VARARGS.H(7) Miscellaneous Information Manual VARARGS.H(7)

NAME

varargs.h — handle variable argument list (LEGACY)

SYNOPSIS

#include <varargs.h>

va_alist
va_dcl
void va_start(pvar)
va_list pvar;
type va_arg(pvar, type)
va_list pvar;
void va_end(pvar)
va_list pvar;

DESCRIPTION

The <varargs.h> header contains a set of macros which allows portable procedures that accept variable argument lists to be written. Routines that have variable argument lists (such as printf(3) but do not use <varargs.h> are inherently non-portable, as different machines use different argument-passing conventions.

va_alist: Used as the parameter list in a function header.
va_dcl: A declaration for va_alist . No semicolon should follow va_dcl .
va_list: A type defined for the variable used to traverse the list.
va_start(): Called to initialise pvar to the beginning of the list.
va_arg(): Will return the next argument in the list pointed to by The argument 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, as it cannot be determined at run time.
va_end(): Used to clean up.

Multiple traversals, each bracketed by va_start() va_end(), are possible.

EXAMPLES

This example is a possible implementation of exec(3) .

#include <varargs.h>

#define MAXARGS    100

/*    execl is called by
 *        execl(file, arg1, arg2, ..., (char *)0);
 */
execl(va_alist)
va_dcl
{
    va_list ap;
    char *file;
    char *args[MAXARGS];
    int argno = 0;

    va_start(ap);
    file = va_arg(ap, char *);
    while ((args[argno++] = va_arg(ap, char *)) != (char *)0)
        ;
    va_end(ap);
    return execv(file, args);
}

APPLICATION USAGE

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, exec(3) is passed a zero pointer to signal the end of the list. The printf(3) function can tell how many arguments are there by the format.

It is non-portable to specify a second argument of char, short or float to va_arg(), since arguments seen by the called function are not type char, short or float. C language converts type char and short arguments to int and converts type float arguments to double before passing them to a function.

For backward compatibility with Issue 3, XSI-conformant systems support <varargs.h> as well as <stdarg.h>. Use of <varargs.h> is not recommended.

FUTURE DIRECTIONS

None.