except the last byte, so just focus on those.

Variable	Stored at Address	Value Stored There
`x`	`0x7ffd96a07b94`	`3490`—the value from the code
`p`	`0x7ffd96a07b98`	`0x7ffd96a07b94`—the address of `x`!
`q`	`0x7ffd96a07ba0`	`0x7ffd96a07b98`—the address of `p`!

On my system, ints are 4 bytes, which is why we’re seeing the address go up by 4 from x to p¹⁴² and then goes up by 8 from p to q. On my system, all pointers are 8 bytes.

Does it matter if it’s an int* or an int**? Is one more bytes than the other? Nope! Remember that all pointers are addresses, that is indexes into memory. And on my machine you can represent an index with 8 bytes… doesn’t matter what’s stored at that index.

Now check out what we did there on line 9 of the previous example: we double dereferenced q to get back to our 3490.

So you can think of & as being used to make pointers, and * being the inverse—it goes the opposite direction of &—to get to the thing pointed to.

In terms of type, each time you &, that adds another pointer level to the type.

If you have	Then you run	The result type is
`int x`	`&x`	`int *`
`int *x`	`&x`	`int **`
`int **x`	`&x`	`int ***`
`int ***x`	`&x`	`int ****`

If you have	Then you run	The result type is
`int ****x`	`*x`	`int ***`
`int ***x`	`*x`	`int **`
`int **x`	`*x`	`int *`
`int *x`	`*x`	`int`

Note that you can use multiple *s in a row to quickly dereference, just like we saw in the example code with **q, above. Each one strips away one level of indirection.

If you have	Then you run	The result type is
`int ****x`	`***x`	`int *`
`int ***x`	`**x`	`int *`
`int **x`	`**x`	`int`

But & doesn’t work the same way—you can only do those one at a time, and have to store the result in an intermediate variable:

int x = 3490;     // Type: int
int *p = &x;      // Type: int *, pointer to an int
int **q = &p;     // Type: int **, pointer to pointer to int
int ***r = &q;    // Type: int ***, pointer to pointer to pointer to int
int ****s = &r;   // Type: int ****, you get the idea
int *****t = &s;  // Type: int *****

23.1.1 Pointer Pointers and const

means that you can’t modify p. Trying to p++ would give you a compile-time error.

But how does that work with int ** or int ***? Where does the const go, and what does it mean?

Let’s start with the simple bit. The const right next to the variable name refers to that variable. So if you want an int*** that you can’t change, you can do this:

int ***const p;

p++;  // Not allowed

int main(void)
{
    int x = 3490;
    int *const p = &x;
    int **q = &p;
}

warning: initialization discards ‘const’ qualifier from pointer target type
    7 |     int **q = &p;
      |               ^

What’s going on? The compiler is telling us here that we had a variable that was const, and we’re assigning its value into another variable that is not const in the same way. The “constness” is discarded, which probably isn’t what we wanted to do.

The type of p is int *const p, and so &p is type int *const *. And we try to assign that into q.

But q is int **! A type with different constness on the first *! So we get a warning that the const in p’s int *const * is being ignored and thrown away.

int x = 3490;
int *const p = &x;
int *const *q = &p;

We could make q even more const. As it is, above, we’re saying, “q isn’t itself const, but the thing it points to is const.” But we could make them both const:

int x = 3490;
int *const p = &x;
int *const *const q = &p;  // More const!

23.2 Multibyte Values

We kinda hinted at this in a variety of places earlier, but clearly not every value can be stored in a single byte of memory. Things take up multiple bytes of memory (assuming they’re not chars). You can tell how many bytes by using sizeof. And you can tell which address in memory is the first byte of the object by using the standard & operator, which always returns the address of the first byte.

And here’s another fun fact! If you iterate over the bytes of any object, you get its object representation. Two things with the same object representation in memory are equal.

