You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
154 lines
6.9 KiB
HTML
154 lines
6.9 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1987-2018 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 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>The preprocessing language (The C Preprocessor)</title>
|
|
|
|
<meta name="description" content="The preprocessing language (The C Preprocessor)">
|
|
<meta name="keywords" content="The preprocessing language (The C Preprocessor)">
|
|
<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="Overview.html#Overview" rel="up" title="Overview">
|
|
<link href="Header-Files.html#Header-Files" rel="next" title="Header Files">
|
|
<link href="Tokenization.html#Tokenization" rel="prev" title="Tokenization">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
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.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<a name="The-preprocessing-language"></a>
|
|
<div class="header">
|
|
<p>
|
|
Previous: <a href="Tokenization.html#Tokenization" accesskey="p" rel="prev">Tokenization</a>, Up: <a href="Overview.html#Overview" accesskey="u" rel="up">Overview</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="The-preprocessing-language-1"></a>
|
|
<h3 class="section">1.4 The preprocessing language</h3>
|
|
<a name="index-directives"></a>
|
|
<a name="index-preprocessing-directives"></a>
|
|
<a name="index-directive-line"></a>
|
|
<a name="index-directive-name"></a>
|
|
|
|
<p>After tokenization, the stream of tokens may simply be passed straight
|
|
to the compiler’s parser. However, if it contains any operations in the
|
|
<em>preprocessing language</em>, it will be transformed first. This stage
|
|
corresponds roughly to the standard’s “translation phase 4” and is
|
|
what most people think of as the preprocessor’s job.
|
|
</p>
|
|
<p>The preprocessing language consists of <em>directives</em> to be executed
|
|
and <em>macros</em> to be expanded. Its primary capabilities are:
|
|
</p>
|
|
<ul>
|
|
<li> Inclusion of header files. These are files of declarations that can be
|
|
substituted into your program.
|
|
|
|
</li><li> Macro expansion. You can define <em>macros</em>, which are abbreviations
|
|
for arbitrary fragments of C code. The preprocessor will replace the
|
|
macros with their definitions throughout the program. Some macros are
|
|
automatically defined for you.
|
|
|
|
</li><li> Conditional compilation. You can include or exclude parts of the
|
|
program according to various conditions.
|
|
|
|
</li><li> Line control. If you use a program to combine or rearrange source files
|
|
into an intermediate file which is then compiled, you can use line
|
|
control to inform the compiler where each source line originally came
|
|
from.
|
|
|
|
</li><li> Diagnostics. You can detect problems at compile time and issue errors
|
|
or warnings.
|
|
</li></ul>
|
|
|
|
<p>There are a few more, less useful, features.
|
|
</p>
|
|
<p>Except for expansion of predefined macros, all these operations are
|
|
triggered with <em>preprocessing directives</em>. Preprocessing directives
|
|
are lines in your program that start with ‘<samp>#</samp>’. Whitespace is
|
|
allowed before and after the ‘<samp>#</samp>’. The ‘<samp>#</samp>’ is followed by an
|
|
identifier, the <em>directive name</em>. It specifies the operation to
|
|
perform. Directives are commonly referred to as ‘<samp>#<var>name</var></samp>’
|
|
where <var>name</var> is the directive name. For example, ‘<samp>#define</samp>’ is
|
|
the directive that defines a macro.
|
|
</p>
|
|
<p>The ‘<samp>#</samp>’ which begins a directive cannot come from a macro
|
|
expansion. Also, the directive name is not macro expanded. Thus, if
|
|
<code>foo</code> is defined as a macro expanding to <code>define</code>, that does
|
|
not make ‘<samp>#foo</samp>’ a valid preprocessing directive.
|
|
</p>
|
|
<p>The set of valid directive names is fixed. Programs cannot define new
|
|
preprocessing directives.
|
|
</p>
|
|
<p>Some directives require arguments; these make up the rest of the
|
|
directive line and must be separated from the directive name by
|
|
whitespace. For example, ‘<samp>#define</samp>’ must be followed by a macro
|
|
name and the intended expansion of the macro.
|
|
</p>
|
|
<p>A preprocessing directive cannot cover more than one line. The line
|
|
may, however, be continued with backslash-newline, or by a block comment
|
|
which extends past the end of the line. In either case, when the
|
|
directive is processed, the continuations have already been merged with
|
|
the first line to make one long line.
|
|
</p>
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
Previous: <a href="Tokenization.html#Tokenization" accesskey="p" rel="prev">Tokenization</a>, Up: <a href="Overview.html#Overview" accesskey="u" rel="up">Overview</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>
|