Is it possible to use the value of a define as part of an identifier?

Question

Is it possible to include a pre-defined id in an identifier?

The header:

/* myfile.h */
#define func MYID##_func
static void func() {}
#undef func

The c file:

/* myfile.c */
#define MYID FOO
#include "myfile.h"
#undef MYID

#define MYID BAR
#include "myfile.h"
#undef MYID

/* check the functions exist */
void main()
{
    FOO_func();
    BAR_func();
}

My aim here is to create 2 functions: (FOO_func, BAR_func).

---

I tried to compile the code above, and the functions are identified as MYID_func (which fails since there are 2 of them).

I could do:

#define func FOO_func
#include "myfile.h"
#undef func

... But this doesn't scale so nicely when the file has many identifiers, since they would all have to be defined first.

---

The goal is to be able to avoid naming collisions when the file is included multiple times in the same C file.

Answer 1

You can use couple of layers of macros to accomplish what you want to.

/* myfile.h */
#define CONCAT2(A, B) A ## B
#define CONCAT(A, B) CONCAT2(A, B)
#define func() CONCAT(MYID, _func)()
static void func() {}
#undef func

You need the extra layers to make sure that MYID is expanded as a macro.

Answer 2

The operator ## don't expand macros. If you want to do that use an extra define:

#define CONCAT_AUX(MACRO_ARG1, MACRO_ARG2) MACRO_ARG1 ## MACRO_ARG2
#define CONCAT(MACRO_ARG1, MACRO_ARG2) CONCAT_AUX(MACRO_ARG1, MACRO_ARG2)

#define func CONCAT(MYID, _func)