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<HTML><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><!-- Created on September, 12 2004 by texi2html 1.64 --><!-- Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author) Karl Berry <karl@freefriends.org> Olaf Bachmann <obachman@mathematik.uni-kl.de> and many others.Maintained by: Olaf Bachmann <obachman@mathematik.uni-kl.de>Send bugs and suggestions to <texi2html@mathematik.uni-kl.de> --><HEAD><TITLE>Autoconf, Automake, and Libtool: Position Independent Code</TITLE><META NAME="description" CONTENT="Autoconf, Automake, and Libtool: Position Independent Code"><META NAME="keywords" CONTENT="Autoconf, Automake, and Libtool: Position Independent Code"><META NAME="resource-type" CONTENT="document"><META NAME="distribution" CONTENT="global"><META NAME="Generator" CONTENT="texi2html 1.64"><script language="Javascript"><!-- // Check the browser version. function checkVersion() { if (navigator.appVersion.charAt(0)>=3) return true; if (navigator.appVersion.charAt(0)>=4) return true; 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There are occasional architectures which don'tmake the distinction, usually because all object code is positionindependent by virtue of the ABI<A NAME="DOCF15" HREF="autobook_fot.html#FOOT15">(15)</A>, or less often because the loadaddress of the object is fixed at compile time (which implies thatshared libraries are not supported by such a platform).<A NAME="IDX7"></A>If an object is compiled as position independent code (PIC), thenthe operating system can load the object at <EM>any</EM> address inpreparation for execution. This involves a time overhead, in replacingdirect address references with relative addresses at compile time, and aspace overhead, in maintaining information to help the runtime loaderfill in the unresolved addresses at runtime. Consequently, PICobjects are usually slightly larger and slower at runtime than theequivalent non-PIC object. The advantage of sharing library codeon disk and in memory outweigh these problems as soon as the PICobject code in shared libraries is reused.<P><A NAME="IDX8"></A>PIC compilation is exactly what is required for objects which willbecome part of a shared library. Consequently, <CODE>libtool</CODE> buildsPIC objects for use in shared libraries and non-PIC objectsfor use in static libraries. Whenever <CODE>libtool</CODE> instructs thecompiler to generate a PIC object, it also defines the preprocessorsymbol, <SAMP>`PIC'</SAMP>, so that assembly code can be aware of whether itwill reside in a PIC object or not.</P><P>Typically, as <CODE>libtool</CODE> is compiling sources, it will generate a <SAMP>`.lo'</SAMP> object, as PIC, and a <SAMP>`.o'</SAMP> object, as non-PIC,and then it will use the appropriate one of the pair when linkingexecutables and libraries of various sorts. On architectures wherethere is no distinction, the <SAMP>`.lo'</SAMP> file is just a soft link to the<SAMP>`.o'</SAMP> file.</P><P>In practice, you can link PIC objects into a static archive for asmall overhead in execution and load speed, and often you can similarlylink non-PIC objects into shared archives. If you find that youneed to do this, <CODE>libtool</CODE> provides several ways to override thedefault behavior (see section <A HREF="autobook_69.html#SEC69">10.1 Creating <CODE>libtool</CODE></A>).</P><P><A NAME="Creating Shared Libraries with libtool"></A></TR></TABLE><BR> <FONT SIZE="-1">This document was generatedby <I>Gary V. Vaughan</I> on <I>September, 12 2004</I>using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html"><I>texi2html</I></A></BODY></HTML>⌨️ 快捷键说明
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