the problem about different treatment to __VA_ARGS__ when using VS 2008 and GCC

[Liu Liu]

Hi,

I am trying to identify a problem because of an unusual usage of
variadic macros. Here is the hypothetic macro:

#define va(c, d, ...) c(d, __VA_ARGS__)
#define var(a, b, ...)  va(__VA_ARGS__, a, b)

var(2, 3, printf, “%d %d %d\n”, 1);

For gcc, the preprocessor will output

printf("%d %d %d\n", 1, 2, 3)

but for VS 2008, the output is

printf, “%d %d %d\n”, 1(2, 3);

I suspect the difference is caused by the different treatment to
__VA_ARGS__, for gcc, it will first expand the expression to
va(printf, "%d %d %d\n", 1, 2, 3), and treat 1, 2, 3 as the
__VA_ARGS__ for macro va. But for VS 2008, it will first treat b as
__VA_ARGS__ for macro va, and then do the expansion.

Which one is correct interpretation for C99 variadic macro? or my
usage falls into an undefined behavior? Thank you.


--
Liu Liu
http://liuliu.me/