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124 lines
6.5 KiB
HTML
124 lines
6.5 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Preprocessor Output - 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="prev" href="Other-Directives.html#Other-Directives" title="Other Directives">
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<link rel="next" href="Traditional-Mode.html#Traditional-Mode" title="Traditional Mode">
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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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-->
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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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pre.display { font-family:inherit }
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pre.format { font-family:inherit }
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pre.smalldisplay { font-family:inherit; font-size:smaller }
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pre.smallformat { font-family:inherit; font-size:smaller }
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span.sc { font-variant:small-caps }
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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="Preprocessor-Output"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Traditional-Mode.html#Traditional-Mode">Traditional Mode</a>,
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Previous: <a rel="previous" accesskey="p" href="Other-Directives.html#Other-Directives">Other Directives</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<hr>
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</div>
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<h2 class="chapter">9 Preprocessor Output</h2>
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<p>When the C preprocessor is used with the C, C++, or Objective-C
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compilers, it is integrated into the compiler and communicates a stream
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of binary tokens directly to the compiler's parser. However, it can
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also be used in the more conventional standalone mode, where it produces
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textual output.
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<!-- FIXME: Document the library interface. -->
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<p><a name="index-output-format-109"></a>The output from the C preprocessor looks much like the input, except
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that all preprocessing directive lines have been replaced with blank
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lines and all comments with spaces. Long runs of blank lines are
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discarded.
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<p>The ISO standard specifies that it is implementation defined whether a
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preprocessor preserves whitespace between tokens, or replaces it with
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e.g. a single space. In GNU CPP, whitespace between tokens is collapsed
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to become a single space, with the exception that the first token on a
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non-directive line is preceded with sufficient spaces that it appears in
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the same column in the preprocessed output that it appeared in the
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original source file. This is so the output is easy to read.
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See <a href="Differences-from-previous-versions.html#Differences-from-previous-versions">Differences from previous versions</a>. CPP does not insert any
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whitespace where there was none in the original source, except where
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necessary to prevent an accidental token paste.
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<p><a name="index-linemarkers-110"></a>Source file name and line number information is conveyed by lines
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of the form
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<pre class="smallexample"> # <var>linenum</var> <var>filename</var> <var>flags</var>
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</pre>
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<p class="noindent">These are called <dfn>linemarkers</dfn>. They are inserted as needed into
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the output (but never within a string or character constant). They mean
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that the following line originated in file <var>filename</var> at line
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<var>linenum</var>. <var>filename</var> will never contain any non-printing
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characters; they are replaced with octal escape sequences.
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<p>After the file name comes zero or more flags, which are ‘<samp><span class="samp">1</span></samp>’,
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‘<samp><span class="samp">2</span></samp>’, ‘<samp><span class="samp">3</span></samp>’, or ‘<samp><span class="samp">4</span></samp>’. If there are multiple flags, spaces
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separate them. Here is what the flags mean:
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<dl>
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<dt>‘<samp><span class="samp">1</span></samp>’<dd>This indicates the start of a new file.
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<br><dt>‘<samp><span class="samp">2</span></samp>’<dd>This indicates returning to a file (after having included another file).
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<br><dt>‘<samp><span class="samp">3</span></samp>’<dd>This indicates that the following text comes from a system header file,
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so certain warnings should be suppressed.
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<br><dt>‘<samp><span class="samp">4</span></samp>’<dd>This indicates that the following text should be treated as being
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wrapped in an implicit <code>extern "C"</code> block.
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<!-- maybe cross reference NO_IMPLICIT_EXTERN_C -->
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</dl>
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<p>As an extension, the preprocessor accepts linemarkers in non-assembler
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input files. They are treated like the corresponding ‘<samp><span class="samp">#line</span></samp>’
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directive, (see <a href="Line-Control.html#Line-Control">Line Control</a>), except that trailing flags are
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permitted, and are interpreted with the meanings described above. If
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multiple flags are given, they must be in ascending order.
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<p>Some directives may be duplicated in the output of the preprocessor.
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These are ‘<samp><span class="samp">#ident</span></samp>’ (always), ‘<samp><span class="samp">#pragma</span></samp>’ (only if the
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preprocessor does not handle the pragma itself), and ‘<samp><span class="samp">#define</span></samp>’ and
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‘<samp><span class="samp">#undef</span></samp>’ (with certain debugging options). If this happens, the
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‘<samp><span class="samp">#</span></samp>’ of the directive will always be in the first column, and there
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will be no space between the ‘<samp><span class="samp">#</span></samp>’ and the directive name. If macro
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expansion happens to generate tokens which might be mistaken for a
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duplicated directive, a space will be inserted between the ‘<samp><span class="samp">#</span></samp>’ and
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the directive name.
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</body></html>
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