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

       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  step-
       ping through a list of arguments whose number and types are not known to the called

       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

       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  unde-
       fined.   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 noth-
       ing 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  indica-
       tion  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() macro.

       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 convert-
       ing 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).

       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);

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

                                  2001-10-14                         STDARG(3)

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