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120 lines
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<a name="The-preprocessing-language"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Tokenization.html#Tokenization">Tokenization</a>,
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Up: <a rel="up" accesskey="u" href="Overview.html#Overview">Overview</a>
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<h3 class="section">1.4 The preprocessing language</h3>
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<p><a name="index-directives-21"></a><a name="index-preprocessing-directives-22"></a><a name="index-directive-line-23"></a><a name="index-directive-name-24"></a>
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After tokenization, the stream of tokens may simply be passed straight
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to the compiler's parser. However, if it contains any operations in the
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<dfn>preprocessing language</dfn>, it will be transformed first. This stage
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corresponds roughly to the standard's “translation phase 4” and is
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what most people think of as the preprocessor's job.
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<p>The preprocessing language consists of <dfn>directives</dfn> to be executed
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and <dfn>macros</dfn> to be expanded. Its primary capabilities are:
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<ul>
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<li>Inclusion of header files. These are files of declarations that can be
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substituted into your program.
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<li>Macro expansion. You can define <dfn>macros</dfn>, which are abbreviations
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for arbitrary fragments of C code. The preprocessor will replace the
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macros with their definitions throughout the program. Some macros are
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automatically defined for you.
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<li>Conditional compilation. You can include or exclude parts of the
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program according to various conditions.
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<li>Line control. If you use a program to combine or rearrange source files
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into an intermediate file which is then compiled, you can use line
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control to inform the compiler where each source line originally came
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from.
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<li>Diagnostics. You can detect problems at compile time and issue errors
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or warnings.
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</ul>
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<p>There are a few more, less useful, features.
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<p>Except for expansion of predefined macros, all these operations are
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triggered with <dfn>preprocessing directives</dfn>. Preprocessing directives
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are lines in your program that start with ‘<samp><span class="samp">#</span></samp>’. Whitespace is
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allowed before and after the ‘<samp><span class="samp">#</span></samp>’. The ‘<samp><span class="samp">#</span></samp>’ is followed by an
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identifier, the <dfn>directive name</dfn>. It specifies the operation to
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perform. Directives are commonly referred to as ‘<samp><span class="samp">#</span><var>name</var></samp>’
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where <var>name</var> is the directive name. For example, ‘<samp><span class="samp">#define</span></samp>’ is
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the directive that defines a macro.
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<p>The ‘<samp><span class="samp">#</span></samp>’ which begins a directive cannot come from a macro
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expansion. Also, the directive name is not macro expanded. Thus, if
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<code>foo</code> is defined as a macro expanding to <code>define</code>, that does
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not make ‘<samp><span class="samp">#foo</span></samp>’ a valid preprocessing directive.
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<p>The set of valid directive names is fixed. Programs cannot define new
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preprocessing directives.
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<p>Some directives require arguments; these make up the rest of the
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directive line and must be separated from the directive name by
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whitespace. For example, ‘<samp><span class="samp">#define</span></samp>’ must be followed by a macro
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name and the intended expansion of the macro.
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<p>A preprocessing directive cannot cover more than one line. The line
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may, however, be continued with backslash-newline, or by a block comment
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which extends past the end of the line. In either case, when the
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directive is processed, the continuations have already been merged with
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the first line to make one long line.
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</body></html>
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