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