Use of undeclared identifier 'true'

Question

Why am I getting this error:

infinite.c:5:12: error: use of undeclared identifier 'true'
    while (true) {

1 error generated.
make: *** [infinite] Error 1

... when I try to compile this simple code for an infinite loop?

#include <stdio.h>

int main(void) {
    int x = 0;
    while (true) {
        printf("%i\n", x);
    }
}

Answer 1

The identifier true is not declared by default. To use it, two solutions :

1. Compile in C99 and include <stdbool.h>.
2. Define this identifier by yourself.

However, the infinite loop for (;;) is often considered as better style.

Answer 2

C has no built-in boolean types. So it doesn't know what true is. You have to declare it on your own in this way:

#define TRUE 1
#define FALSE 0

[...]
while (TRUE) {
     [...]
}

Answer 3

Include stdbool.h to use C99 booleans.
If you want to stick with C89 define it yourself:

typedef enum
{
    true=1, false=0
}bool;