C pre-processor macro abuse to prepend number of arguments to variadic functions

macros.h

#ifndef _MACROS_H
#define _MACROS_H

#include <stdint.h>
#include <stdarg.h>

/*
    The following macros are used to prepend the number of arguments for
    variadic functions.
*/

#define _NARG(...)      _NARG_( __VA_ARGS__, _RSEQ() )
#define _NARG_(...)     _SEQ( __VA_ARGS__ )
#define _SEQ( \
          _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,_64,_65,_66,_67,_68,_69,_70, \
         _71,_72,_73,_74,_75,_76,_77,_78,_79,_80, \
         _81,_82,_83,_84,_85,_86,_87,_88,_89,_90, \
         _91,_92,_93,_94,_95,_96,_97,_98,_99,_100, \
         _101,_102,_103,_104,_105,_106,_107,_108,_109,_110, \
         _111,_112,_113,_114,_115,_116,_117,_118,_119,_120, \
         _121,_122,_123,_124,_125,_126,_127,N,...) N
#define _RSEQ() \
         127,126,125,124,123,122,121,120, \
         119,118,117,116,115,114,113,112,111,110, \
         109,108,107,106,105,104,103,102,101,100, \
         99,98,97,96,95,94,93,92,91,90, \
         89,88,87,86,85,84,83,82,81,80, \
         79,78,77,76,75,74,73,72,71,70, \
         69,68,67,66,65,64,63,62,61,60, \
         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

/*
    In the example below, the variadic function has the prototype of ( size_t, int, ... ). 
    The above definitions are then used to create a macro which permits the 
    calling of this function with the prototype ( int, ... ). When expanded, this
    macro will prepend the number of arguments to the function call. This 
    additional value can then be used within the function to appropriately 
    process the arguments. 
    
    #define add(...) _add(_NARG(__VA_ARGS__), __VA_ARGS__)
    
    int _add(size_t Arg, int Value1, ...) {
        va_list sArg;
        int nSum;
        
        nSum = Value1;
        va_start(sArg, Value1);
        while(--Arg > 0) {
            nValue = va_arg(sArg, int);
            nSum += nValue;
        }
        va_end(sArg)
        return nSum;
    }
    
    printf("%u\n", add(1, 4, 5, 6)); // 16
*/

#endif  // _MACROS_H