Using the system header file stdbool.h allows you to use bool as a Boolean data type. true evaluates to 1 and false evaluates to 0.

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

int main(void) {
    bool x = true;  /* equivalent to bool x = 1; */
    bool y = false; /* equivalent to bool y = 0; */
    if (x)  /* Functionally equivalent to if (x != 0) or if (x != false) */
    {
        puts("This will print!");
    }
    if (!y) /* Functionally equivalent to if (y == 0) or if (y == false) */
    {
        puts("This will also print!");
    }
}

bool is just a nice spelling for the data type _Bool (opens new window). It has special rules when numbers or pointers are converted to it.

C of all versions, will effectively treat any integer value other than 0 as true for comparison operators and the integer value 0 as false. If you don't have _Bool or bool as of C99 available, you could simulate a Boolean data type in C using #define macros, and you might still find such things in legacy code.

#include <stdio.h>

#define bool int
#define true 1
#define false 0

int main(void) {
    bool x = true;  /* Equivalent to int x = 1; */
    bool y = false; /* Equivalent to int y = 0; */
    if (x) /* Functionally equivalent to if (x != 0) or if (x != false) */
    {
        puts("This will print!");
    }
    if (!y) /* Functionally equivalent to if (y == 0) or if (y == false) */
    {
        puts("This will also print!");
    }
}

Don't introduce this in new code since the definition of these macros might clash with modern uses of <stdbool.h>.

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 (opens new window). 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> (opens new window).

All integers or pointers can be used in an expression that is interpreted as "truth value".

int main(int argc, char* argv[]) {
  if (argc % 4) {
    puts("arguments number is not divisible by 4");
  } else {
    puts("argument number is divisible by 4");
  }
...

The expression argc % 4 is evaluated and leads to one of the values 0, 1, 2 or 3. The first, 0 is the only value that is "false" and brings execution into the else part. All other values are "true" and go into the if part.

double* A = malloc(n*sizeof *A);
if (!A) {
   perror("allocation problems");
   exit(EXIT_FAILURE);
}

Here the pointer A is evaluated and if it is a null pointer, an error is detected and the program exits.

Many people prefer to write something as A == NULL, instead, but if you have such pointer comparisons as part of other complicated expressions, things become quickly difficult to read.

char const* s = ....;   /* some pointer that we receive */
if (s != NULL && s[0] != '\0' && isalpha(s[0])) {
   printf("this starts well, %c is alphabetic\n", s[0]);
}

For this to check, you'd have to scan a complicated code in the expression and be sure about operator preference.

char const* s = ....;   /* some pointer that we receive */
if (s && s[0] && isalpha(s[0])) {
   printf("this starts well, %c is alphabetic\n", s[0]);
}

is relatively easy to capture: if the pointer is valid we check if the first character is non-zero and then check if it is a letter.

Considering that most debuggers are not aware of #define macros, but can check enum constants, it may be desirable to do something like this:

#if __STDC_VERSION__ < 199900L
typedef enum { false, true } bool;
/* Modern C code might expect these to be macros. */
# ifndef bool
#  define bool bool
# endif
# ifndef true
#  define true true
# endif
# ifndef false
#  define false false
# endif
#else
# include <stdbool.h>
#endif

/* Somewhere later in the code ... */
bool b = true;

This allows compilers for historic versions of C to function, but remains forward compatible if the code is compiled with a modern C compiler.

For more information on typedef, see Typedef (opens new window), for more on enum see Enumerations (opens new window)

Remarks

To use the predefined type _Bool and the header <stdbool.h>, you must be using the C99/C11 versions of C.

To avoid compiler warnings and possibly errors, you should only use the typedef/define example if you're using C89 and previous versions of the language.