stdarg.h

These functions maintain a list of arguments to be accessed within functions that accept a variable number of arguments (for example the vprintf function). va_list is a type of variable argument list defined in stdarg.h. Variable argument lists can be processed by a function when it does not know the number of arguments being passed. The va_list array holds information required by va_arg and va_end. When a called function takes a variable argument list, it declares the last parameter of type va_list.

va_start is first called to initialize the argument list, for which the parameter arg_ptr points to the first argument in the va_list. The other parameter prev_parm is the parameter preceding the first argument. After a call to va_start, a call to va_arg will evaluate data type from the location pointed to by arg_ptr and increment arg_ptr. va_end resets arg_ptr to NULL.

/* 	Example for va_arg 
	Also demonstrates va_end, va_start 
*/ 

#include <stdlib.h>
#include <stdio.h>
#include <stdarg.h>

void miniprintf(char * formatstr, ...) 
{   
   va_list marker; 
   int pos = 0; 

   va_start (marker, formatstr); 

   while (formatstr[pos]) 
   {   
      if (formatstr[pos] == '% ') 
      {   
	 pos++; 
	 switch (formatstr[pos]) 
	 {   
	    case 'S' : 
	    case 's' : 
	       printf ("% s", va_arg (marker, 
			char *)); 
	       break; 
	    case 'I' : 
	    case 'i' : 
	       printf ("% d", va_arg (marker, 
			int)); 
	} 
      } 
      else 
	 putchar (formatstr[pos]); 
      pos++; 
   } 
   va_end (marker); 
} 

void main () 
{   
   miniprintf (" This is a string: \"% s\"\nAnd this is an int: (% i)\n", 
      "String", 12); 
} 

This is a string: "String"
And this is an int: (12)