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123 lines
5.7 KiB
HTML
123 lines
5.7 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Line Numbering - The GNU C Preprocessor Internals</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="The GNU C Preprocessor Internals">
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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="Token-Spacing.html#Token-Spacing" title="Token Spacing">
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<link rel="next" href="Guard-Macros.html#Guard-Macros" title="Guard Macros">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<style type="text/css"><!--
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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="Line-Numbering"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Guard-Macros.html#Guard-Macros">Guard Macros</a>,
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Previous: <a rel="previous" accesskey="p" href="Token-Spacing.html#Token-Spacing">Token Spacing</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="unnumbered">Line numbering</h2>
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<p><a name="index-line-numbers-17"></a>
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<h3 class="section">Just which line number anyway?</h3>
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<p>There are three reasonable requirements a cpplib client might have for
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the line number of a token passed to it:
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<ul>
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<li>The source line it was lexed on.
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<li>The line it is output on. This can be different to the line it was
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lexed on if, for example, there are intervening escaped newlines or
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C-style comments. For example:
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<pre class="smallexample"> foo /* <span class="roman">A long
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comment</span> */ bar \
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baz
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⇒
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foo bar baz
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</pre>
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<li>If the token results from a macro expansion, the line of the macro name,
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or possibly the line of the closing parenthesis in the case of
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function-like macro expansion.
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</ul>
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<p>The <code>cpp_token</code> structure contains <code>line</code> and <code>col</code>
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members. The lexer fills these in with the line and column of the first
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character of the token. Consequently, but maybe unexpectedly, a token
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from the replacement list of a macro expansion carries the location of
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the token within the <code>#define</code> directive, because cpplib expands a
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macro by returning pointers to the tokens in its replacement list. The
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current implementation of cpplib assigns tokens created from built-in
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macros and the ‘<samp><span class="samp">#</span></samp>’ and ‘<samp><span class="samp">##</span></samp>’ operators the location of the most
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recently lexed token. This is a because they are allocated from the
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lexer's token runs, and because of the way the diagnostic routines infer
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the appropriate location to report.
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<p>The diagnostic routines in cpplib display the location of the most
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recently <em>lexed</em> token, unless they are passed a specific line and
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column to report. For diagnostics regarding tokens that arise from
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macro expansions, it might also be helpful for the user to see the
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original location in the macro definition that the token came from.
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Since that is exactly the information each token carries, such an
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enhancement could be made relatively easily in future.
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<p>The stand-alone preprocessor faces a similar problem when determining
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the correct line to output the token on: the position attached to a
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token is fairly useless if the token came from a macro expansion. All
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tokens on a logical line should be output on its first physical line, so
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the token's reported location is also wrong if it is part of a physical
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line other than the first.
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<p>To solve these issues, cpplib provides a callback that is generated
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whenever it lexes a preprocessing token that starts a new logical line
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other than a directive. It passes this token (which may be a
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<code>CPP_EOF</code> token indicating the end of the translation unit) to the
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callback routine, which can then use the line and column of this token
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to produce correct output.
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<h3 class="section">Representation of line numbers</h3>
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<p>As mentioned above, cpplib stores with each token the line number that
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it was lexed on. In fact, this number is not the number of the line in
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the source file, but instead bears more resemblance to the number of the
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line in the translation unit.
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<p>The preprocessor maintains a monotonic increasing line count, which is
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incremented at every new line character (and also at the end of any
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buffer that does not end in a new line). Since a line number of zero is
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useful to indicate certain special states and conditions, this variable
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starts counting from one.
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<p>This variable therefore uniquely enumerates each line in the translation
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unit. With some simple infrastructure, it is straight forward to map
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from this to the original source file and line number pair, saving space
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whenever line number information needs to be saved. The code the
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implements this mapping lies in the files <samp><span class="file">line-map.c</span></samp> and
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<samp><span class="file">line-map.h</span></samp>.
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<p>Command-line macros and assertions are implemented by pushing a buffer
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containing the right hand side of an equivalent <code>#define</code> or
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<code>#assert</code> directive. Some built-in macros are handled similarly.
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Since these are all processed before the first line of the main input
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file, it will typically have an assigned line closer to twenty than to
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one.
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</body></html>
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