c - Chained macro invocations. Argument in parentheses treated differently? -

#define a(p1, p2, p3, p4) foo(p1, p2, p3, p4) #define b(s) a(p1, p2, (s), p4) 

here, a() macros binding, aimed increase portability should ever need call bar(p1, p2, p3, p4) , not want rewrite whole code base.

now trying define b() make writing easier on me, p1, p2 , p4 have same values. this, however, not work, unless remove parentheses around s. what going on? passing a?

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turns out that:

#define foo(p1, p2, p3, p4) p1 ## p2 ## p3 ## p4() 

i not sure p4 defined, though, have valid value. so, when pass (s) instead of s, p1p2 instead of p1p2sp4.

so passing "(" + + ")"? right! , (which concatinates it's inputs simple p1 ## p2 ## p3 ## p4) stops before third argument

yes, foo produces

p1 ## p2 ## ( s ) ## p4 

and pasting parentheses p4 resp. p1 ## p2 produces invalid tokens whole thing refused.

for token-pasting, parentheses big no-no.