How to create C macro name using another macro?

Question

I have been given a library that defines the following macros:

#define FOO_0(A, B) (A + B)
#define FOO_1(A, B) (A - B)

Now I'd like to create a new macro MY_FOO that takes a third argument x and use it build the name of the macro to be called (e.g. FOO_<x>)

Here is my experiment:

#define MY_FOO(X, A, B) FOO_## X ##(A, B)

However, when I try to use it in my code:

int main(void) {
    int a = 2, b = 3, x = 0;
    printf("FOO_%d(%d, %d) = %d", x, a, b, MY_FOO(x, a, b));
    return 0;
}

I get the following error:

prog.c: In function ‘main’:
prog.c:6:25: error: pasting "FOO_x" and "(" does not give a valid preprocessing token
 #define MY_FOO(X, A, B) FOO_## X ##(A, B)
                         ^
prog.c:11:41: note: in expansion of macro ‘MY_FOO’
  printf("FOO_%d(%d, %d) = %d", x, a, b, MY_FOO(x, a, b));
                                         ^~~~~~
prog.c:6:25: warning: implicit declaration of function ‘FOO_x’ [-Wimplicit-function-declaration]
 #define MY_FOO(X, A, B) FOO_## X ##(A, B)
                         ^
prog.c:11:41: note: in expansion of macro ‘MY_FOO’
  printf("FOO_%d(%d, %d) = %d", x, a, b, MY_FOO(x, a, b));
                                         ^~~~~~

Is there a way to workaround this?

In C, there should never be a need to create macros with completely variable names. Macro names are for the programmer, not for the program. If you somehow come up with such a need still, you should rather run an external script that generates the C code. — Lundin
– Lundin, Commented Aug 8, 2017 at 9:52
Generating the code proposed by @bipll in the post below automatically could actually be a very good idea because I have to use the same pattern for a lot of macros — Muffo
– Muffo, Commented Aug 8, 2017 at 17:07

bipll · Accepted Answer · 2017-08-08 00:33:44Z

4

If you really need to turn run-time values into macro names, you'll have to do it on your own:

switch(x) {
    case 0: printf("FOO_0(%d, %d) = %d", a, b, FOO_0(a, b));
            break;
}

You can even make it slightly more advanced with local syntax-defs:

switch(x) {
#define CASE(x) case x: printf("FOO_" #x "(%d, %d) = %d", a, b, FOO_ ## x(a, b)); \
                        break
    CASE(0);
#undef CASE
}

answered Aug 8, 2017 at 0:33

bipll

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2 Comments

Muffo Over a year ago

Thanks for the answer! The distinction between run-time and compile-time value is probably the key to this problem.

bipll Over a year ago

Even more than that, macros are parse-time values, before the compilation itself. :)

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