title: Macro to count number of arguments
body: <p>I have a variadic function from a third-party C library:</p>
  <pre><code>int func(int argc, ...);
  </code></pre>
  <p><code>argc</code> indicates the number of passed optional arguments.
  I'm wrapping it with a macro that counts the number of arguments, as suggested <a href="https://groups.google.com/forum/?fromgroups#!topic/comp.std.c/d-6Mj5Lko_s" rel="nofollow noreferrer">here</a>. For reading convenience, here's the macro:</p>
  <pre><code>#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
  
  #define PP_NARG_(...)    PP_ARG_N(__VA_ARGS__)    
  
  #define PP_NARG(...)     PP_NARG_(__VA_ARGS__, PP_RSEQ_N())
  </code></pre>
  <p>and I'm wrapping it like so:</p>
  <pre><code>#define my_func(...)     func(PP_NARG(__VA_ARGS__), __VA_ARGS__)
  </code></pre>
  <p>The <code>PP_NARG</code> macro works great for functions accepting one or more arguments. For instance, <code>PP_NARG(&quot;Hello&quot;, &quot;World&quot;)</code> evaluates to <code>2</code>.<br></p>
  <p>The problem is that when no arguments are passed, <code>PP_NARG()</code> evaluates to <code>1</code> instead of <code>0</code>.<br>
  I understand <a href="https://web.archive.org/web/20201001165923/http://efesx.com/2010/07/17/variadic-macro-to-count-number-of-arguments/" rel="nofollow noreferrer">how this macro works</a>, but I can't come up with an idea to modify it so that it behaves correctly for this case as well.</p>
  <p>Any ideas?</p>
  <hr />
  <p><strong>EDIT</strong>:<br>
  I have found a workaround for <code>PP_NARG</code>, and posted it as an answer.<br>
  I still have problems with wrapping the variadic function though. When <code>__VA_ARGS__</code> is empty, <code>my_func</code> expands to <code>func(0, )</code> which triggers a compilation error.</p>