Using ## with variadic macro?

[Michael B Allen]

Is the following legit?

#define PRINT(err, fmt, ...) \
my_printf(__FILE__ ": %s" fmt, errstr(err), ##__VA_ARGS__)

Without the ## if I call this like PRINT(EFOO, ": foo!") this reduces
to following invalid function call:

my_printf("file.c" ": %s" ": foo!", errstr(err),)

But with gcc at least the ## ensures the trailing comma is removed if
no rest arguments are provided. Is this gcc specific is it valid C99?

Thanks,
Mike

 

[Ian Pilcher]

Is the following legit?

#define PRINT(err, fmt, ...) \
my_printf(__FILE__ ": %s" fmt, errstr(err), ##__VA_ARGS__)

Without the ## if I call this like PRINT(EFOO, ": foo!") this reduces
to following invalid function call:

my_printf("file.c" ": %s" ": foo!", errstr(err),)

But with gcc at least the ## ensures the trailing comma is removed if
no rest arguments are provided. Is this gcc specific is it valid C99?

I've been wrestling with a similar problem. If you compile this code
with -std=c99 -Wall -W -pedantic, I believe that you'll get a warning
about unused rest arguments. Based on this, I believe that this is a
GCC extension.