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| <title>The C Preprocessor: Overview</title> |
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| Next: <a href="Header-Files.html#Header-Files" accesskey="n" rel="next">Header Files</a>, Previous: <a href="index.html#Top" accesskey="p" rel="prev">Top</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="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
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| <a name="Overview-1"></a> |
| <h2 class="chapter">1 Overview</h2> |
| <p>The C preprocessor, often known as <em>cpp</em>, is a <em>macro processor</em> |
| that is used automatically by the C compiler to transform your program |
| before compilation. It is called a macro processor because it allows |
| you to define <em>macros</em>, which are brief abbreviations for longer |
| constructs. |
| </p> |
| <p>The C preprocessor is intended to be used only with C, C++, and |
| Objective-C source code. In the past, it has been abused as a general |
| text processor. It will choke on input which does not obey C’s lexical |
| rules. For example, apostrophes will be interpreted as the beginning of |
| character constants, and cause errors. Also, you cannot rely on it |
| preserving characteristics of the input which are not significant to |
| C-family languages. If a Makefile is preprocessed, all the hard tabs |
| will be removed, and the Makefile will not work. |
| </p> |
| <p>Having said that, you can often get away with using cpp on things which |
| are not C. Other Algol-ish programming languages are often safe |
| (Pascal, Ada, etc.) So is assembly, with caution. <samp>-traditional-cpp</samp> |
| mode preserves more white space, and is otherwise more permissive. Many |
| of the problems can be avoided by writing C or C++ style comments |
| instead of native language comments, and keeping macros simple. |
| </p> |
| <p>Wherever possible, you should use a preprocessor geared to the language |
| you are writing in. Modern versions of the GNU assembler have macro |
| facilities. Most high level programming languages have their own |
| conditional compilation and inclusion mechanism. If all else fails, |
| try a true general text processor, such as GNU M4. |
| </p> |
| <p>C preprocessors vary in some details. This manual discusses the GNU C |
| preprocessor, which provides a small superset of the features of ISO |
| Standard C. In its default mode, the GNU C preprocessor does not do a |
| few things required by the standard. These are features which are |
| rarely, if ever, used, and may cause surprising changes to the meaning |
| of a program which does not expect them. To get strict ISO Standard C, |
| you should use the <samp>-std=c90</samp>, <samp>-std=c99</samp> or |
| <samp>-std=c11</samp> options, depending |
| on which version of the standard you want. To get all the mandatory |
| diagnostics, you must also use <samp>-pedantic</samp>. See <a href="Invocation.html#Invocation">Invocation</a>. |
| </p> |
| <p>This manual describes the behavior of the ISO preprocessor. To |
| minimize gratuitous differences, where the ISO preprocessor’s |
| behavior does not conflict with traditional semantics, the |
| traditional preprocessor should behave the same way. The various |
| differences that do exist are detailed in the section <a href="Traditional-Mode.html#Traditional-Mode">Traditional Mode</a>. |
| </p> |
| <p>For clarity, unless noted otherwise, references to ‘<samp>CPP</samp>’ in this |
| manual refer to GNU CPP. |
| </p> |
| <table class="menu" border="0" cellspacing="0"> |
| <tr><td align="left" valign="top">• <a href="Character-sets.html#Character-sets" accesskey="1">Character sets</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Initial-processing.html#Initial-processing" accesskey="2">Initial processing</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Tokenization.html#Tokenization" accesskey="3">Tokenization</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="The-preprocessing-language.html#The-preprocessing-language" accesskey="4">The preprocessing language</a>:</td><td> </td><td align="left" valign="top"> |
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