A C preprocessor macro that allows executing code depending on the number of arguments in `__VA_ARGS__`. Test with `gcc -std=c99 -E`. Original version of an `_IFEMPTY` macro was discarded because it depended on the GCC extension syntax `## __VA_ARGS__`.

ONEARG.h

/// Usage: _ISONEARG()        -> 1
///        _ISONEARG(A)       -> 1
///        _ISONEARG(A,B,...) -> 0
///
/// Alternative to _IFEMPTY, that works with standard C.
/// Works for up to 100 arguments.
///
/// Examples:
///
/// "_ISONEARG()" -> 1
/// "_ISONEARG(1)" -> 1
/// "_ISONEARG(1,2)" -> 0
/// "_IFONEARG(then, else)" -> then
/// "_IFONEARG(then, else, 1)" -> then
/// "_IFONEARG(then, else, 1, 2)" -> else
///
/// Implementation Notes:
///
/// - The _IFONEARG_PROTECT mechanism was implemented to allow
///   constructs like _IFONEARG(M(),...) where M() expands
///   to something containing a comma (,).

#define _ISONEARG(...) _ISONEARG_(__VA_ARGS__,\
  0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,            \
  0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,            \
  0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,            \
  0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,            \
  0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1)
#define _ISONEARG_(                                 \
  _001,_002,_003,_004,_005,_006,_007,_008,_009,_010,    \
  _011,_012,_013,_014,_015,_016,_017,_018,_019,_020,    \
  _021,_022,_023,_024,_025,_026,_027,_028,_029,_030,    \
  _031,_032,_033,_034,_035,_036,_037,_038,_039,_040,    \
  _041,_042,_043,_044,_045,_046,_047,_048,_049,_050,    \
  _051,_052,_053,_054,_055,_056,_057,_058,_059,_060,    \
  _061,_062,_063,_064,_065,_066,_067,_068,_069,_070,    \
  _071,_072,_073,_074,_075,_076,_077,_078,_079,_080,    \
  _081,_082,_083,_084,_085,_086,_087,_088,_089,_090,    \
  _091,_092,_093,_094,_095,_096,_097,_098,_099,_100,    \
  N,...) N
#define _IFONEARG_PROTECT(...) __VA_ARGS__
#define _IFONEARG(T,E,...)\
 _IFONEARG_C2(_IFONEARG_IFTHEN_,_ISONEARG(__VA_ARGS__))\
 (_IFONEARG_PROTECT(T),_IFONEARG_PROTECT(E))
#define _IFONEARG_C2(A,B) _IFONEARG_C2_(A,B)
#define _IFONEARG_C2_(A,B) A ## B
#define _IFONEARG_IFTHEN_0(T,E) E
#define _IFONEARG_IFTHEN_1(T,E) T