Extract Argument from C Macro

Question

I have a number of definitions consisting of two comma-separated expressions, like this:

#define PIN_ALARM  GPIOC,14

I want to pass the second expression of those definitions (14 in the case above) to unary macros like the following:

#define _PIN_MODE_OUTPUT(n)          (1U << ((n) * 2U))

How can I extract the second number? I want a macro, call it "PICK_RIGHT", which will do this for me:

#define PICK_RIGHT(???)   ???

So that I can make a new macro that can take my "PIN" definitions:

#define PIN_MODE_OUTPUT(???) _PIN_MODE_OUTPUT(PICK_RIGHT(???))

And I can simply do:

#define RESULT   PIN_MODE_OUTPUT(PIN_ALARM)

Answer 1

Do not use macros for this. If you must, the following will work by throwing away the left part first so just the number remains. Use with care. No guarantees.

#define PIN_ALARM  GPIOC,14
#define RIGHTPART_ONLY(a,b) b

#define PIN_MODE_OUTPUT(a) RIGHTPART_ONLY(a)

#define RESULT   PIN_MODE_OUTPUT(PIN_ALARM)

int main (void)
{
    printf ("we'll pick ... %d\n", PIN_MODE_OUTPUT(PIN_ALARM));
    printf ("or maybe %d\n", RESULT);

    return 0;
}

If you want the left part as a string, you can use this (with the same warnings as above), where the left part gets converted to a string by #:

#define LEFTPART_ONLY(a,b) #a
#define PIN_MODE_NAME(a) LEFTPART_ONLY(a)

There is a practical reason this is not entirely without problems. GPIOC is a symbol and as such it is possibly defined elsewhere. Fortunately, it is not a problem if it is undefined, or it is but to a simple type - after all, first thing the macros do is "throw away the left part". But as Jonathan Leffler comments

Note that if GPIOC maps to a macro containing commas, you're likely to get compilation errors.