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.

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, or float to va_arg, because arguments seen by the called function are never char, short, or float.

Pre-ANSI C converts char and short arguments to int and converts float arguments to double before passing them to a function.

STANDARDS CONFORMANCE

<stdarg.h>: AES, SVID3, XPG4, FIPS 151-2, POSIX.1, ANSI C

va_arg: SVID3, XPG4, ANSI C

va_end: SVID3, XPG4, ANSI C

va_list: SVID3, XPG4, ANSI C

va_start: SVID3, XPG4, ANSI C

stdarg(5)

Technical documentation

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