TipsAndTricks/PackagingNonversionedLibrary

How to Package an Unversioned Library

First some background: libraries in Linux systems are generally versioned so that it is possible to have multiple versions of the same library installed, to ease upgrades or support older software. For example in versioned libraries the actual library is called libfoo.so.1.2, and then there will be a libfoo.so.1 symbolic link to libfoo.so.1.2, and finally a libfoo.so symbolic link to libfoo.so.1.2. When linking a binary against a library you typically tell it the unversioned file name (-lfoo to the linker) but the linker will follow the symbolic link and actually link against the versioned filename. The unversioned symbolic link is only used at development time, so is packaged along with the headers in the development package ${PN}-dev, with the actual library and versioned symbolic links in ${PN}. As versioned libraries are far more common than unversioned libraries, the default packaging rules assume versioned libraries.

However this means that packaging an unversioned library means a bit of work in the recipe, as by default libfoo.so will get packaged into ${PN}-dev (triggerring a QA warning that a non-symlink library is in a -dev package), and binaries in the same recipe will link to the library in ${PN}-dev (triggering more QA warnings). To solve this the unversioned library needs to be packaged into ${PN} where it belongs. These are the (abridged) default FILES variables in bitbake.conf:

SOLIBS = ".so.*"
SOLIBSDEV = ".so"
FILES_${PN} = "... ${libdir}/lib*${SOLIBS} ..."
FILES_SOLIBSDEV ?= "... ${libdir}/lib*${SOLIBSDEV} ..."
FILES_${PN}-dev = "... ${FILES_SOLIBSDEV} ..."

SOLIBS defines a pattern that matches real shared object libraries, and SOLIBSDEV matches the development form (unversioned symlink). These are then used in FILES_${PN} and FILES_${PN}-dev which puts the real libraries into ${PN} and the unversioned symbolic link into PN-dev. To package unversioned libraries, this needs to be modified as follows in the recipe:

SOLIBS = ".so"
FILES_SOLIBSDEV = ""

This says that .so is the real library, and unsets FILES_SOLIBSDEV so that no libraries get packaged into PN-dev. This is required because unless PACKAGES is changed PN-dev collects files before PN, so PN-dev must not collect any of the files we want in PN.

When installing the library, use the install command rather than the oe_soinstall helper function. oe_soinstall will cause an error due to recursive symbolic linking (see its implementation in utils.bbclass for more details).

Finally loadable modules, essentially unversioned libraries that are linked at runtime using dlopen() instead of at build time, should generally be installed in a private directory. However if they are installed in ${libdir} then they can be treated as unversioned libraries.