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Editorial Team
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Asked: 2026-05-17T16:20:34+00:00

Example = #define a 100 #define b 200 main() { int c=a+b; } After

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Example = 

#define a 100
#define b 200
main()
{
  int c=a+b;
}

After preprocessing

Output-

#define a 100
main()
{
 int c=a+200;
}
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1 Answer

  1. Editorial Team

    You could try refactoring the macros to allow external configuration, i.e.

    /* config.h */

/* set defaults for a and b */
#ifndef a
#define a 100
#endif

#ifndef b
#define b 200 
#endif

    and

    /* main.c */

#include "config.h"

int main(void) 
{
    int c = a + b;
}

    Then you can set the macros externally when building, for instance like this: 

    $ gcc -E -Da=a main.c
# 1 "main.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "main.c"


# 1 "config.h" 1
# 4 "main.c" 2

int main(void)
{
    int c = a + 200;
}
$ gcc -E -Db=b main.c
# 1 "main.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "main.c"


# 1 "config.h" 1
# 4 "main.c" 2

int main(void)
{
    int c = 100 + b;
}

    Now, of course you don’t have to use a separate configuration header, but I’d recommend it from a maintenance perspective, it will make it easier to keep track of available configuration settings and their defaults.

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