Count number of parameters in C variable argument method call

Question

When using va_start(), va_arg() and va_end() to read parameters passed to a method, is there a way to count how many arguments there are?

According to the man page i

Answer 1

Here is a fairly astonishing trick that allows you to build what you want using C99's variadic macros:

PP_NARG()
Returns the number of arguments contained in __VA_ARGS__

You can use this to write a macro wrapping your real function that adds a count to the front of the real function's arguments.

See also this question. However probably the classical approaches are still more sensible for a year or three yet!

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The code, so that this answer is useful even without the Usenet archive...

/* The PP_NARG macro returns the number of arguments that have been
  * passed to it.
  */

#define PP_NARG(...) \
         PP_NARG_(__VA_ARGS__,PP_RSEQ_N())
#define PP_NARG_(...) \
         PP_ARG_N(__VA_ARGS__)
#define PP_ARG_N( \
          _1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
         _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
         _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
         _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
         _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
         _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
         _61,_62,_63,N,...) N
#define PP_RSEQ_N() \
         63,62,61,60,                   \
         59,58,57,56,55,54,53,52,51,50, \
         49,48,47,46,45,44,43,42,41,40, \
         39,38,37,36,35,34,33,32,31,30, \
         29,28,27,26,25,24,23,22,21,20, \
         19,18,17,16,15,14,13,12,11,10, \
         9,8,7,6,5,4,3,2,1,0