va_start

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 func-
       tion or an array type.

   va_arg()
       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.

   va_end()
       Each invocation of va_start() must be matched by a corresponding  invo-
       cation of va_end() in the same function.  After the call va_end(ap) the
       variable ap is undefined.  Multiple traversals of the list, each brack-
       eted  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  argu-
       ment  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 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.

ATTRIBUTES
       For an  explanation  of  the  terms  used  in  this  section,  see  at-
       tributes(7).

       +----------------------+---------------+-----------------+
       |Interface             | Attribute     | Value           |
       +----------------------+---------------+-----------------+
       |va_start(), va_end(), | Thread safety | MT-Safe         |
       |va_copy()             |               |                 |
       +----------------------+---------------+-----------------+
       |va_arg()              | Thread safety | MT-Safe race:ap |
       +----------------------+---------------+-----------------+
CONFORMING TO
       The va_start(), va_arg(), and va_end() macros conform to C89.  C99  de-
       fines the va_copy() macro.

BUGS
       Unlike  the  historical 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 associated with each format character based on the type.

       #include <stdio.h>
       #include <stdarg.h>

       void
       foo(char *fmt, ...)   /* '...' is C syntax for a variadic function */

       {
           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 5.05 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
       https://www.kernel.org/doc/man-pages/.

                                  2019-05-09                         STDARG(3)