If you want to iterate over the object representation, you should do it with pointers to unsigned char.

void *my_memcpy(void *dest, const void *src, size_t n)
{
    // Make local variables for src and dest, but of type unsigned char

    const unsigned char *s = src;
    unsigned char *d = dest;

    while (n-- > 0)   // For the given number of bytes
        *d++ = *s++;  // Copy source byte to dest byte

    // Most copy functions return a pointer to the dest as a convenience
    // to the caller

    return dest;
}

(There are some good examples of post-increment and post-decrement in there for you to study, as well.)

It’s important to note that the version, above, is probably less efficient than the one that comes with your system.

But you can pass pointers to anything into it, and it’ll copy those objects. Could be int*, struct animal*, or anything.

Let’s do another example that prints out the object representation bytes of a struct so we can see if there’s any padding in there and what values it has¹⁴⁵.

#include <stdio.h>

struct foo {
    char a;
    int b;
};

int main(void)
{
    struct foo x = {0x12, 0x12345678};
    unsigned char *p = (unsigned char *)&x;

    for (size_t i = 0; i < sizeof x; i++) {
        printf("%02X\n", p[i]);
    }
}

What we have there is a struct foo that’s built in such a way that should encourage a compiler to inject padding bytes (though it doesn’t have to). And then we get an unsigned char * to the first byte of the struct foo variable x.

From there, all we need to know is the sizeof x and we can loop through that many bytes, printing out the values (in hex for ease).

Running this gives a bunch of numbers as output. I’ve annotated it below to identify where the values were stored:

12  | x.a == 0x12

AB  |
BF  | padding bytes with "random" value
26  |

78  |
56  | x.b == 0x12345678
34  |
12  |

On all systems, sizeof(char) is 1, and we see that first byte at the top of the output holding the value 0x12 that we stored there.

Finally, on my system, sizeof(int) is 4, and we can see those 4 bytes at the end. Notice how they’re the same bytes as are in the hex value 0x12345678, but strangely in reverse order¹⁴⁶.

So that’s a little peek under the hood at the bytes of a more complex entity in memory.

23.3 The NULL Pointer and Zero

Personally, I always use NULL when I mean NULL, but you might see some other variants from time to time. Though '\0' (a byte with all bits set to zero) will also compare equal, it’s weird to compare it to a pointer; you should compare NULL against the pointer. (Of course, lots of times in string processing, you’re comparing the thing the pointer points to to '\0', and that’s right.)

0 is called the null pointer constant, and, when compared to or assigned into another pointer, it is converted to a null pointer of the same type.

23.4 Pointers as Integers

You can cast pointers to integers and vice-versa (since a pointer is just an index into memory), but you probably only ever need to do this if you’re doing some low-level hardware stuff. The results of such machinations are implementation-defined, so they aren’t portable. And weird things could happen.

C does make one guarantee, though: you can convert a pointer to a uintptr_t type and you’ll be able to convert it back to a pointer without losing any data.

Additionally, if you feel like being signed, you can use intptr_t to the same effect.

23.5 Casting Pointers to other Pointers

If you cast to a pointer of another type and then access the object it points to, the behavior is undefined due to something called strict aliasing.

Plain old aliasing refers to the ability to have more than one way to access the same object. The access points are aliases for each other.

Strict aliasing says you are only allowed to access an object via pointers to compatible types to that object.

int a = 1;
int *p = &a;

p is a pointer to an int, and it points to a compatible type—namely int—so we’re golden.

But the following isn’t good because int and float are not compatible types:

int a = 1;
float *p = (float *)&a;

Here’s a demo program that does some aliasing. It takes a variable v of type int32_t and aliases it to a pointer to a struct words. That struct has two int16_ts in it. These types are incompatible, so we’re in violation of strict aliasing rules. The compiler will assume that these two pointers never point to the same object… but we’re making it so they do. Which is naughty of us.

#include <stdio.h>
#include <stdint.h>

struct words {
    int16_t v[2];
};

void fun(int32_t *pv, struct words *pw)
{
    for (int i = 0; i < 5; i++) {
        (*pv)++;

        // Print the 32-bit value and the 16-bit values:

        printf("%x, %x-%x\n", *pv, pw->v[1], pw->v[0]);
    }
}

int main(void)
{
    int32_t v = 0x12345678;

    struct words *pw = (struct words *)&v;  // Violates strict aliasing

    fun(&v, pw);
}

See how I pass in the two incompatible pointers to fun()? One of the types is int32_t* and the other is struct words*.

