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

       stdarg - 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 parameter last is the name of the last parameter before  the  vari-
       able argument list, i.e., the last parameter of which the calling func-
       tion knows the type.

       Because the address of this parameter  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.

       The va_arg macro expands to an expression that has the type  and  value
       of  the  next argument in the call.  The parameter 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 parameter type is a type name
       specified so that the type of a pointer to an object that has the spec-
       ified 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  invoca-
       tion  of  va_end  in  the  same function. After the call va_end(ap) the
       variable ap is undefined.  Multiple  transversals  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 a pointer to  the  stack
       frame  of the variadic function.  In such a setup (by far the most com-
       mon) 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 parameters are passed in registers, it may be
       necessary for va_start to allocate memory, store the parameters  there,
       and  also  an indication of which parameter 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  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.

       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, *p, *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);

       The va_start, va_arg, and va_end macros  conform  to  ANSI  X3.159-1989
       (``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>

              void foo(va_alist) va_dcl {
                   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

                                  2001-10-14                         STDARG(3)