244 lines
11 KiB
HTML
244 lines
11 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2016 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled "GNU Free Documentation License".
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The C Preprocessor: Initial processing</title>
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<meta name="description" content="The C Preprocessor: Initial processing">
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<meta name="keywords" content="The C Preprocessor: Initial processing">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Overview.html#Overview" rel="up" title="Overview">
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<link href="Tokenization.html#Tokenization" rel="next" title="Tokenization">
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<link href="Character-sets.html#Character-sets" rel="prev" title="Character sets">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smalllisp {font-size: smaller}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Initial-processing"></a>
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<div class="header">
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<p>
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Next: <a href="Tokenization.html#Tokenization" accesskey="n" rel="next">Tokenization</a>, Previous: <a href="Character-sets.html#Character-sets" accesskey="p" rel="prev">Character sets</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>
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</div>
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<hr>
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<a name="Initial-processing-1"></a>
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<h3 class="section">1.2 Initial processing</h3>
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<p>The preprocessor performs a series of textual transformations on its
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input. These happen before all other processing. Conceptually, they
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happen in a rigid order, and the entire file is run through each
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transformation before the next one begins. CPP actually does them
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all at once, for performance reasons. These transformations correspond
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roughly to the first three “phases of translation” described in the C
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standard.
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</p>
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<ol>
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<li> <a name="index-line-endings"></a>
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The input file is read into memory and broken into lines.
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<p>Different systems use different conventions to indicate the end of a
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line. GCC accepts the ASCII control sequences <kbd>LF</kbd>, <kbd>CR LF<!-- /@w --></kbd> and <kbd>CR</kbd> as end-of-line markers. These are the canonical
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sequences used by Unix, DOS and VMS, and the classic Mac OS (before
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OSX) respectively. You may therefore safely copy source code written
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on any of those systems to a different one and use it without
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conversion. (GCC may lose track of the current line number if a file
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doesn’t consistently use one convention, as sometimes happens when it
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is edited on computers with different conventions that share a network
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file system.)
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</p>
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<p>If the last line of any input file lacks an end-of-line marker, the end
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of the file is considered to implicitly supply one. The C standard says
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that this condition provokes undefined behavior, so GCC will emit a
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warning message.
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</p>
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</li><li> <a name="index-trigraphs"></a>
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<a name="trigraphs"></a>If trigraphs are enabled, they are replaced by their
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corresponding single characters. By default GCC ignores trigraphs,
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but if you request a strictly conforming mode with the <samp>-std</samp>
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option, or you specify the <samp>-trigraphs</samp> option, then it
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converts them.
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<p>These are nine three-character sequences, all starting with ‘<samp>??</samp>’,
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that are defined by ISO C to stand for single characters. They permit
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obsolete systems that lack some of C’s punctuation to use C. For
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example, ‘<samp>??/</samp>’ stands for ‘<samp>\</samp>’, so <tt>'??/n'</tt> is a character
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constant for a newline.
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</p>
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<p>Trigraphs are not popular and many compilers implement them
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incorrectly. Portable code should not rely on trigraphs being either
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converted or ignored. With <samp>-Wtrigraphs</samp> GCC will warn you
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when a trigraph may change the meaning of your program if it were
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converted. See <a href="Invocation.html#Wtrigraphs">Wtrigraphs</a>.
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</p>
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<p>In a string constant, you can prevent a sequence of question marks
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from being confused with a trigraph by inserting a backslash between
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the question marks, or by separating the string literal at the
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trigraph and making use of string literal concatenation. <tt>"(??\?)"</tt>
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is the string ‘<samp>(???)</samp>’, not ‘<samp>(?]</samp>’. Traditional C compilers
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do not recognize these idioms.
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</p>
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<p>The nine trigraphs and their replacements are
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</p>
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<div class="smallexample">
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<pre class="smallexample">Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
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Replacement: [ ] { } # \ ^ | ~
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</pre></div>
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</li><li> <a name="index-continued-lines"></a>
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<a name="index-backslash_002dnewline"></a>
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Continued lines are merged into one long line.