But they both point to the same object: the 32-bit value initialized to 0x12345678.

So if we look at the fields in the struct words, we should see the two 16-bit halves of that number. Right?

And in the fun() loop, we increment the pointer to the int32_t. That’s it. But since the struct points to that same memory, it, too, should be updated to the same value.

So let’s run it and get this, with the 32-bit value on the left and the two 16-bit portions on the right. It should match¹⁴⁸:

12345679, 1234-5679
1234567a, 1234-567a
1234567b, 1234-567b
1234567c, 1234-567c
1234567d, 1234-567d

12345679, 1234-5678
1234567a, 1234-5679
1234567b, 1234-567a
1234567c, 1234-567b
1234567d, 1234-567c

They’re off by one! But they point to the same memory! How could this be? Answer: it’s undefined behavior to alias memory like that. Anything is possible, except not in a good way.

If your code violates strict aliasing rules, whether it works or not depends on how someone decides to compile it. And that’s a bummer since that’s beyond your control. Unless you’re some kind of omnipotent deity.

GCC can be forced to not use the strict aliasing rules with -fno-strict-aliasing. Compiling the demo program, above, with -O3 and this flag causes the output to be as expected.

Lastly, type punning is using pointers of different types to look at the same data. Before strict aliasing, this kind of things was fairly common:

int a = 0x12345678;
short b = *((short *)&a);   // Violates strict aliasing

If you want to do type punning (relatively) safely, see the section on Unions and Type Punning.

23.6 Pointer Differences

As you know from the section on pointer arithmetic, you can subtract one pointer from another¹⁴⁹ to get the difference between them in count of array elements.

Now the type of that difference is something that’s up to the implementation, so it could vary from system to system.

To be more portable, you can store the result in a variable of type ptrdiff_t defined in <stddef.h>.

int cats[100];

int *f = cats + 20;
int *g = cats + 60;

ptrdiff_t d = g - f;  // difference is 40

printf("%td\n", d);  // Print decimal: 40
printf("%tX\n", d);  // Print hex:     28

23.7 Pointers to Functions

Functions are just collections of machine instructions in memory, so there’s no reason we can’t get a pointer to the first instruction of the function.

This can be useful for passing a pointer to a function into another function as an argument. Then the second one could call whatever was passed in.

The tricky part with these, though, is that C needs to know the type of the variable that is the pointer to the function.

Like “this is a pointer to a function that takes two int arguments and returns void”.

Well, it turns out it looks very much like a function prototype, except with some extra parentheses:

// Declare p to be a pointer to a function.
// This function returns a float, and takes two ints as arguments.

float (*p)(int, int);

Also notice that you don’t have to give the parameters names. But you can if you want; they’re just ignored.

// Declare p to be a pointer to a function.
// This function returns a float, and takes two ints as arguments.

float (*p)(int a, int b);

So now that we know how to declare a variable, how do we know what to assign into it? How do we get the address of a function?

Turns out there’s a shortcut just like with getting a pointer to an array: you can just refer to the bare function name without parens. (You can put an & in front of this if you like, but it’s unnecessary and not idiomatic.)

Once you have a pointer to a function, you can call it just by adding parens and an argument list.

Let’s do a simple example where I effectively make an alias for a function by setting a pointer to it. Then we’ll call it.

#include <stdio.h>

void print_int(int n)
{
    printf("%d\n", n);
}

int main(void)
{
    // Assign p to point to print_int:

    void (*p)(int) = print_int;

    p(3490);          // Call print_int via the pointer
}

Notice how the type of p represents the return value and parameter types of print_int. It has to, or else C will complain about incompatible pointer types.

One more example here shows how we might pass a pointer to a function as an argument to another function.

We’ll write a function that takes a couple integer arguments, plus a pointer to a function that operates on those two arguments. Then it prints the result.

