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| <a name="Line-Numbering"></a> |
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| <a name="Line-numbering"></a> |
| <h2 class="unnumbered">Line numbering</h2> |
| <a name="index-line-numbers"></a> |
| |
| <a name="Just-which-line-number-anyway_003f"></a> |
| <h3 class="section">Just which line number anyway?</h3> |
| |
| <p>There are three reasonable requirements a cpplib client might have for |
| the line number of a token passed to it: |
| </p> |
| <ul> |
| <li> The source line it was lexed on. |
| </li><li> The line it is output on. This can be different to the line it was |
| lexed on if, for example, there are intervening escaped newlines or |
| C-style comments. For example: |
| |
| <div class="smallexample"> |
| <pre class="smallexample">foo /* <span class="roman">A long |
| comment</span> */ bar \ |
| baz |
| ⇒ |
| foo bar baz |
| </pre></div> |
| |
| </li><li> If the token results from a macro expansion, the line of the macro name, |
| or possibly the line of the closing parenthesis in the case of |
| function-like macro expansion. |
| </li></ul> |
| |
| <p>The <code>cpp_token</code> structure contains <code>line</code> and <code>col</code> |
| members. The lexer fills these in with the line and column of the first |
| character of the token. Consequently, but maybe unexpectedly, a token |
| from the replacement list of a macro expansion carries the location of |
| the token within the <code>#define</code> directive, because cpplib expands a |
| macro by returning pointers to the tokens in its replacement list. The |
| current implementation of cpplib assigns tokens created from built-in |
| macros and the ‘<samp>#</samp>’ and ‘<samp>##</samp>’ operators the location of the most |
| recently lexed token. This is a because they are allocated from the |
| lexer’s token runs, and because of the way the diagnostic routines infer |
| the appropriate location to report. |
| </p> |
| <p>The diagnostic routines in cpplib display the location of the most |
| recently <em>lexed</em> token, unless they are passed a specific line and |
| column to report. For diagnostics regarding tokens that arise from |
| macro expansions, it might also be helpful for the user to see the |
| original location in the macro definition that the token came from. |
| Since that is exactly the information each token carries, such an |
| enhancement could be made relatively easily in future. |
| </p> |
| <p>The stand-alone preprocessor faces a similar problem when determining |
| the correct line to output the token on: the position attached to a |
| token is fairly useless if the token came from a macro expansion. All |
| tokens on a logical line should be output on its first physical line, so |
| the token’s reported location is also wrong if it is part of a physical |
| line other than the first. |
| </p> |
| <p>To solve these issues, cpplib provides a callback that is generated |
| whenever it lexes a preprocessing token that starts a new logical line |
| other than a directive. It passes this token (which may be a |
| <code>CPP_EOF</code> token indicating the end of the translation unit) to the |
| callback routine, which can then use the line and column of this token |
| to produce correct output. |
| </p> |
| <a name="Representation-of-line-numbers"></a> |
| <h3 class="section">Representation of line numbers</h3> |
| |
| <p>As mentioned above, cpplib stores with each token the line number that |
| it was lexed on. In fact, this number is not the number of the line in |
| the source file, but instead bears more resemblance to the number of the |
| line in the translation unit. |
| </p> |
| <p>The preprocessor maintains a monotonic increasing line count, which is |
| incremented at every new line character (and also at the end of any |
| buffer that does not end in a new line). Since a line number of zero is |
| useful to indicate certain special states and conditions, this variable |
| starts counting from one. |
| </p> |
| <p>This variable therefore uniquely enumerates each line in the translation |
| unit. With some simple infrastructure, it is straight forward to map |
| from this to the original source file and line number pair, saving space |
| whenever line number information needs to be saved. The code the |
| implements this mapping lies in the files <samp>line-map.c</samp> and |
| <samp>line-map.h</samp>. |
| </p> |
| <p>Command-line macros and assertions are implemented by pushing a buffer |
| containing the right hand side of an equivalent <code>#define</code> or |
| <code>#assert</code> directive. Some built-in macros are handled similarly. |
| Since these are all processed before the first line of the main input |
| file, it will typically have an assigned line closer to twenty than to |
| one. |
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