For example I want to write my own printf() alternative, but I have to perform calculations on the variable arguments:
#define log(fmt_string, ...) my_log(fmt_string, pack_args(__VA_ARGS__), __VA_ARGS__)
where pack_args(...) - is a macro too. How should I change this code to handle the only fmt_string presence scenario?
log("Some message here");
In P99 I have two macros
#define P00_ARG( \
_1, _2, _3, _4, _5, _6, _7, _8, \
_9, _10, _11, _12, _13, _14, _15, _16, \
... etc ... \
_153, _154, _155, _156, _157, _158, _159, \
...) _159
#define P99_HAS_COMMA(...) P00_ARG(__VA_ARGS__, \
1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, \
... etc .... \
1, 1, 1, 1, 1, 1, 0, 0)
You can use this to determine if your argument has a comma (so there are more arguments than your format) or not (only a format). You can then use that to construct a call to one of two macros:
#define log(...) log2(P99_HAS_COMMA(__VA_ARGS__), __VA_ARGS__)
#define log2(N, ...) log3(N, __VA_ARGS__)
#define log3(N, ...) log ## N(__VA_ARGS__)
#define log0(FMT) /* your version with format only goes here */
#define log1(FMT, __VA_ARGS__) /* your version with more goes here */
How should I change this code to [handle] the only fmt_string presence scenario?
You cannot do this at all with a variadic macro in standard C. The standard explicitly specifies that in the invocation of a variadic macro "there shall be more arguments in the invocation than there are parameters in the macro definition (excluding the ...)" (C2011, 6.10.3/4). You could allow the macro to be used with just one argument by changing it to ...
#define log(...) /* ... */
... but then you could not separate the format string from the other arguments -- at least not without re-introducing the same problem you have now.
You'll need to use a bona fide function if you need to support a zero-length variable argument list.
