I dont want to preprocess specific macros.It should not replace it with its value.Is it possible?

Question

Example =

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

After preprocrssing

Output-

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

Answer 1

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

Answer 2

No. What's your real goal? If you want to do, it's better to make some script to do that.

Answer 3

under unix you can play with the unifdef command

Answer 4

If you don't want to preprocess a macro, just don't use it:

#define a 100
//#define b 200

int main()
{
    int c = a + 200;

    return 0;
}

Answer 5

The purpose of preprocessing is to process the preprocessing directives. I don't know of any way to keep a specific preprocessing directive, especially because the next step, compilation, doesn't know anything about these.

This is another reason why you should be careful when using macros -- when the name is found in the source, then it is substituted, no matter what:

#define a 100

main()
{
    int a; // Compilation error happens here
    ....
}

Answer 6

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.

Answer 7

Regarding Peter Miehle's answer, unifdef can be used to partially process #if and #ifdef directives, but it cannot be used to selectively expand macros. See http://dotat.at/prog/unifdef