Include Guard Pattern

Include guards define a value once even if guarded text is seen again.

include_guard_pattern.c

#include <stdio.h>

#ifndef CONFIG_LIMIT
#define CONFIG_LIMIT 6
#endif

int main(void) {
    int used = ;
    int limit = 6;
    int remaining = limit - used;

    printf("remaining=%d\n", remaining);
    return 0;
}

#include <stdio.h>

#ifndef CONFIG_LIMIT
#define CONFIG_LIMIT 6
#endif

int main(void) {
    int used = ;
    int limit = 6;
    int remaining = limit - used;

    printf("remaining=%d\n", remaining);
    return 0;
}

#include <stdio.h>

#ifndef CONFIG_LIMIT
#define CONFIG_LIMIT 6
#endif

int main(void) {
    int used = ;
    int limit = 6;
    int remaining = limit - used;

    printf("remaining=%d\n", remaining);
    return 0;
}

guard macro `#ifndef` checks whether a guard name has already been defined.

one definition The guarded block supplies one default value for the rest of the source.

expanded value The executable trace uses the guarded value after preprocessing.