<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
<html>
|
<!-- Copyright (C) 1987-2016 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. A copy of
|
the license is included in the
|
section entitled "GNU Free Documentation License".
|
|
This manual contains no Invariant Sections. The Front-Cover Texts are
|
(a) (see below), and the Back-Cover Texts are (b) (see below).
|
|
(a) The FSF's Front-Cover Text is:
|
|
A GNU Manual
|
|
(b) The FSF's Back-Cover Text is:
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
software. Copies published by the Free Software Foundation raise
|
funds for GNU development. -->
|
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
|
<head>
|
<title>The C Preprocessor: If</title>
|
|
<meta name="description" content="The C Preprocessor: If">
|
<meta name="keywords" content="The C Preprocessor: If">
|
<meta name="resource-type" content="document">
|
<meta name="distribution" content="global">
|
<meta name="Generator" content="makeinfo">
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
<link href="index.html#Top" rel="start" title="Top">
|
<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
<link href="Conditional-Syntax.html#Conditional-Syntax" rel="up" title="Conditional Syntax">
|
<link href="Defined.html#Defined" rel="next" title="Defined">
|
<link href="Ifdef.html#Ifdef" rel="prev" title="Ifdef">
|
<style type="text/css">
|
<!--
|
a.summary-letter {text-decoration: none}
|
blockquote.smallquotation {font-size: smaller}
|
div.display {margin-left: 3.2em}
|
div.example {margin-left: 3.2em}
|
div.indentedblock {margin-left: 3.2em}
|
div.lisp {margin-left: 3.2em}
|
div.smalldisplay {margin-left: 3.2em}
|
div.smallexample {margin-left: 3.2em}
|
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
|
div.smalllisp {margin-left: 3.2em}
|
kbd {font-style:oblique}
|
pre.display {font-family: inherit}
|
pre.format {font-family: inherit}
|
pre.menu-comment {font-family: serif}
|
pre.menu-preformatted {font-family: serif}
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
pre.smallexample {font-size: smaller}
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
pre.smalllisp {font-size: smaller}
|
span.nocodebreak {white-space:nowrap}
|
span.nolinebreak {white-space:nowrap}
|
span.roman {font-family:serif; font-weight:normal}
|
span.sansserif {font-family:sans-serif; font-weight:normal}
|
ul.no-bullet {list-style: none}
|
-->
|
</style>
|
|
|
</head>
|
|
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
|
<a name="If"></a>
|
<div class="header">
|
<p>
|
Next: <a href="Defined.html#Defined" accesskey="n" rel="next">Defined</a>, Previous: <a href="Ifdef.html#Ifdef" accesskey="p" rel="prev">Ifdef</a>, Up: <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="u" rel="up">Conditional Syntax</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
|
</div>
|
<hr>
|
<a name="If-1"></a>
|
<h4 class="subsection">4.2.2 If</h4>
|
|
<p>The ‘<samp>#if</samp>’ directive allows you to test the value of an arithmetic
|
expression, rather than the mere existence of one macro. Its syntax is
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">#if <var>expression</var>
|
|
<var>controlled text</var>
|
|
#endif /* <var>expression</var> */
|
</pre></div>
|
|
<p><var>expression</var> is a C expression of integer type, subject to stringent
|
restrictions. It may contain
|
</p>
|
<ul>
|
<li> Integer constants.
|
|
</li><li> Character constants, which are interpreted as they would be in normal
|
code.
|
|
</li><li> Arithmetic operators for addition, subtraction, multiplication,
|
division, bitwise operations, shifts, comparisons, and logical
|
operations (<code>&&</code> and <code>||</code>). The latter two obey the usual
|
short-circuiting rules of standard C.
|
|
</li><li> Macros. All macros in the expression are expanded before actual
|
computation of the expression’s value begins.
|
|
</li><li> Uses of the <code>defined</code> operator, which lets you check whether macros
|
are defined in the middle of an ‘<samp>#if</samp>’.
|
|
</li><li> Identifiers that are not macros, which are all considered to be the
|
number zero. This allows you to write <code>#if MACRO<!-- /@w --></code> instead of
|
<code>#ifdef MACRO<!-- /@w --></code>, if you know that MACRO, when defined, will
|
always have a nonzero value. Function-like macros used without their
|
function call parentheses are also treated as zero.
|
|
<p>In some contexts this shortcut is undesirable. The <samp>-Wundef</samp>
|
option causes GCC to warn whenever it encounters an identifier which is
|
not a macro in an ‘<samp>#if</samp>’.
|
</p></li></ul>
|
|
<p>The preprocessor does not know anything about types in the language.
|
Therefore, <code>sizeof</code> operators are not recognized in ‘<samp>#if</samp>’, and
|
neither are <code>enum</code> constants. They will be taken as identifiers
|
which are not macros, and replaced by zero. In the case of
|
<code>sizeof</code>, this is likely to cause the expression to be invalid.
|
</p>
|
<p>The preprocessor calculates the value of <var>expression</var>. It carries
|
out all calculations in the widest integer type known to the compiler;
|
on most machines supported by GCC this is 64 bits. This is not the same
|
rule as the compiler uses to calculate the value of a constant
|
expression, and may give different results in some cases. If the value
|
comes out to be nonzero, the ‘<samp>#if</samp>’ succeeds and the <var>controlled
|
text</var> is included; otherwise it is skipped.
|
</p>
|
<hr>
|
<div class="header">
|
<p>
|
Next: <a href="Defined.html#Defined" accesskey="n" rel="next">Defined</a>, Previous: <a href="Ifdef.html#Ifdef" accesskey="p" rel="prev">Ifdef</a>, Up: <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="u" rel="up">Conditional Syntax</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
|
</div>
|
|
|
|
</body>
|
</html>
|
