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<title>The C Preprocessor: Preprocessor Output</title>
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<a name="Preprocessor-Output"></a>
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<p>
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Next: <a href="Traditional-Mode.html#Traditional-Mode" accesskey="n" rel="next">Traditional Mode</a>, Previous: <a href="Other-Directives.html#Other-Directives" accesskey="p" rel="prev">Other Directives</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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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<hr>
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<a name="Preprocessor-Output-1"></a>
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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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</p>
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<a name="index-output-format"></a>
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<p>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>
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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>
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<a name="index-linemarkers"></a>
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<p>Source file name and line number information is conveyed by lines
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of the form
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</p>
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<div class="smallexample">
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<pre class="smallexample"># <var>linenum</var> <var>filename</var> <var>flags</var>
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</pre></div>
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<p>These are called <em>linemarkers</em>. 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>
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<p>After the file name comes zero or more flags, which are ‘<samp>1</samp>’,
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‘<samp>2</samp>’, ‘<samp>3</samp>’, or ‘<samp>4</samp>’. If there are multiple flags, spaces
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separate them. Here is what the flags mean:
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</p>
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<dl compact="compact">
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<dt>‘<samp>1</samp>’</dt>
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<dd><p>This indicates the start of a new file.
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</p></dd>
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<dt>‘<samp>2</samp>’</dt>
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<dd><p>This indicates returning to a file (after having included another file).
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</p></dd>
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<dt>‘<samp>3</samp>’</dt>
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<dd><p>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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</p></dd>
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<dt>‘<samp>4</samp>’</dt>
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<dd><p>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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</p></dd>
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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>#line</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>
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<p>Some directives may be duplicated in the output of the preprocessor.
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These are ‘<samp>#ident</samp>’ (always), ‘<samp>#pragma</samp>’ (only if the
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preprocessor does not handle the pragma itself), and ‘<samp>#define</samp>’ and
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‘<samp>#undef</samp>’ (with certain debugging options). If this happens, the
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‘<samp>#</samp>’ of the directive will always be in the first column, and there
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will be no space between the ‘<samp>#</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>#</samp>’ and
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the directive name.
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</p>
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<div class="header">
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<p>
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Next: <a href="Traditional-Mode.html#Traditional-Mode" accesskey="n" rel="next">Traditional Mode</a>, Previous: <a href="Other-Directives.html#Other-Directives" accesskey="p" rel="prev">Other Directives</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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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