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       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.

       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.


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


       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>

               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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       project, and information about reporting bugs, can be found at

                                            2001-10-14                                  STDARG(3)