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213 lines
7.4 KiB
HTML
213 lines
7.4 KiB
HTML
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<title>Libtool Inter-library Dependencies</title>
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<body bgcolor="#ffffff">
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<center><h1><img src="/graphics/libtool.gif" width=477 height=192
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alt="Libtool"></h1></center>
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<h1>Inter-library Dependencies</h1>
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<p>About twice a week, for the last five weeks, I've been receiving
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bug reports which tell me that libtool's inter-library dependency
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handling is broken.
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<p>I know. I broke it intentionally, until I have the time to fix it
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myself, or somebody else takes the time to help me with it.
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<p>These same people often give me a simple one-line patch which
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re-enables my old, simplistic inter-library dependencies, but nobody
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seems to want to test things thoroughly and come up with a real
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solution.
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<p>If you don't care about the history, and you just want to help me
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out, jump to <a href="#solution">the bottom of this document</a>.
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<h2>Background</h2>
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<p>Libtool's basic premise is to make static and shared libraries
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behave the same way from a programmer's point of view. This allows
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users to build a libtoolized package with or without shared libraries,
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determined at configuration time. It does this by using a
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<dfn>libtool object</dfn> (<samp>.lo</samp>) and <dfn>libtool
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archive</dfn> (<samp>.la</samp>) abstraction, so that the package
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maintainer can use libtool to operate on these files without making
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any assumptions about their underlying representation.
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<p>For the most part, this abstraction works well, and has made
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libtool as popular as it is today. Without this abstraction, it would
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be significantly harder to port libtool to new platforms.
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<p>Unfortunately, what this abstraction has also done is reveal some
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fundamental inconsistencies with most shared library implementations.
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Every shared library implementation works well for `hello world'-type
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examples, but very few are robust and well-designed so that libtool
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doesn't need special tricks in order to build correct, featureful
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shared libraries.
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<p>Providing inter-library dependencies is one feature that has
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revealed these kinds of inconsistencies.
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<h2>The problem</h2>
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<p>My orginal inter-library dependency code received rigourous testing
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in beta releases of <a href="http://www.red-bean.com/guile/">GNU
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Guile</a>. As soon as the Guile people started using my code, I
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received a flood of bug reports. People were reporting that
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<samp>libguile</samp> (Guile's main shared library) was failing to
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link, or that programs linked against <samp>libguile</samp> were
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dumping core.
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<p>Not good.
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<p>The Guile people chased this bug down to the following scenario:
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<ol>
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<li>The user's system has a static regexp library installed,
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<samp>librx.a</samp>.
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<li>Guile's <samp>configure</samp> script detects that the
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<samp>-lrx</samp> linker option can be used to link in
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<samp>librx</samp>.
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<li>When <samp>libguile</samp> is built, the <samp>-lrx</samp> linker
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option is used.
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<li>Some linkers fail at this point, because they don't allow shared
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libraries to contain or depend on static libraries.
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<li>If the linker didn't fail, then a few programs are linked against
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<samp>libguile</samp>.
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<li>On some systems, these programs core dump because
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<samp>libguile</samp> is a shared library that contains non-PIC code
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(from <samp>librx</samp>).
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</ol>
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<h2>The interim solution</h2>
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<p>I needed some way to respond to these reports. I saw my options (in
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order of my preference) as:
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<ol>
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<li>Write code in <samp>ltmain.sh</samp> to prevent static libraries
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from appearing in inter-library dependencies. This would take some
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work, but obviously is the best solution.
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<li>Find the systems that fail, and turn off inter-library
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dependencies on only those systems.
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<li>Force the package maintainer to guarantee that static libraries
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never appear in inter-library dependencies.
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</ol>
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<p>I immediately vetoed the last solution, because that would violate
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the whole point of using libtool, and would cause a lot of people to
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waste time solving a problem that really should be fixed by libtool.
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<p>I preferred the first solution, but at the time of the reports, I
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didn't see an obviously simple mechanism for detecting the difference
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between shared and static libraries.
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<p>So, in the meantime, I tried turning off inter-library dependencies
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on the systems that failed.
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<p>I quickly discovered, to my chagrin, that many systems fail. So,
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it was be simpler for me to turn off <em>all</em> inter-library
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dependencies, then find out which systems work, rather than vice
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versa.
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<h2>The current situation</h2>
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<p>I've been busy trying to avoid bankruptcy. It's been over three
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months since I first turned off inter-library dependencies, and I
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still haven't completed the solution I want.
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<p>I've been gearing up for the 1.1 release of libtool, because there
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is a high demand for a stable public release. So, I'm not going to
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introduce any destabilizing changes to the inter-library dependency
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code until after 1.1 is released.
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<h2><a name="solution">The solution</a></h2>
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<p>So, I want to tell you how you can help me solve the various
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dilemmas surrounding this issue:
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<ol>
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<li>I need to find out more about the nature of the problems I ran
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into with Guile. Unfortunately, I cannot reproduce them in simple
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tests on my own platform (i586-pc-linux-gnulibc1), even though I think
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they were reported here. I need to find out which platforms already
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have perfect inter-library dependency support, how to work around the
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problem on other platforms, and, more importantly, exactly
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<em>why</em> some systems give me problems and others don't.
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<p>On correct platforms, you can link <em>any</em> static library
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against a shared library via the <samp>-lNAME</samp> option without
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the linker complaining, then link a program against this library and
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run it without dumping core. I know that this scenario will always
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work fine on the following systems:
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<ul>
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<li>None reported yet.
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</ul>
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<p>I also know that on some systems, you can create a shared library
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linked against a static one, but running programs linked against such
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a library will dump core:
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<ul>
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<li>None reported yet.
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</ul>
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<p>Finally, there are some systems which won't even allow you to link
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a shared library against a static one:
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<ul>
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<li>Solaris 2.x
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</ul>
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<li>Help me figure out a good, portable way to detect if a given
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<samp>-lNAME</samp> option refers to a shared library or not, since
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that is needed as a workaround to the problem. Some suggestions so
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far have been:
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<ul>
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<li>Link the library against a tiny test program, and:
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<ul>
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<li>run <samp>ldd(1)</samp> on the test program and search for
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<samp>libNAME</samp> in the ldd output.
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<li>use the <samp>-verbose</samp> flag for GNU ld in order to see
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which library is actually linked.
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<li>run some sort of other program to determine if the library was
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dynamic.
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</ul>
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<li>Track the <samp>-LDIR</samp> flags, do a search for the library,
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and then check whether it is shared by:
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<ul>
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<li>using the <samp>file(1)</samp> program.
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<li>looking at its suffix.
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</ul>
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</ul>
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<li>Contact any people you know who might be interested, get them to
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read this page, so that they can help me solve the problem.
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<li>Send me money, so that I can devote more of my time to solving the
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problem.
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</ol>
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<p>Thank you for your help, and have fun.
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<hr>
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<a href="http://www.gimp.org/"><img src="/graphics/gfx_by_gimp.gif"
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align="right" border=0></a>
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<p><a href="libtool.html">Back to the libtool home page.</a>
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<address>
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Gordon Matzigkeit <a href="mailto:gord@profitpress.com"><gord@profitpress.com></a>
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</address>
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</html>
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