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


   #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);


   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().

   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.

   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.

   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.

   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);

   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.


   For   an   explanation   of   the  terms  used  in  this  section,  see

   Interface              Attribute      Value           
   va_start(), va_end(),  Thread safety  MT-Safe         
   va_arg()               Thread safety  MT-Safe race:ap 


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


   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 <varargs.h>

           va_list ap;

           while (...) {
               x = va_arg(ap, type);

   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.


   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


   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>

   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);
           case 'd':              /* int */
               d = va_arg(ap, int);
               printf("int %d\n", d);
           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);


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   latest    version    of    this    page,    can     be     found     at

                              2015-03-02                         STDARG(3)


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