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Ubuntu Policy Manual
Chapter 8 - Shared libraries


Packages containing shared libraries must be constructed with a little care to make sure that the shared library is always available. This is especially important for packages whose shared libraries are vitally important, such as the C library (currently libc6).

Packages involving shared libraries should be split up into several binary packages. This section mostly deals with how this separation is to be accomplished; rules for files within the shared library packages are in Libraries, Section 10.2 instead.


8.1 Run-time shared libraries

The run-time shared library needs to be placed in a package whose name changes whenever the shared object version changes.[50] The most common mechanism is to place it in a package called librarynamesoversion, where soversion is the version number in the soname of the shared library[51]. Alternatively, if it would be confusing to directly append soversion to libraryname (e.g. because libraryname itself ends in a number), you may use libraryname-soversion and libraryname-soversion-dev instead.

If you have several shared libraries built from the same source tree you may lump them all together into a single shared library package, provided that you change all of their sonames at once (so that you don't get filename clashes if you try to install different versions of the combined shared libraries package).

The package should install the shared libraries under their normal names. For example, the libgdbm3 package should install libgdbm.so.3.0.0 as /usr/lib/libgdbm.so.3.0.0. The files should not be renamed or re-linked by any prerm or postrm scripts; dpkg will take care of renaming things safely without affecting running programs, and attempts to interfere with this are likely to lead to problems.

Shared libraries should not be installed executable, since the dynamic linker does not require this and trying to execute a shared library usually results in a core dump.

The run-time library package should include the symbolic link that ldconfig would create for the shared libraries. For example, the libgdbm3 package should include a symbolic link from /usr/lib/libgdbm.so.3 to libgdbm.so.3.0.0. This is needed so that the dynamic linker (for example ld.so or ld-linux.so.*) can find the library between the time that dpkg installs it and the time that ldconfig is run in the postinst script.[52]


8.1.1 ldconfig

Any package installing shared libraries in one of the default library directories of the dynamic linker (which are currently /usr/lib and /lib) or a directory that is listed in /etc/ld.so.conf[53] must use ldconfig to update the shared library system.

The package maintainer scripts must only call ldconfig under these circumstances:

[54]


8.2 Shared library support files

If your package contains files whose names do not change with each change in the library shared object version, you must not put them in the shared library package. Otherwise, several versions of the shared library cannot be installed at the same time without filename clashes, making upgrades and transitions unnecessarily difficult.

It is recommended that supporting files and run-time support programs that do not need to be invoked manually by users, but are nevertheless required for the package to function, be placed (if they are binary) in a subdirectory of /usr/lib, preferably under /usr/lib/package-name. If the program or file is architecture independent, the recommendation is for it to be placed in a subdirectory of /usr/share instead, preferably under /usr/share/package-name. Following the package-name naming convention ensures that the file names change when the shared object version changes.

Run-time support programs that use the shared library but are not required for the library to function or files used by the shared library that can be used by any version of the shared library package should instead be put in a separate package. This package might typically be named libraryname-tools; note the absence of the soversion in the package name.

Files and support programs only useful when compiling software against the library should be included in the development package for the library.[55]


8.3 Static libraries

The static library (libraryname.a) is usually provided in addition to the shared version. It is placed into the development package (see below).

In some cases, it is acceptable for a library to be available in static form only; these cases include:


8.4 Development files

The development files associated to a shared library need to be placed in a package called librarynamesoversion-dev, or if you prefer only to support one development version at a time, libraryname-dev.

In case several development versions of a library exist, you may need to use dpkg's Conflicts mechanism (see Conflicting binary packages - Conflicts, Section 7.4) to ensure that the user only installs one development version at a time (as different development versions are likely to have the same header files in them, which would cause a filename clash if both were installed).

The development package should contain a symlink for the associated shared library without a version number. For example, the libgdbm-dev package should include a symlink from /usr/lib/libgdbm.so to libgdbm.so.3.0.0. This symlink is needed by the linker (ld) when compiling packages, as it will only look for libgdbm.so when compiling dynamically.


8.5 Dependencies between the packages of the same library

Typically the development version should have an exact version dependency on the runtime library, to make sure that compilation and linking happens correctly. The ${binary:Version} substitution variable can be useful for this purpose. [56]


8.6 Dependencies between the library and other packages - the shlibs system

If a package contains a binary or library which links to a shared library, we must ensure that when the package is installed on the system, all of the libraries needed are also installed. This requirement led to the creation of the shlibs system, which is very simple in its design: any package which provides a shared library also provides information on the package dependencies required to ensure the presence of this library, and any package which uses a shared library uses this information to determine the dependencies it requires. The files which contain the mapping from shared libraries to the necessary dependency information are called shlibs files.

Thus, when a package is built which contains any shared libraries, it must provide a shlibs file for other packages to use, and when a package is built which contains any shared libraries or compiled binaries, it must run dpkg-shlibdeps on these to determine the libraries used and hence the dependencies needed by this package.[57]

In the following sections, we will first describe where the various shlibs files are to be found, then how to use dpkg-shlibdeps, and finally the shlibs file format and how to create them if your package contains a shared library.


