NAME    [Toc]    [Back]

       stdarg - variable argument lists

SYNOPSIS    [Toc]    [Back]

       #include <stdarg.h>

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

DESCRIPTION    [Toc]    [Back]

       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  variable
 argument list, i.e., the last parameter 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.   Successive  invocations  return  the
       values of the remaining arguments.

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

       The va_end macro returns no value.

EXAMPLES    [Toc]    [Back]

       The function foo takes a string of format characters and prints out the
       argument associated with each format character 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);
			     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);
	      }

CONFORMING TO    [Toc]    [Back]

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

COMPATIBILITY    [Toc]    [Back]

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

BUGS    [Toc]    [Back]

       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
       vfprintf(3).



BSD MANPAGE			  1993-11-29			     STDARG(3)