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RedHat 9 (Linux i386) - man page for va_arg (redhat section 3)

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 variable argument list,
       i.e., the last parameter of which the calling function 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 function 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	specified
       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 invocation of va_end in the
       same function. After the call va_end(ap) the variable ap is undefined. Multiple	transver-
       sals  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  vari-
       adic  function.	 In  such a setup (by far the most common) 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 allo-
       cated 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 invocation 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 associ-
       ated 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 compat-
       ible 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)


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