<html lang="en">
<head>
<title>Calling Variadics - The GNU C Library</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="The GNU C Library">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="How-Variadic.html#How-Variadic" title="How Variadic">
<link rel="prev" href="How-Many-Arguments.html#How-Many-Arguments" title="How Many Arguments">
<link rel="next" href="Argument-Macros.html#Argument-Macros" title="Argument Macros">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
This file documents the GNU C library.

This is Edition 0.12, last updated 2007-10-27,
of `The GNU C Library Reference Manual', for version
2.8 (Sourcery G++ Lite 2011.03-41).

Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
2003, 2007, 2008, 2010 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Free Software Needs Free Documentation''
and ``GNU Lesser General Public License'', the Front-Cover texts being
``A GNU Manual'', and with the Back-Cover Texts as in (a) below.  A
copy of the license is included in the section entitled "GNU Free
Documentation License".

(a) The FSF's Back-Cover Text is: ``You have the freedom to
copy and modify this GNU manual.  Buying copies from the FSF
supports it in developing GNU and promoting software freedom.''-->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
  pre.display { font-family:inherit }
  pre.format  { font-family:inherit }
  pre.smalldisplay { font-family:inherit; font-size:smaller }
  pre.smallformat  { font-family:inherit; font-size:smaller }
  pre.smallexample { font-size:smaller }
  pre.smalllisp    { font-size:smaller }
  span.sc    { font-variant:small-caps }
  span.roman { font-family:serif; font-weight:normal; } 
  span.sansserif { font-family:sans-serif; font-weight:normal; } 
--></style>
<link rel="stylesheet" type="text/css" href="../cs.css">
</head>
<body>
<div class="node">
<a name="Calling-Variadics"></a>
<p>
Next:&nbsp;<a rel="next" accesskey="n" href="Argument-Macros.html#Argument-Macros">Argument Macros</a>,
Previous:&nbsp;<a rel="previous" accesskey="p" href="How-Many-Arguments.html#How-Many-Arguments">How Many Arguments</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="How-Variadic.html#How-Variadic">How Variadic</a>
<hr>
</div>

<h5 class="subsubsection">A.2.2.4 Calling Variadic Functions</h5>

<p><a name="index-variadic-functions_002c-calling-3721"></a><a name="index-calling-variadic-functions-3722"></a><a name="index-declaring-variadic-functions-3723"></a>
You don't have to do anything special to call a variadic function. 
Just put the arguments (required arguments, followed by optional ones)
inside parentheses, separated by commas, as usual.  But you must declare
the function with a prototype and know how the argument values are converted.

   <p>In principle, functions that are <em>defined</em> to be variadic must also
be <em>declared</em> to be variadic using a function prototype whenever
you call them.  (See <a href="Variadic-Prototypes.html#Variadic-Prototypes">Variadic Prototypes</a>, for how.)  This is because
some C compilers use a different calling convention to pass the same set
of argument values to a function depending on whether that function
takes variable arguments or fixed arguments.

   <p>In practice, the GNU C compiler always passes a given set of argument
types in the same way regardless of whether they are optional or
required.  So, as long as the argument types are self-promoting, you can
safely omit declaring them.  Usually it is a good idea to declare the
argument types for variadic functions, and indeed for all functions. 
But there are a few functions which it is extremely convenient not to
have to declare as variadic&mdash;for example, <code>open</code> and
<code>printf</code>.

   <p><a name="index-default-argument-promotions-3724"></a><a name="index-argument-promotion-3725"></a>Since the prototype doesn't specify types for optional arguments, in a
call to a variadic function the <dfn>default argument promotions</dfn> are
performed on the optional argument values.  This means the objects of
type <code>char</code> or <code>short&nbsp;int</code><!-- /@w --> (whether signed or not) are
promoted to either <code>int</code> or <code>unsigned&nbsp;int</code><!-- /@w -->, as
appropriate; and that objects of type <code>float</code> are promoted to type
<code>double</code>.  So, if the caller passes a <code>char</code> as an optional
argument, it is promoted to an <code>int</code>, and the function can access
it with <code>va_arg (</code><var>ap</var><code>, int)</code>.

   <p>Conversion of the required arguments is controlled by the function
prototype in the usual way: the argument expression is converted to the
declared argument type as if it were being assigned to a variable of
that type.

   </body></html>