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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: Library Versioning</TITLE><META NAME="description" CONTENT="Autoconf, Automake, and Libtool: Library Versioning"><META NAME="keywords" CONTENT="Autoconf, Automake, and Libtool: Library Versioning"><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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<TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_92.html#SEC92" onMouseover="img_act('back')" onMouseout="img_inact('back')"><IMG SRC="back.png" BORDER="0" ALT="FastBack: Convenience Libraries" ALIGN="MIDDLE" NAME="back"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_83.html#SEC83" onMouseover="img_act('up')" onMouseout="img_inact('up')"><IMG SRC="up.png" BORDER="0" ALT="Up: Using GNU Libtool" ALIGN="MIDDLE" NAME="up"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_93.html#SEC93" onMouseover="img_act('forward')" onMouseout="img_inact('forward')"><IMG SRC="forward.png" BORDER="0" ALT="FastForward: A Large GNU Autotools Project" ALIGN="MIDDLE" NAME="forward"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook.html#SEC_Top" onMouseover="img_act('home')" onMouseout="img_inact('home')"><IMG SRC="home.png" BORDER="0" ALT="Top: Autoconf, Automake, and Libtool" ALIGN="MIDDLE" NAME="home"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_toc.html#SEC_Contents" onMouseover="img_act('toc')" onMouseout="img_inact('toc')"><IMG SRC="toc.png" BORDER="0" ALT="Contents: Table of Contents" ALIGN="MIDDLE" NAME="toc"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_285.html#SEC285" onMouseover="img_act('index')" onMouseout="img_inact('index')"><IMG SRC="index.png" BORDER="0" ALT="Index: Index" ALIGN="MIDDLE" NAME="index"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_abt.html#SEC_About" onMouseover="img_act('help')" onMouseout="img_inact('help')"><IMG SRC="help.png" BORDER="0" ALT="About: About this document" ALIGN="MIDDLE" NAME="help"></A></TD></TR></TABLE></TD><TD ALIGN="LEFT"><H2> 11.4 Library Versioning </H2><!--docid::SEC91::--><P>It is important to note from the outset that the version number of yourproject is a very different thing to the version number of any librariesshipped with your project.  It is a common error for maintainers to tryto force their libraries to have the same version number as the currentrelease version of the package as a whole.  At best, they will breakbinary compatibility unnecessarily, so that their users won't gain thebenefits of the changes in their latest revision without relinking allapplications that use it.  At worst, they will allow the runtimelinker to load binary incompatible libraries, causing applications tocrash.</P><P>Far better, the Libtool versioning system will build native sharedlibraries with the correct <EM>native</EM> library version numbers.Although different architectures use various numbering schemes,Libtool abstracts these away behind the system described here.The various native library version numbering schemes are designed sothat when an executable is started, the runtime loader can, whereappropriate, choose a more recent installed library version than the onewith which the executable was actually built.  This allows you to fixbugs in your library, and having  built it with the correct Libtoolversion number, have those fixes propagate into any executables thatwere built with the old buggy version.  This can only work if theruntime loader can tell whether it can load the new library into the oldexecutable and expect them to work together.  The library versionnumbers give this information to the runtime loader, so it is veryimportant to set them correctly.</P><P>The version scheme used by Libtool tracks <EM>interfaces</EM>,where an interface is the set of exported entry points into the library.All Libtool libraries start with <SAMP>`-version-info'</SAMP> set to<SAMP>`0:0:0'</SAMP> -- this will be the default version number if you don'texplicitly set it on the Libtool link command line.  The meaning ofthese numbers (from left to right) is as follows:</P><P><DL COMPACT><DT><VAR>current</VAR><DD>The number of the current interface exported by the library.  A<VAR>current</VAR> value of <SAMP>`0'</SAMP>, means that you are calling theinterface exported by this library <EM>interface 0</EM>.<P><DT><VAR>revision</VAR><DD>The implementation number of the most recent interface exported by thislibrary.  In this case, a <VAR>revision</VAR> value of <SAMP>`0'</SAMP> means thatthis is the first implementation of the interface.