diff options
| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
|---|---|---|
| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
| commit | abdaadbcae30fe0c9a66c7516798279fdfd97750 (patch) | |
| tree | 00a54a6e25601e43876d03c1a4a12a749d4a914c /share/doc/cpp/If.html | |
https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads
Change-Id: I7303388733328cd98ab9aa3c30236db67f2e9e9c
Diffstat (limited to 'share/doc/cpp/If.html')
| -rw-r--r-- | share/doc/cpp/If.html | 146 |
1 files changed, 146 insertions, 0 deletions
diff --git a/share/doc/cpp/If.html b/share/doc/cpp/If.html new file mode 100644 index 0000000..55b5152 --- /dev/null +++ b/share/doc/cpp/If.html @@ -0,0 +1,146 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1987-2023 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 5.1, 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="previous" 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="previous">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="previous">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> |