Added in the C standard version C99, _Bool is also a native C data type. It is capable of holding the values 0 (for false) and 1 (for true).

#include <stdio.h>

int main(void) {
    _Bool x = 1; 
    _Bool y = 0;
    if(x) /* Equivalent to if (x == 1) */
    {
        puts("This will print!");
    }
    if (!y) /* Equivalent to if (y == 0) */
    {
        puts("This will also print!");
    }
}

_Bool is an integer type but has special rules for conversions from other types. The result is analogous to the usage of other types in if expressions. In the following

_Bool z = X;

• If X has an arithmetic type (is any kind of number), z becomes 0 if X == 0. Otherwise z becomes 1.
• If X has a pointer type, z becomes 0 if X is a null pointer and 1 otherwise.

To use nicer spellings bool, false and true you need to use <stdbool.h>.