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<p>A continued line is a line which ends with a backslash, ‘<samp>\</samp>’. The
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backslash is removed and the following line is joined with the current
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one. No space is inserted, so you may split a line anywhere, even in
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the middle of a word. (It is generally more readable to split lines
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only at white space.)
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</p>
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<p>The trailing backslash on a continued line is commonly referred to as a
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<em>backslash-newline</em>.
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</p>
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<p>If there is white space between a backslash and the end of a line, that
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is still a continued line. However, as this is usually the result of an
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editing mistake, and many compilers will not accept it as a continued
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line, GCC will warn you about it.
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</p>
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</li><li> <a name="index-comments"></a>
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<a name="index-line-comments"></a>
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<a name="index-block-comments"></a>
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All comments are replaced with single spaces.
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<p>There are two kinds of comments. <em>Block comments</em> begin with
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‘<samp>/*</samp>’ and continue until the next ‘<samp>*/</samp>’. Block comments do not
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nest:
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</p>
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<div class="smallexample">
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<pre class="smallexample">/* <span class="roman">this is</span> /* <span class="roman">one comment</span> */ <span class="roman">text outside comment</span>
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</pre></div>
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<p><em>Line comments</em> begin with ‘<samp>//</samp>’ and continue to the end of the
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current line. Line comments do not nest either, but it does not matter,
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because they would end in the same place anyway.
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</p>
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<div class="smallexample">
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<pre class="smallexample">// <span class="roman">this is</span> // <span class="roman">one comment</span>
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<span class="roman">text outside comment</span>
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</pre></div>
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</li></ol>
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<p>It is safe to put line comments inside block comments, or vice versa.
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</p>
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<div class="smallexample">
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<pre class="smallexample">/* <span class="roman">block comment</span>
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// <span class="roman">contains line comment</span>
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<span class="roman">yet more comment</span>
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*/ <span class="roman">outside comment</span>
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// <span class="roman">line comment</span> /* <span class="roman">contains block comment</span> */
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</pre></div>
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<p>But beware of commenting out one end of a block comment with a line
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comment.
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</p>
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<div class="smallexample">
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<pre class="smallexample"> // <span class="roman">l.c.</span> /* <span class="roman">block comment begins</span>
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<span class="roman">oops! this isn’t a comment anymore</span> */
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</pre></div>
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<p>Comments are not recognized within string literals.
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<tt>"/* blah */"<!-- /@w --></tt> is the string constant ‘<samp>/* blah */<!-- /@w --></samp>’, not
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an empty string.
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</p>
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<p>Line comments are not in the 1989 edition of the C standard, but they
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are recognized by GCC as an extension. In C++ and in the 1999 edition
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of the C standard, they are an official part of the language.
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</p>
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<p>Since these transformations happen before all other processing, you can
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split a line mechanically with backslash-newline anywhere. You can
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comment out the end of a line. You can continue a line comment onto the
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next line with backslash-newline. You can even split ‘<samp>/*</samp>’,
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‘<samp>*/</samp>’, and ‘<samp>//</samp>’ onto multiple lines with backslash-newline.
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For example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">/\
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*
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*/ # /*
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*/ defi\
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ne FO\
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O 10\
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20
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</pre></div>
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<p>is equivalent to <code>#define FOO 1020<!-- /@w --></code>. All these tricks are
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extremely confusing and should not be used in code intended to be
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readable.
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</p>
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<p>There is no way to prevent a backslash at the end of a line from being
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interpreted as a backslash-newline. This cannot affect any correct
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program, however.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Tokenization.html#Tokenization" accesskey="n" rel="next">Tokenization</a>, Previous: <a href="Character-sets.html#Character-sets" accesskey="p" rel="prev">Character sets</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>
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</div>
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</body>
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</html>
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