Include Guard Pattern

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

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.

                            used
                            used = 4 (default)used = 2used = 8
                        

                            include_guard_pattern.c
                        
Replay: real traced execution (multi-file project)

                            #include <stdio.h>

#ifndef CONFIG_LIMIT
#define CONFIG_LIMIT 6
#endif

int main(void) {
    int used = 4;
    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 = 2;
    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 = 8;
    int limit = 6;
    int remaining = limit - used;

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

                        
1used ← 4, limit ← 6, remaining ← 2⋯7int main(void) {8    int used→ 4 = 4; //@used=2, 89    int limit→ 6 = 6;10    int remaining→ 2 = limit6 - used4;1112    printf("remaining=%d\n", remaining2);13    return 0;14}⋯outputremaining=2
1used ← 2, limit ← 6, remaining ← 4⋯7int main(void) {8    int used→ 2 = 2;9    int limit→ 6 = 6;10    int remaining→ 4 = limit6 - used2;1112    printf("remaining=%d\n", remaining4);13    return 0;14}⋯outputremaining=4
1used ← 8, limit ← 6, remaining ← -2⋯7int main(void) {8    int used→ 8 = 8;9    int limit→ 6 = 6;10    int remaining→ -2 = limit6 - used8;1112    printf("remaining=%d\n", remaining-2);13    return 0;14}⋯outputremaining=-2