How to generate function name using c macro

Question

Macro "VER" is defined as "((u_long)1)" in some other header file which I can't change.

In my code I need to compose function "test_1" using "test" and the VER. However compiler reported error since it was the "test_((u_long)1)" instead of "test_1" generated.

My question is: How to write macro so that it can generate "test_1"?

Thanks in advance!

#define VER ((u_long)1)      /* This is defined in some other header file which I can't change*/

#define paste(x, y, z) x ## y ## z
#define paste2(x, y, z) paste(x, y, z)
#define fcall(fname) paste2(fname, _, VER)

int test_1() {
  return 0;
}

int main() {
  fcall( test )();
  return 0;
}

Angew is no longer proud of SO · Accepted Answer · 2013-07-23 11:23:14Z

6

I can't guarantee it's 100% portable, but it should work:

#define VER ((u_long)1)

#define STRIP1(x) STRIP2 x

#define STRIP2(x) STRIP3 x

#define STRIP3(x)

#define paste(x, y, z) x ## y ## z
#define paste2(x, y, z) paste(x, y, z)
#define fcall(fname) paste2(fname, _, STRIP1(VER))

Live example

It works by interpreting the parentheses inside the definition of VER as macro invocation. Here's the individual expansions, as they occur:

STRIP1(VER) // STRIP1 with argument VER

STRIP2 ((u_long)1) // STRIP2 with argument (u_long)1

STRIP3 (u_long)1  // STRIP3 with argument u_long, followed by 1

1

If we re-arrange the whitespace (it's insignificant anyway), we get this:

STRIP1(VER)

STRIP2((u_long) 1)

STRIP3(u_long) 1

1

edited Jul 23, 2013 at 11:23

answered Jul 23, 2013 at 10:28

Angew is no longer proud of SO

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3 Comments

brian beuning Over a year ago

That is very cool. But if someone changes the definition of VER (say to (1)), it breaks.

Angew is no longer proud of SO Over a year ago

@brianbeuning Of course. But then again, normally during development you are aware when "external code I can't modify" changes (e.g. because you've upgraded to a new version), so you can adapt the STRIP macros accordingly.

Angew is no longer proud of SO Over a year ago

@hhwangcocora You might want to mark the answer as accepted, then (using the green tick-mark next to it). That's how SO works.

Mats Petersson · Accepted Answer · 2013-07-23 10:19:22Z

0

With that source, you probably can't, since macro expansion is textual, so the ((u_long)1) can not possibly become 1.

One solution would of course be to actually write some code to print the actual value of VER. The other alternative is to convince the people that own the file you can't change that they should change it for you...

answered Jul 23, 2013 at 10:19

Mats Petersson

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Comments

BLUEPIXY · Accepted Answer · 2013-07-23 10:42:32Z

0

As modified Angew version

#define VER ((u_long)1)

#define _(x) x
#define id(x) _(x)

#define STRIP3(x)
#define STRIP2(x) id(STRIP3 x)
#define STRIP1(x) id(STRIP2 x)

#define paste(x, y, z) x ## y ## z
#define paste2(x, y, z) paste(x, y, z)
#define fcall(fname) paste2(fname, _, STRIP1(VER))

answered Jul 23, 2013 at 10:42

BLUEPIXY

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