stdarg.5 (2010 09)

s
stdarg(5) stdarg(5)
NAME
stdarg.h - macros for handling variable argument lists
SYNOPSIS
#include <stdarg.h>
void va_start(va_list pvar, argN);
type va_arg(va_list pvar, type);
void va_end(va_list pvar);
DESCRIPTION
The <stdarg.h> header contains a set of macros that can be used to write portable procedures that
accept variable argument lists. Routines that have variable argument lists (such as
printf()) but do
not use stdarg are inherently nonportable, because different machines use different argument-passing
conventions.
va_list is a type defined for the variable used to traverse the list.
va_start is called to initialize pvar to the beginning of the list. The type of argN should be the same as
the argument to the function just before the variable portion of the argument list.
va_arg returns the next argument in the list pointed to by pvar . type is the type the argument is
expected to be. Different types can be mixed, but it is up to the routine to know what type of argument is
expected, because it cannot be determined at runtime.
va_end is used to clean up.
Multiple traversals, each bracketed by
va_start ... va_end, are possible.
NOTE: The
<stdarg.h> header file supercedes the <varargs.h> header, and contains all of the
varargs macros. <varargs.h> is provided for compatibility with pre-ANSI compilers and earlier
releases of HP C/HP-UX.
EXAMPLE
This example is a possible implementation of
execl (see exec (2)):
#include <stdarg.h>
#define MAXARGS 100
/* execl is called by
execl(file, arg1, arg2, ..., (char *)0);
*/
execl(const char *file, const char *args, ...)
{
va_list ap;
char *array[MAXARGS];
int argno = 0;
va_start(ap, args);
if ((array[0] = args) != 0)
while ((array[argno++] = va_arg(ap, char *)) != 0)
;
va_end(ap);
return execv(file, array);
}
WARNINGS
It is up to the calling routine to specify how many arguments there are, since it is not always possible to
determine this from the stack frame. For example,
execl() is passed a zero pointer to signal the end of
the list, and printf() can tell how many arguments are there by the format string.
Unless ANSI C is used, it is non-portable to specify a second argument of char , short ,orfloat to va_arg ,
because arguments seen by the called function are never char , short ,orfloat .
Pre-ANSI C converts char and short arguments to int and converts float arguments to double before pass-
ing them to a function.
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1