<P>If the next release of this library exports the same interface, but hasa different implementation (perhaps some bugs have been fixed), the<VAR>revision</VAR> number will be higher, but <VAR>current</VAR> number will bethe same.  In that case, when given a choice, the library with thehighest <VAR>revision</VAR> will always be used by the runtime loader.</P><P><DT><VAR>age</VAR><DD>The number of previous additional interfaces supported by this library.If <VAR>age</VAR> were <SAMP>`2'</SAMP>, then this library can be linked intoexecutables which were built with a release of this library thatexported the current interface number, <VAR>current</VAR>, or any of theprevious two interfaces.<P>By definition <VAR>age</VAR> must be less than or equal to <VAR>current</VAR>.At the outset, only the first ever interface is implemented, so<VAR>age</VAR> can only be <SAMP>`0'</SAMP>.</DL><P>For later releases of a library, the <SAMP>`-version-info'</SAMP> argumentneeds to be set correctly depending on any interface changes you havemade. This is quite straightforward when you understand what the threenumbers mean:</P><P><OL><LI>If you have changed any of the sources for this library, the<VAR>revision</VAR> number must be incremented.  <STRONG>This is a newrevision of the current interface</STRONG>.<P><LI>If the interface has changed, then <VAR>current</VAR> must be incremented,and <VAR>revision</VAR> reset to <SAMP>`0'</SAMP>.  <STRONG>This is the first revisionof a new interface</STRONG>.<P><LI>If the new interface is a superset of the previous interface (that is,if the previous interface has not been broken by the changes in this newrelease), then <VAR>age</VAR> must be incremented.  <STRONG>This releaseis backwards compatible with the previous release</STRONG>.<P><LI>If the new interface has removed elements with respect to the previousinterface, then you have broken backward compatibility and <VAR>age</VAR>must be reset to <SAMP>`0'</SAMP>.  <STRONG>This release has a new, but backwardsincompatible interface</STRONG>.</OL><P><BLOCKQUOTE>For example, if the next release of the library included some newcommands for an existing socket protocol, you would use<CODE>-version-info 1:0:1</CODE>.  <STRONG>This is the first revision of anew interface.  This release is backwards compatible with the previousrelease</STRONG>.<P>Later, you implement a faster way of handling part of the algorithm atthe core of the library, and release it with <CODE>-version-info1:1:1</CODE>. <STRONG>This is a new revision of the current interface</STRONG>.</P><P>Unfortunately the speed of your new implementation can only be fullyexploited by changing the API to access the structures at a lowerlevel, which breaks compatibility with the previous interface, so yourelease it as <CODE>-version-info 2:0:0</CODE>.  <STRONG>This release has anew, but backwards incompatible interface</STRONG>.</BLOCKQUOTE><P>When deciding which numbers to change in the <CODE>-version-info</CODE>argument for a new release, you must remember that an interface changeis not limited to the API of the library.  The notion of aninterface must include any method by which a user (code or human) caninteract with the library: adding new builtin commands to a shelllibrary; the format used in an output file; the handshake protocolrequired for a client connecting over a socket, and so on.</P><P>Additionally, If you use a development model which has both a stable andan unstable tree being developed in parallel, for example, and you don'tmind forcing your users to relink all of the applications which use oneof your Libtool libraries every time you make a release, then<CODE>libtool</CODE> provides the <SAMP>`-release'</SAMP> flag to encode theproject version number in the name of the library, See section <A HREF="autobook_88.html#SEC88">11.2.1 Creating Libtool Libraries with Automake</A>. This can save you librarycompatibility problems later if you need to, say, make a patch releaseof an older revision of your library, but the library version numberthat you should use has already been taken by another earlier release.In this case, you could be fairly certain that library releases from theunstable branch will not be binary compatible with the stable releases,so you could make all the stable releases with <SAMP>`-release 1.0'</SAMP> andbegin the first unstable release with <SAMP>`-release 1.1'</SAMP>.</P><P><A NAME="Convenience Libraries"></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>
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