#include <stdio.h>

int add(int a, int b)
{
    return a + b;
}

int mult(int a, int b)
{
    return a * b;
}

void print_math(int (*op)(int, int), int x, int y)
{
    int result = op(x, y);

    printf("%d\n", result);
}

int

1 Foreword

1.1 Audience

1.2 How to Read This Book

1.3 Platform and Compiler

1.4 Official Homepage

1.5 Email Policy

1.6 Mirroring

1.7 Note for Translators

1.8 Copyright and Distribution

1.9 Dedication

2 Hello, World!

2.1 What to Expect from C

2.2 Hello, World!

2.3 Compilation Details

2.4 Building with gcc

2.5 Building with clang

2.6 Building from IDEs

2.7 C Versions

3 Variables and Statements

3.1 Variables

3.1.1 Variable Names

3.1.2 Variable Types

3.1.3 Boolean Types

3.2 Operators and Expressions

3.2.1 Arithmetic

3.2.2 Ternary Operator

3.2.3 Pre-and-Post Increment-and-Decrement

3.2.4 The Comma Operator

3.2.5 Conditional Operators

3.2.6 Boolean Operators

3.2.7 The sizeof Operator

3.3 Flow Control

3.3.1 The if-else statement

3.3.2 The while statement

3.3.3 The do-while statement

3.3.4 The for statement

3.3.5 The switch Statement

4 Functions

4.1 Passing by Value

4.2 Function Prototypes

4.3 Empty Parameter Lists

5 Pointers—Cower In Fear!

5.1 Memory and Variables

5.2 Pointer Types

5.3 Dereferencing

5.4 Passing Pointers as Arguments

5.5 The NULL Pointer

5.6 A Note on Declaring Pointers

5.7 sizeof and Pointers

6 Arrays

6.1 Easy Example

6.2 Getting the Length of an Array

6.3 Array Initializers

6.4 Out of Bounds!

6.5 Multidimensional Arrays

6.6 Arrays and Pointers

6.6.1 Getting a Pointer to an Array

6.6.2 Passing Single Dimensional Arrays to Functions

6.6.3 Changing Arrays in Functions

6.6.4 Passing Multidimensional Arrays to Functions

7 Strings

7.1 String Literals

7.2 String Variables

7.3 String Variables as Arrays

7.4 String Initializers

7.5 Getting String Length

7.6 String Termination

7.7 Copying a String

8 Structs

8.1 Declaring a Struct

8.2 Struct Initializers

8.3 Passing Structs to Functions

8.4 The Arrow Operator

8.5 Copying and Returning structs

8.6 Comparing structs

9 File Input/Output

9.1 The FILE* Data Type

9.2 Reading Text Files

9.3 End of File: EOF

9.3.1 Reading a Line at a Time

2.4 Building with `gcc`

2.5 Building with `clang`

3.2.7 The `sizeof` Operator

3.3.1 The `if`-`else` statement

3.3.2 The `while` statement

3.3.3 The `do-while` statement

3.3.4 The `for` statement

3.3.5 The `switch` Statement

5.5 The `NULL` Pointer

5.7 `sizeof` and Pointers

8.5 Copying and Returning `struct`s

8.6 Comparing `struct`s

9.1 The `FILE*` Data Type

9.3 End of File: `EOF`

9.6.1 `struct` and Number Caveats

10 `typedef`: Making New Types

10.1 `typedef` in Theory

10.2 `typedef` in Practice

10.2.1 `typedef` and `struct`s

10.2.2 `typedef` and Other Types

10.2.3 `typedef` and Pointers

10.2.4 `typedef` and Capitalization

10.3 Arrays and `typedef`

11.3 `void` Pointers

12.1 Allocating and Deallocating, `malloc()` and `free()`

12.4 An Alternative: `calloc()`

12.5 Changing Allocated Size with `realloc()`

12.5.2 `realloc()` with `NULL`

13.3 `for`-loop Scope

14.3 More Integer Types: `short`, `long`, `long long`

14.4 More Float: `double` and `long double`

15.2 `char` Conversions

15.4.3 `void*`

16.1.1 `const`