1<html lang="en"> 2<head> 3<title>Conditional Uses - The C Preprocessor</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="The C Preprocessor"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Conditionals.html#Conditionals" title="Conditionals"> 9<link rel="next" href="Conditional-Syntax.html#Conditional-Syntax" title="Conditional Syntax"> 10<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 11<!-- 12Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 131997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 142008, 2009, 2010, 2011 15Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation. A copy of 20the license is included in the 21section entitled ``GNU Free Documentation License''. 22 23This manual contains no Invariant Sections. The Front-Cover Texts are 24(a) (see below), and the Back-Cover Texts are (b) (see below). 25 26(a) The FSF's Front-Cover Text is: 27 28 A GNU Manual 29 30(b) The FSF's Back-Cover Text is: 31 32 You have freedom to copy and modify this GNU Manual, like GNU 33 software. Copies published by the Free Software Foundation raise 34 funds for GNU development. 35--> 36<meta http-equiv="Content-Style-Type" content="text/css"> 37<style type="text/css"><!-- 38 pre.display { font-family:inherit } 39 pre.format { font-family:inherit } 40 pre.smalldisplay { font-family:inherit; font-size:smaller } 41 pre.smallformat { font-family:inherit; font-size:smaller } 42 pre.smallexample { font-size:smaller } 43 pre.smalllisp { font-size:smaller } 44 span.sc { font-variant:small-caps } 45 span.roman { font-family:serif; font-weight:normal; } 46 span.sansserif { font-family:sans-serif; font-weight:normal; } 47--></style> 48<link rel="stylesheet" type="text/css" href="../cs.css"> 49</head> 50<body> 51<div class="node"> 52<a name="Conditional-Uses"></a> 53<p> 54Next: <a rel="next" accesskey="n" href="Conditional-Syntax.html#Conditional-Syntax">Conditional Syntax</a>, 55Up: <a rel="up" accesskey="u" href="Conditionals.html#Conditionals">Conditionals</a> 56<hr> 57</div> 58 59<h3 class="section">4.1 Conditional Uses</h3> 60 61<p>There are three general reasons to use a conditional. 62 63 <ul> 64<li>A program may need to use different code depending on the machine or 65operating system it is to run on. In some cases the code for one 66operating system may be erroneous on another operating system; for 67example, it might refer to data types or constants that do not exist on 68the other system. When this happens, it is not enough to avoid 69executing the invalid code. Its mere presence will cause the compiler 70to reject the program. With a preprocessing conditional, the offending 71code can be effectively excised from the program when it is not valid. 72 73 <li>You may want to be able to compile the same source file into two 74different programs. One version might make frequent time-consuming 75consistency checks on its intermediate data, or print the values of 76those data for debugging, and the other not. 77 78 <li>A conditional whose condition is always false is one way to exclude code 79from the program but keep it as a sort of comment for future reference. 80</ul> 81 82 <p>Simple programs that do not need system-specific logic or complex 83debugging hooks generally will not need to use preprocessing 84conditionals. 85 86 </body></html> 87 88