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Language Ccpp Cref Man Va_start.htm

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<h3>STDARG(3)           Linux Programmer's Manual           STDARG(3)

</h3>       stdarg - variable argument lists

</h3>       #include &lt;stdarg.h&gt;

       void va_start( va_list ap, last);
       typeva_arg( va_list ap, type);
       void va_end( va_list ap);

</h3>       A  function  may  be called with a varying number of argu-
       ments  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 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 variable argument list, i.e., the last parame-
       ter of which the calling function knows the type.

       Because the address of  this  parameter  is  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_start macro returns no value.

       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 specified type  can  be  obtained  simply  by
       adding a * to type.

       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.

       The first use of  the  va_arg  macro  after  that  of  the
       va_start  macro  returns the argument after last.  Succes-
       sive invocations return the values of the remaining  argu-

       The va_end macro handles a normal return from the function
       whose variable argument list was initialized by  va_start.

<h3>BSD MANPAGE              29 November 1993                       1

<h3>STDARG(3)           Linux Programmer's Manual           STDARG(3)

       The va_end macro returns no value.

</h3>       The  function  foo takes a string of format characters and
       prints out the argument associated with each format  char-
       acter based on the type.
              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 */
                             c = va_arg(ap, char);
                             printf("char %c\n", c);

</h3>       The  va_start,  va_arg,  and va_end macros conform to ANSI
       C3.159-1989 (``ANSI C'').

</h3>       These macros are not compatible with the  historic  macros
       they  replace.  A backward compatible version can be found
       in the include file varargs.h.

</h3>       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  argu-
       ments on to a function that takes a va_list argument, such
       as vfprintf(3).

<h3>BSD MANPAGE              29 November 1993                       2

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