stdarg.h

va_arg, va_end, va_start

Header

stdarg.h

Prototype

type va_arg(va_list arg_ptr, type);
void va_end(va_list arg_ptr);
void va_start(va_list arg_ptr, prev_parm);

Description

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.

Return Value

va_arg returns the current argument. va_start and va_end evaluate to void.

Compatibility

DOS Windows 3.x Phar Lap DOSX Win32

Example

	/* 	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); 
	}

Output

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