Conditional Flags

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

conditional_flags.c

#include <stdio.h>

#define USE_BONUS 1

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

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

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.