C preprocessor: include based on define

Question

How can I include a file or another, based on the value of a defined string?

This doesn't work:

#define VAR VALUE_A

#if VAR == "VALUE_A"
    #include "a.h"
#elif VAR == "VALUE_B"
    #include "b.h"
#endif

If it's important, I'm not actually defining VAR, I'm passing it down from the command-line via gcc -D NAME=VALUE.

Answer 1

The == operator does not compare strings. But you have a couple of other options to configure your includes. In addition to the solutions already mentioned in other answers, I like this one because I think it is quite self-explanatory.

/* Constant identifying the "alpha" library. */
#define LIBRARY_ALPHA 1

/* Constant identifying the "beta" library. */
#define LIBRARY_BETA 2

/* Provide a default library if the user does not select one. */
#ifndef LIBRARY_TO_USE
#define LIBRARY_TO_USE LIBRARY_ALPHA
#endif

/* Include the selected library while handling errors properly. */
#if LIBRARY_TO_USE == LIBRARY_ALPHA
#include <alpha.h>
#elif LIBRARY_TO_USE == LIBRARY_BETA
#define BETA_USE_OPEN_MP 0  /* You can do more stuff than simply include a header if needed. */
#include <beta.h>
#else
#error "Invalid choice for LIBRARY_TO_USE (select LIBRARY_ALPHA or LIBRARY_BETA)"
#endif

Your users can now compile with:

$ cc -DLIBRARY_TO_USE=LIBRARY_BETA whatever.c