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196 lines
9.6 KiB
HTML
196 lines
9.6 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Initial processing - The C Preprocessor</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="The C Preprocessor">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Overview.html#Overview" title="Overview">
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<link rel="prev" href="Character-sets.html#Character-sets" title="Character sets">
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<link rel="next" href="Tokenization.html#Tokenization" title="Tokenization">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1987-2015 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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<meta http-equiv="Content-Style-Type" content="text/css">
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<style type="text/css"><!--
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span.roman { font-family:serif; font-weight:normal; }
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span.sansserif { font-family:sans-serif; font-weight:normal; }
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Initial-processing"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Tokenization.html#Tokenization">Tokenization</a>,
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Previous: <a rel="previous" accesskey="p" href="Character-sets.html#Character-sets">Character sets</a>,
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Up: <a rel="up" accesskey="u" href="Overview.html#Overview">Overview</a>
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<hr>
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</div>
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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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<ol type=1 start=1>
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<li><a name="index-line-endings-1"></a>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>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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<li><a name="index-trigraphs-2"></a><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><span class="option">-std</span></samp>
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option, or you specify the <samp><span class="option">-trigraphs</span></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><span class="samp">??</span></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><span class="samp">??/</span></samp>’ stands for ‘<samp><span class="samp">\</span></samp>’, so <tt>'??/n'</tt> is a character
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constant for a newline.
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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><span class="option">-Wtrigraphs</span></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="Wtrigraphs.html#Wtrigraphs">Wtrigraphs</a>.
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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><span class="samp">(???)</span></samp>’, not ‘<samp><span class="samp">(?]</span></samp>’. Traditional C compilers
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do not recognize these idioms.
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<p>The nine trigraphs and their replacements are
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<pre class="smallexample"> Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
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Replacement: [ ] { } # \ ^ | ~
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</pre>
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<li><a name="index-continued-lines-3"></a><a name="index-backslash_002dnewline-4"></a>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><span class="samp">\</span></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>The trailing backslash on a continued line is commonly referred to as a
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<dfn>backslash-newline</dfn>.
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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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<li><a name="index-comments-5"></a><a name="index-line-comments-6"></a><a name="index-block-comments-7"></a>All comments are replaced with single spaces.
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<p>There are two kinds of comments. <dfn>Block comments</dfn> begin with
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‘<samp><span class="samp">/*</span></samp>’ and continue until the next ‘<samp><span class="samp">*/</span></samp>’. Block comments do not
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nest:
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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>
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<p><dfn>Line comments</dfn> begin with ‘<samp><span class="samp">//</span></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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<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>
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</ol>
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<p>It is safe to put line comments inside block comments, or vice versa.
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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>
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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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<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>
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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><span class="samp">/* blah */<!-- /@w --></span></samp>’, not
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an empty string.
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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>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><span class="samp">/*</span></samp>’,
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‘<samp><span class="samp">*/</span></samp>’, and ‘<samp><span class="samp">//</span></samp>’ onto multiple lines with backslash-newline.
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For example:
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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>
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<p class="noindent">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>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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</body></html>
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