Conditional Flags

Conditional compilation includes one branch of source before the compiler runs.

compile-time branch `#if` chooses source text at build time, not while the program is running.

runtime selector The selector changes data that flows through the compiled branch.

                            base
                            base = 10 (default)base = 5base = 12
                        

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

                            #include <stdio.h>

#define USE_BONUS 1

int main(void) {
    int base = 10;
#if USE_BONUS
    int score = base + 3;
#else
    int score = base;
#endif

    printf("score=%d\n", score);
    return 0;
}
#include <stdio.h>

#define USE_BONUS 1

int main(void) {
    int base = 5;
#if USE_BONUS
    int score = base + 3;
#else
    int score = base;
#endif

    printf("score=%d\n", score);
    return 0;
}
#include <stdio.h>

#define USE_BONUS 1

int main(void) {
    int base = 12;
#if USE_BONUS
    int score = base + 3;
#else
    int score = base;
#endif

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

                        
1base ← 10, score ← 13⋯5int main(void) {6    int base→ 10 = 10; //@base=5, 127#if USE_BONUS8    int score→ 13 = base10 + 3;9#else10    int score = base;11#endif1213    printf("score=%d\n", score13);14    return 0;15}⋯outputscore=13
1base ← 5, score ← 8⋯5int main(void) {6    int base→ 5 = 5;7#if USE_BONUS8    int score→ 8 = base5 + 3;9#else10    int score = base;11#endif1213    printf("score=%d\n", score8);14    return 0;15}⋯outputscore=8
1base ← 12, score ← 15⋯5int main(void) {6    int base→ 12 = 12;7#if USE_BONUS8    int score→ 15 = base12 + 3;9#else10    int score = base;11#endif1213    printf("score=%d\n", score15);14    return 0;15}⋯outputscore=15