<html lang="en">
<head>
<title>Overview - The C Preprocessor</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="The C Preprocessor">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="prev" href="index.html#Top" title="Top">
<link rel="next" href="Header-Files.html#Header-Files" title="Header Files">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996,
1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,
2008, 2009, 2010
Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation.  A copy of
the license is included in the
section entitled ``GNU Free Documentation License''.

This manual contains no Invariant Sections.  The Front-Cover Texts are
(a) (see below), and the Back-Cover Texts are (b) (see below).

(a) The FSF's Front-Cover Text is:

     A GNU Manual

(b) The FSF's Back-Cover Text is:

     You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development.
-->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
  pre.display { font-family:inherit }
  pre.format  { font-family:inherit }
  pre.smalldisplay { font-family:inherit; font-size:smaller }
  pre.smallformat  { font-family:inherit; font-size:smaller }
  pre.smallexample { font-size:smaller }
  pre.smalllisp    { font-size:smaller }
  span.sc    { font-variant:small-caps }
  span.roman { font-family:serif; font-weight:normal; } 
  span.sansserif { font-family:sans-serif; font-weight:normal; } 
--></style>
<link rel="stylesheet" type="text/css" href="../cs.css">
</head>
<body>
<div class="node">
<a name="Overview"></a>
<p>
Next:&nbsp;<a rel="next" accesskey="n" href="Header-Files.html#Header-Files">Header Files</a>,
Previous:&nbsp;<a rel="previous" accesskey="p" href="index.html#Top">Top</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="index.html#Top">Top</a>
<hr>
</div>

<h2 class="chapter">1 Overview</h2>

<!-- man begin DESCRIPTION -->
<p>The C preprocessor, often known as <dfn>cpp</dfn>, is a <dfn>macro processor</dfn>
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 <dfn>macros</dfn>, which are brief abbreviations for longer
constructs.

   <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>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><span class="option">-traditional-cpp</span></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>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>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><span class="option">-std=c90</span></samp> or <samp><span class="option">-std=c99</span></samp> options, depending
on which version of the standard you want.  To get all the mandatory
diagnostics, you must also use <samp><span class="option">-pedantic</span></samp>.  See <a href="Invocation.html#Invocation">Invocation</a>.

   <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>For clarity, unless noted otherwise, references to &lsquo;<samp><span class="samp">CPP</span></samp>&rsquo; in this
manual refer to GNU CPP. 
<!-- man end -->

<ul class="menu">
<li><a accesskey="1" href="Character-sets.html#Character-sets">Character sets</a>
<li><a accesskey="2" href="Initial-processing.html#Initial-processing">Initial processing</a>
<li><a accesskey="3" href="Tokenization.html#Tokenization">Tokenization</a>
<li><a accesskey="4" href="The-preprocessing-language.html#The-preprocessing-language">The preprocessing language</a>
</ul>

   </body></html>