To all port maintainers: libtool

Baptiste Daroussin bapt at FreeBSD.org
Wed May 7 23:16:41 UTC 2014


On Thu, May 08, 2014 at 12:24:20AM +0200, Tijl Coosemans wrote:
> Hi,
> 
> I've been asked to write something about USES=libtool to clarify a few
> things about what it does and why.
> 
> 
> First: what is libtool?
> 
> Libtool is a build script that acts as a wrapper around a compiler.  You
> can compile code or link a library (or executable) with libtool and it will
> invoke the right compiler, linker and other commands for you.  It supports
> multiple languages, compilers and platforms and simplifies writing
> makefiles when used in combination with automake.  Many ports use it.
> 
> 
> The problems with libtool on FreeBSD.
> 
> Libraries created with libtool have three version numbers X.Y.Z.  How they
> are calculated is not straightforward but that's not important right now.
> The important thing is that the major version number X is the only one
> that really matters.  An executable that links to libA will ask for
> libA.so.X and doesn't care about Y and Z.  Only when the major version
> number of a library changes does everything that link to it need to be
> recompiled.  On FreeBSD however libtool uses X+Y as major version number
> which means that changes in Y also require rebuilding all ports that
> depend on the library.
> 
> A second problem is that libtool flattens the dependency tree.  When you
> link an executable or library with libA while libA in turn links to libB
> then libtool will make the executable or library also link with libB.
> This means that when the major version number of libB changes, the
> executable or library will also have to be rebuilt.  Some libraries get
> propagated into hundreds and even thousands of ports this way while only
> a few actually use them directly.
> 
> These two problems make major version number changes too frequent and too
> painful, not only for users who build their own ports, but also for
> package users, because too many packages need to be rebuilt, mirrored and
> downloaded every week than strictly necessary.
> 
> A third problem is with ports that have USE_AUTOTOOLS=libtool in their
> Makefile.  Normally the libtool script is generated by the configure
> script so it uses the compiler, linker and other tools that you configure
> a port with.  Ports with USE_AUTOTOOLS=libtool however use
> /use/local/bin/libtool.  This is the libtool script generated by the
> configure script of devel/libtool.  This mechanism was meant for ports
> that ship with a version of libtool that doesn't work properly, but the
> problem is that such ports aren't necessarily built with the same tools as
> devel/libtool.  An upcoming feature that allows cross-building ports and
> that will be used to provide ARM packages is currently blocked by this
> because these ports don't use the right cross compiler and linker.
> 
> 
> The solution.
> 
> All ports that use libtool need to have USES=libtool added to their
> Makefile and USE_AUTOTOOLS=libtool removed.  USES=libtool will patch the
> port to address the problems above along with a few smaller problems with
> older versions of libtool that some ports still ship with.  USES=libtool
> also replaces USE_GNOME=ltverhack so that should be removed as well.  In
> fact, all libtool related hacks and patches that a port contains can
> probably be removed (typically patches or post-patch commands for files
> like ltconfig, ltmain.sh and configure).
> 
> The easiest way to know if a port uses libtool is to build it first.  You
> can then search the work directory for a file named "libtool":
> 
> # cd some/port
> # make
> # find work -name libtool
> 
> Another giveaway is the installation of files with .la extension.
> 
> 
> USES=libtool modifiers :keepla and :oldver.
> 
> One of the ways libtool propagates links is through .la libraries.  These
> are ordinary text files that contain some meta-data about a library,
> including a record of all dependencies.  These files are normally not
> needed and USES=libtool removes them from the staging area.  For the rare
> cases where a port does need these files USES=libtool:keepla can be used.
> In this case .la files are kept but the dependency record is cleared so
> dependencies won't be propagated to other ports.
> 
> Another important reason to keep .la files is that in this transition
> period other ports may still contain references to them in the dependency
> record of their .la files.  These ports first need to have some form of
> USES=libtool added to their Makefile such that these references are
> removed.  So, as long as there are dependent ports that install .la files
> but do not have some form of USES=libtool, a port must use
> USES=libtool:keepla.
> 
> Finally, because the major version number goes from X+Y to X, if Y is
> non-zero, the library version will go backwards.  This is not a problem
> in the sense of API compatibility (because of the way libtool versioning
> works everything between X and X+Y is backwards compatible and a new
> release that isn't backwards compatible will see its major version number
> jump to newX=oldX+oldY+1), but it may be a problem if due to link
> propagation many ports depend on the library and they all need to get a
> PORTREVISION bump.  To avoid this, USES=libtool:oldver can be used.  A
> port will then continue to use the old X+Y version scheme but it will
> no longer propagate its own dependencies to other ports.  Once enough
> ports use some form of USES=libtool the number of dependent ports will
> drop to a point that removing :oldver becomes possible.  Note that
> USES=libtool:oldver is a transitional measure and support for it will
> eventually be removed.

Thank you very much for this write up! and for all your work helping us
solving a very long standing major problem in the ports tree!

regards,
Bapt
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