Passing an empty parameter to a C macro

#include <stdio.h>

#define nothing

#define RET_VAL_IF_NULL(p,v)   \
   if (p == NULL)              \
      return v;

#define RET_IF_NULL(p)         \
   RET_VAL_IF_NULL(p, nothing) /* *** HERE *** */

static void func1(char* ptr)
{
   RET_IF_NULL(ptr)
   printf("* %s from func1\n", ptr);
}

static int func2(char* ptr)
{
   RET_VAL_IF_NULL(ptr,1)
   printf("* %s from func2\n", ptr);
   return 0;
}

void main(void)
{
   char* p1 = 0, *p2 = "string";
   func1(p1);
   func1(p2);
   func2(p1);
   func2(p2);
}

I'm a long-time software developer living in Brazil.

I've been developing software for retail and banking automation in C/C++ for many years now. In the old days I even did some COBOL programming, and some assembly for the 8080.

My experience ranges from low level software such as interface code for serial devices for DOS and Windows (bar code scanners, printers, cash dispensers, etc) and goes to writing end user applications for POS terminals and bank ATMs. In between I've done a great deal of TCP/IP programming using the basic Berkeley sockets interface, which is my main interest nowadays.