| <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: <a rel="next" accesskey="n" href="Argument-Macros.html#Argument-Macros">Argument Macros</a>, |
| Previous: <a rel="previous" accesskey="p" href="How-Many-Arguments.html#How-Many-Arguments">How Many Arguments</a>, |
| Up: <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—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 int</code><!-- /@w --> (whether signed or not) are |
| promoted to either <code>int</code> or <code>unsigned 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> |
| |
