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20 <stdbool.h> Boolean Types

This is a small header file that defines a number of convenient Boolean macros. If you really need that kind of thing.

Macro Description
bool Type for Boolean, expands to _Bool
true True value, expands to 1
false False value, expands to 0

There’s one more macro that I’m not putting in the table because it’s such a long name it’ll blow up the table alignment:

__bool_true_false_are_defined

which expands to 1.

20.1 Example

Here’s a lame example that shows off these macros.

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

int main(void)
{
    bool x;

    x = (3 > 2);

    if (x == true)
        printf("The universe still makes sense.\n");

    x = false;

    printf("x is now %d\n", x);  // 0
}

Output:

The universe still makes sense.
x is now 0

20.2 _Bool?

What’s the deal with _Bool? Why didn’t they just make it bool?

Well, there was a lot of C code out there where people had defined their own bool type and adding an official bool would have broken those typedefs.

But C has already reserved all identifiers that start with an underscore followed by a capital letter, so it was clear to make up a new _Bool type and go with that.

And, if you know your code can handle it, you can include this header to get all this juicy syntax.

One more note on conversions: unlike converting to int, the only thing that converts to false in a _Bool is a scalar zero value. Anything at all that’s not zero, like -3490, 0.12, or NaN, converts to true.

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