Counting arguments in variadic macros

va_nargs.c

// RUN: cc -Wall -Wextra %s && ./a.out

#include <assert.h>

#if defined(__GNUC__) || defined(__clang__)
  // Supports 0-10 arguments
  #define VA_NARGS_IMPL(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, N, ...) N
  // ## deletes preceding comma if _VA_ARGS__ is empty (GCC, Clang)
  #define VA_NARGS(...) VA_NARGS_IMPL(_, ## __VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#else
  // Supports 1-10 arguments
  #define VA_NARGS_IMPL(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, N, ...) N
  #define VA_NARGS(...) VA_NARGS_IMPL(__VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
#endif

#define VA_NARGS2(...) ((int)(sizeof((int[]){ __VA_ARGS__ })/sizeof(int)))

#define TEST(...)   VA_NARGS(__VA_ARGS__)
#define TEST2(...)  VA_NARGS2(__VA_ARGS__)

void test(void)
{
    assert(TEST() == 0);
    assert(TEST(1) == 1);
    assert(TEST(1, 2) == 2);
    assert(TEST(1, 2, 3) == 3);
    assert(TEST(1, 2, 3, 'a') == 4);
    assert(TEST(1, 2, 3, 'a', 'b') == 5);
    assert(TEST(1, 2, 3, 'a', 'b', 'c') == 6);
    assert(TEST(1, 2, 3, 'a', 'b', 'c', 4.0) == 7);
    assert(TEST(1, 2, 3, 'a', 'b', 'c', 4.0, 5.0) == 8);
    assert(TEST(1, 2, 3, 'a', 'b', 'c', 4.0, 5.0, 6.0) == 9);
    assert(TEST(1, 2, 3, 'a', 'b', 'c', 4.0, 5.0, 6.0, 0) == 10);
    //assert(TEST(1, 2, 3, 'a', 'b', 'c', 4.0, 5.0, 6.0, 0, 0) == 11); // fails

    assert(TEST2() == 0);
    assert(TEST2(1) == 1);
    assert(TEST2(1, 2) == 2);
    assert(TEST2(1, 2, 3) == 3);
    assert(TEST2(1, 2, 3, 'a') == 4);
    assert(TEST2(1, 2, 3, 'a', 'b') == 5);
    assert(TEST2(1, 2, 3, 'a', 'b', 'c') == 6);
    assert(TEST2(1, 2, 3, 'a', 'b', 'c', 4.0) == 7);
    assert(TEST2(1, 2, 3, 'a', 'b', 'c', 4.0, 5.0) == 8);
    assert(TEST2(1, 2, 3, 'a', 'b', 'c', 4.0, 5.0, 6.0) == 9);
    assert(TEST2(1, 2, 3, 'a', 'b', 'c', 4.0, 5.0, 6.0, 0) == 10);
    assert(TEST2(1, 2, 3, 'a', 'b', 'c', 4.0, 5.0, 6.0, 0, 0) == 11);
}

int main(void)
{
    test();

    return 0;
}

Small improvement:

#if defined(__GNUC__) || defined(__clang__) // supports passing 0 arguments using gcc/clang non-standard features
#define VA_NARGS_IMPL(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, N, ...) N
#define VA_NARGS(...) VA_NARGS_IMPL(_, ## __VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#else
#define VA_NARGS_IMPL(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, N, ...) N // 1 or more arguments only
#define VA_NARGS(...) VA_NARGS_IMPL(__VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
#endif

Thanks, @sammonius, I'll update the gist.