STDARG(3)		   Linux Programmers Manual		    STDARG(3)



NAME
       stdarg, va_start, va_arg, va_end, va_copy - variable argument lists

SYNOPSIS
       #include 

       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  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 argument last is the name of the last argument before the  variable
       argument list, that is, the last argument of which the calling function
       knows the type.

       Because the address of this argument 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 argument 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 argument 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  invo
       cation of va_end() in the same function.  After the call va_end(ap) the
       variable ap is undefined.  Multiple traversals of the list, each brack
       eted  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  be  a  pointer  to  the
       stack frame of the variadic 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 arguments are passed in registers, it may be
       necessary for va_start() to allocate memory, store the arguments there,
       and  also an indication of which argument 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.

CONFORMING TO
       The  va_start(),  va_arg(),  and  va_end()  macros conform to C89.  C99
       defines the va_copy() macro.

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

       The historic setup is:

	   #include 

	   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).

EXAMPLE
       The function foo takes a string of format characters and prints out the
       argument associated with each format character based on the type.

       #include 
       #include 

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

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



				  2001-10-14			     STDARG(3)