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



NAME
       stdarg, va_start, va_arg, va_end, va_copy - 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 argument last is the name of the last argument before the variable argument list, that is, the  last  argu-
       ment 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 regis-
       ter 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
       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.  Suc-
       cessive 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  pro-
       moted 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 traversals 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 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 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.

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 <varargs.h>.

       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  argu-
       ments.   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 <stdio.h>
       #include <stdarg.h>

       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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



                                  2001-10-14                         STDARG(3)