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STDARG(3)		   Linux Programmer's Manual		     STDARG(3)

NAME
       stdarg -	variable argument lists

SYNOPSIS
       #include	<stdarg.h>

       void va_start(va_list ap, last);
       type va_arg(va_list ap, type);
       void va_end(va_list ap);
       void va_copy(va_list dest, va_list src);

DESCRIPTION
       A  function may be called with a	varying	number of arguments of varying
       types.  The include file	stdarg.h declares a type va_list  and  defines
       three  macros for stepping through a list of arguments whose number and
       types are not known to the called function.

       The called function must	declare	an object of  type  va_list  which  is
       used by the macros va_start, va_arg, and	va_end.

   va_start
       The  va_start  macro  initializes  ap  for subsequent use by va_arg and
       va_end, and must	be called first.

       The parameter last is the name of the last parameter before  the	 vari-
       able argument list, i.e., the last parameter of which the calling func-
       tion knows the type.

       Because the address of this parameter  may  be  used  in	 the  va_start
       macro,  it should not be	declared as a register variable, or as a func-
       tion or an array	type.

   va_arg
       The va_arg macro	expands	to an expression that has the type  and	 value
       of  the	next argument in the call.  The	parameter ap is	the va_list ap
       initialized by va_start.	 Each call to va_arg modifies ap so  that  the
       next call returns the next argument.  The parameter type	is a type name
       specified so that the type of a pointer to an object that has the spec-
       ified type can be obtained simply by adding a * to type.

       The  first  use	of  the	 va_arg	macro after that of the	va_start macro
       returns the argument after last.	  Successive  invocations  return  the
       values of the remaining arguments.

       If  there  is  no  next argument, or if type is not compatible with the
       type of the actual next argument	(as promoted according to the  default
       argument	promotions), random errors will	occur.

       If  ap is passed	to a function that uses	va_arg(ap,type)	then the value
       of ap is	undefined after	the return of that function.

   va_end
       Each invocation of va_start must	be matched by a	corresponding  invoca-
       tion  of	 va_end	 in  the  same function. After the call	va_end(ap) the
       variable	ap is undefined.  Multiple  transversals  of  the  list,  each
       bracketed  by  va_start and va_end are possible.	 va_end	may be a macro
       or a function.

   va_copy
       An obvious implementation would have a va_list a	pointer	to  the	 stack
       frame  of the variadic function.	 In such a setup (by far the most com-
       mon) there seems	nothing	against	an assignment
		   va_list aq =	ap;
       Unfortunately, there are	also systems that make it an array of pointers
       (of length 1), and there	one needs
		   va_list aq;
		   *aq = *ap;
       Finally,	on systems where parameters are	passed in registers, it	may be
       necessary for va_start to allocate memory, store	the parameters	there,
       and  also  an indication	of which parameter is next, so that va_arg can
       step through the	list. Now va_end can free the allocated	memory	again.
       To  accommodate	this  situation, C99 adds a macro va_copy, so that the
       above assignment	can be replaced	by
		   va_list aq;
		   va_copy(aq, ap);
		   ...
		   va_end(aq);
       Each invocation of va_copy must be matched by a	corresponding  invoca-
       tion  of	 va_end	in the same function.  Some systems that do not	supply
       va_copy have __va_copy instead, since that was the  name	 used  in  the
       draft proposal.

EXAMPLES
       The function foo	takes a	string of format characters and	prints out the
       argument	associated with	each format character based on the type.
	      #include <stdio.h>
	      #include <stdarg.h>

	      void foo(char *fmt, ...) {
		   va_list ap;
		   int d;
		   char	c, *p, *s;

		   va_start(ap,	fmt);
		   while (*fmt)
			switch(*fmt++) {
			case 's':	    /* string */
			     s = va_arg(ap, char *);
			     printf("string %s\n", s);
			     break;
			case 'd':	    /* int */
			     d = va_arg(ap, int);
			     printf("int %d\n",	d);
			     break;
			case 'c':	    /* char */
			     /*	need a cast here since va_arg only
				takes fully promoted types */
			     c = (char)	va_arg(ap, int);
			     printf("char %c\n", c);
			     break;
			}
		   va_end(ap);
	      }

CONFORMING TO
       The va_start, va_arg, and va_end	macros	conform	 to  ANSI  X3.159-1989
       (``C89'').  C99 defines the va_copy macro.

COMPATIBILITY
       These  macros are not compatible	with the historic macros they replace.
       A backward  compatible  version	can  be	 found	in  the	 include  file
       varargs.h.

COMPARISON
       The historic setup is:
	      #include <varargs.h>

	      void foo(va_alist) va_dcl	{
		   va_list ap;

		   va_start(ap);
		   while(...) {
			...
			x = va_arg(ap, type);
			...
		   }
		   va_end(ap);
	      }
       On  some	 systems,  va_end  contains  a	closing	 '}' matching a	'{' in
       va_start, so that both macros must occur	in the same function, and in a
       way that	allows this.

BUGS
       Unlike  the varargs macros, the stdarg macros do	not permit programmers
       to code a function with no fixed	 arguments.   This  problem  generates
       work  mainly  when  converting varargs code to stdarg code, but it also
       creates difficulties for	variadic functions that	wish to	 pass  all  of
       their arguments on to a function	that takes a va_list argument, such as
       vfprintf(3).

				  2001-10-14			     STDARG(3)

NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | CONFORMING TO | COMPATIBILITY | COMPARISON | BUGS

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