21 <stddef.h> A Few Standard Definitions

Despite its name, I’ve haven’t seen this frequently included.

It includes several types and macros.

21.1 ptrdiff_t

This holds the different between two pointers. You could store this in another type, but the result of a pointer subtraction is an implementation-defined type; you can be maximally portable by using ptrdiff_t.

#include <stdio.h>
#include <stddef.h>

int main(void)
{
    int cats[100];

    int *f = cats + 20;
    int *g = cats + 60;

    ptrdiff_t d = g - f;  // difference is 40

And you can print it by prefixing the integer format specifier with t:

    printf("%td\n", d);  // Print decimal: 40
    printf("%tX\n", d);  // Print hex:     28
}

21.2 size_t

This is the type returned by sizeof and used in a few other places. It’s an unsigned integer.

You can print it using the z prefix in printf():

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

int main(void)
{
    size_t x;

    x = sizeof(int);

    printf("%zu\n", x);

Some functions return negative numbers cast to size_t as error values (such as mbrtoc16()). If you want to print these as negative values, you can do it with %zd:

    char16_t a;
    mbstate_t mbs;
    memset(&mbs, 0, sizeof mbs);

    x = mbrtoc16(&a, "b", 8, &mbs);

    printf("%zd\n", x);
}

21.3 max_align_t

As far as I can tell, this exists to allow the runtime computation of the maximum fundamental alignment⁴⁵ on the current platform. Someone please mail me if there’s another use.

Maybe you need this if you’re writing your own memory allocator or somesuch.

#include <stddef.h>
#include <stdio.h>      // For printf()
#include <stdalign.h>   // For alignof

int main(void)
{
    int max = alignof(max_align_t);

    printf("Maximum fundamental alignment: %d\n", max);
}

On my system, this prints:

Maximum fundamental alignment: 16

See also alignas, alignof.

21.4 wchar_t

This is analogous to char, except it’s for wide characters.

It’s an integer type that has enough range to hold unique values for all characters in all supported locales.

The value 0 is the wide NUL character.

Finally, the values of character constants from the basic character set will be the same as their corresponding wchar_t values… unless __STDC_MB_MIGHT_NEQ_WC__ is defined.

21.5 offsetof

If you have a struct or union, you can use this to get the byte offset of fields within that type.

Usage is:

offsetof(type, fieldname);

The resulting value has type size_t.

Here’s an example that prints the field offsets of a struct:

#include <stdio.h>
#include <stddef.h>

struct foo {
    int a;
    char b;
    char c;
    float d;
};

int main(void)
{
    printf("a: %zu\n", offsetof(struct foo, a));
    printf("b: %zu\n", offsetof(struct foo, b));
    printf("c: %zu\n", offsetof(struct foo, c));
    printf("d: %zu\n", offsetof(struct foo, d));
}

On my system, this outputs:

a: 0
b: 4
c: 5
d: 8

And you can’t use offsetof on a bitfield, so don’t get your hopes up.