Using true and false in C

Just use 0 or 1 directly in the code.

For C programmers, this is as intuitive as true or false.

I usually do a:

typedef enum {FALSE = 0, TRUE} boolean;

Just include <stdbool.h> if your system provides it. That defines a number of macros, including bool, false, and true (defined to _Bool, 0, and 1 respectively). See section 7.16 of C99 for more details.