Macro expansion issue with gcc

Question

In gcc, it appears that referencing the results of a macro expansion later inside that same expansion doesn't work. For example:

#define TESTMACRO(name) \
        static int name##_func(int solNo) { \
                return (solNo); \
        }\
        static int name##Thing = {0,##name##_func},NULL,{"", capInvSw##name}}; 

TESTMACRO(stuff)

This results in errors like this:

test.c:7:29: error: pasting "," and "stuff" does not give a valid preprocessing token
  static int name##Thing = {0,##name##_func},NULL,{"", capInvSw##name`}}; 
                             ^
test.c:9:1: note: in expansion of macro ‘TESTMACRO’
 TESTMACRO(stuff)

I would expect to have a function called stuff_func created and passed into stuffThing. I believe that this works in other compilers. What is the equivalent way to do this in gcc?

What does your pre-processor output present for the expansion? — nico
– nico, Commented Mar 7, 2019 at 22:19
Looks like you have a spurious extra ## in the macro body, which is what the error message is complaining about... — Chris Dodd
– Chris Dodd, Commented Mar 8, 2019 at 0:28

nico · Accepted Answer · 2019-03-07 22:13:11Z

1

You can try to run only the pre-processor on your code by passing the -E flag:

gcc -E foo.c

Which evaluates your macro to:

static int stuff_func(int solNo) { return (solNo); } static int stuffThing = {0,stuff_func},NULL,{"", capInvSwstuff`}};

That can be expanded for readability to:

static int stuff_func(int solNo) { 
return (solNo); 
} 

static int stuffThing = {0,stuff_func},NULL,{"", capInvSwstuff`}};

And it appears that you have one extra/missing brackets } in your expanded macro.

Hope it helps.

answered Mar 7, 2019 at 22:13

nico

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1 Comment

MikeLatiolais Over a year ago

Ah, yes, this was an artificial example meant to strip some ambiguity from a real life pinmame issue.

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