How to rename a C preprocessor macro?

Question

Consider a (read-only third-party) header lib.h with:

#define XYZ 42

In a source file, I want to use the word XYZ for an unrelated purpose, and do not want the substitution with 42. But, in the same source file, for other purposes, I also do want to access the value 42 from lib.h without hardcoding it. How do I rename the macro from XYZ to, say, LIB_XYZ?

The following does not work, because preprocessor wants XYZ at the time the LIB_XYZ substitution is made, but XYZ had been undefined:

#include "lib.h"
#define LIB_XYZ XYZ
#undef XYZ

Is there a way to trick the preprocessor into expanding LIB_XYZ to its final value before XYZ is lost?

Answer 1

Not with the pre-processor, at least, not that I am aware of.

However, for simple constants with known type like in your example, there is a workaround.

#include <stdio.h>

// <xyz.h>

#define XYZ 42

// </xyz.h>

enum xyz_constants
{
  LIB_XYZ = XYZ,
};

#undef XYZ

#define XYZ 27

int
main()
{
  printf("old value: %d, new value: %d\n", LIB_XYZ, XYZ);
  return 0;
}

Not showing the fluff from stdio.h, this code is pre-processed to the following.

enum xyz_constants
{
  LIB_XYZ = 42,
};

int
main()
{
  printf("old value: %d, new value: %d\n", LIB_XYZ, 27);
  return 0;
}

You can extend this to some degree to other data types and certain function-like macros but there are of course limits.

Anyway, why do you need the particular identifier XYZ? Can't you use a different name for your macro?