159 lines
7.4 KiB
HTML
159 lines
7.4 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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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The GNU C Preprocessor Internals: Line Numbering</title>
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<meta name="description" content="The GNU C Preprocessor Internals: Line Numbering">
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<meta name="keywords" content="The GNU C Preprocessor Internals: Line Numbering">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="index.html#Top" rel="up" title="Top">
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<link href="Guard-Macros.html#Guard-Macros" rel="next" title="Guard Macros">
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<link href="Token-Spacing.html#Token-Spacing" rel="prev" title="Token Spacing">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Line-Numbering"></a>
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<div class="header">
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<p>
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Next: <a href="Guard-Macros.html#Guard-Macros" accesskey="n" rel="next">Guard Macros</a>, Previous: <a href="Token-Spacing.html#Token-Spacing" accesskey="p" rel="prev">Token Spacing</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="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Line-numbering"></a>
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<h2 class="unnumbered">Line numbering</h2>
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<a name="index-line-numbers"></a>
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<a name="Just-which-line-number-anyway_003f"></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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</p>
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<ul>
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<li> The source line it was lexed on.
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</li><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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<div class="smallexample">
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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></div>
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</li><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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</li></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>#</samp>’ and ‘<samp>##</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>
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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>
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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>
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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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</p>
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<a name="Representation-of-line-numbers"></a>
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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>
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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>
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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>line-map.c</samp> and
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<samp>line-map.h</samp>.
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</p>
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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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</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="Guard-Macros.html#Guard-Macros" accesskey="n" rel="next">Guard Macros</a>, Previous: <a href="Token-Spacing.html#Token-Spacing" accesskey="p" rel="prev">Token Spacing</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="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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