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 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 function or an array type.

   va_arg()
       The  va_arg() macro expands to an expression that has the type and value of the next argu‐
       ment 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 argu‐
       ment 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 tra‐
       versals of the list, each bracketed by va_start() and va_end() are possible.  va_end() may
       be a macro or a function.

   va_copy()
       The va_copy() macro copies the (previously initialized)  variable  argument  list  src  to
       dest.   The behavior is as if va_start() were applied to dest with the same last argument,
       followed by the same number of va_arg() invocations that was used  to  reach  the  current
       state of src.

       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.

ATTRIBUTES
   Multithreading (see pthreads(7))
       The va_start(), va_arg(), va_end(), and va_copy() macros are thread-safe.

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-compat‐
       ible 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 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 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, *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 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.



                                            2013-12-10                                  STDARG(3)