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recently I was browsing SO and I came across this topic, where Sam Jansen is declaring a macro PACKED_STRUCT(name), but in code he is using it once with no arguments and once with the name argument supplied.

I tried to compile a similar sample progam with GCC 4.6.1 and I was surprised it compiled fine with no warnings at all (I was using -std=c99 -Wall -Wextra -pedantic command line switches). However when I tried to make two argument macro and call it with less than two arguments, it did not work.

So my question is if this is a bug in GCC, or it is a feature of GCC, or it is defined somewhere in the standard, that it has to work like this?

According to this page in GCC's documentation, this should not be possible.

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jcxz
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1 Answers1

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Read that page of GCC's documentation again; it's making a distinction between empty arguments and missing arguments. Given

#define ONE(x)    one(x)
#define TWO(x,y)  two(x,y)

all of these are perfectly fine as far as the preprocessor is concerned (expansion in comment):

ONE(1)    /* one(1)   */
ONE()     /* one()    */
TWO(1,2)  /* two(1,2) */
TWO(1,)   /* two(1,)  */
TWO(,2)   /* two(,2)  */
TWO(,)    /* two(,)   */

but this is not okay:

TWO()     /* error: macro "TWO" requires 2 arguments, but only 1 given */
zwol
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  • Thanks for the answer, it seems you are right. Additionally I found [a similar SO post](http://stackoverflow.com/questions/7666344/are-empty-macro-arguments-legal-in-c11), where the accepted answer states that empty macro arguments should be legal even with C99 standard – jcxz Sep 30 '12 at 13:06