8.6.1 The shlibs files present on the system

There are several places where shlibs files are found. The following list gives them in the order in which they are read by dpkg-shlibdeps. (The first one which gives the required information is used.)


8.6.2 How to use dpkg-shlibdeps and the shlibs files

Put a call to dpkg-shlibdeps into your debian/rules file. If your package contains only compiled binaries and libraries (but no scripts), you can use a command such as:

     dpkg-shlibdeps debian/tmp/usr/bin/* debian/tmp/usr/sbin/* \
       debian/tmp/usr/lib/*

Otherwise, you will need to explicitly list the compiled binaries and libraries.[59]

This command puts the dependency information into the debian/substvars file, which is then used by dpkg-gencontrol. You will need to place a ${shlibs:Depends} variable in the Depends field in the control file for this to work.

If dpkg-shlibdeps doesn't complain, you're done. If it does complain you might need to create your own debian/shlibs.local file, as explained below (see Writing the debian/shlibs.local file, Section 8.6.5).

If you have multiple binary packages, you will need to call dpkg-shlibdeps on each one which contains compiled libraries or binaries. In such a case, you will need to use the -T option to the dpkg utilities to specify a different substvars file.

If you are creating a udeb for use in the Debian Installer, you will need to specify that dpkg-shlibdeps should use the dependency line of type udeb by adding -tudeb as option[60]. If there is no dependency line of type udeb in the shlibs file, dpkg-shlibdeps will fall back to the regular dependency line.

For more details on dpkg-shlibdeps, please see dpkg-shlibdeps - calculates shared library dependencies, Section C.1.4 and dpkg-shlibdeps(1).


8.6.3 The shlibs File Format

Each shlibs file has the same format. Lines beginning with # are considered to be comments and are ignored. Each line is of the form:

     [type: ]library-name soname-version dependencies ...

We will explain this by reference to the example of the zlib1g package, which (at the time of writing) installs the shared library /usr/lib/libz.so.1.1.3.

type is an optional element that indicates the type of package for which the line is valid. The only type currently in use is udeb. The colon and space after the type are required.

library-name is the name of the shared library, in this case libz. (This must match the name part of the soname, see below.)

soname-version is the version part of the soname of the library. The soname is the thing that must exactly match for the library to be recognized by the dynamic linker, and is usually of the form name.so.major-version, in our example, libz.so.1.[61] The version part is the part which comes after .so., so in our case, it is 1.

dependencies has the same syntax as a dependency field in a binary package control file. It should give details of which packages are required to satisfy a binary built against the version of the library contained in the package. See Syntax of relationship fields, Section 7.1 for details.

In our example, if the first version of the zlib1g package which contained a minor number of at least 1.3 was 1:1.1.3-1, then the shlibs entry for this library could say:

     libz 1 zlib1g (>= 1:1.1.3)

The version-specific dependency is to avoid warnings from the dynamic linker about using older shared libraries with newer binaries.

As zlib1g also provides a udeb containing the shared library, there would also be a second line:

     udeb: libz 1 zlib1g-udeb (>= 1:1.1.3)

8.6.4 Providing a shlibs file

If your package provides a shared library, you need to create a shlibs file following the format described above. It is usual to call this file debian/shlibs (but if you have multiple binary packages, you might want to call it debian/shlibs.package instead). Then let debian/rules install it in the control area:

     install -m644 debian/shlibs debian/tmp/DEBIAN

or, in the case of a multi-binary package:

     install -m644 debian/shlibs.package debian/package/DEBIAN/shlibs

An alternative way of doing this is to create the shlibs file in the control area directly from debian/rules without using a debian/shlibs file at all,[62] since the debian/shlibs file itself is ignored by dpkg-shlibdeps.

As dpkg-shlibdeps reads the DEBIAN/shlibs files in all of the binary packages being built from this source package, all of the DEBIAN/shlibs files should be installed before dpkg-shlibdeps is called on any of the binary packages.


8.6.5 Writing the debian/shlibs.local file

This file is intended only as a temporary fix if your binaries or libraries depend on a library whose package does not yet provide a correct shlibs file.

We will assume that you are trying to package a binary foo. When you try running dpkg-shlibdeps you get the following error message (-O displays the dependency information on stdout instead of writing it to debian/substvars, and the lines have been wrapped for ease of reading):

     $ dpkg-shlibdeps -O debian/tmp/usr/bin/foo
     dpkg-shlibdeps: warning: unable to find dependency
       information for shared library libbar (soname 1,
       path /usr/lib/libbar.so.1, dependency field Depends)
     shlibs:Depends=libc6 (>= 2.2.2-2)

You can then run ldd on the binary to find the full location of the library concerned:

     $ ldd foo
     libbar.so.1 => /usr/lib/libbar.so.1 (0x4001e000)
     libc.so.6 => /lib/libc.so.6 (0x40032000)
     /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)

So the foo binary depends on the libbar shared library, but no package seems to provide a *.shlibs file handling libbar.so.1 in /var/lib/dpkg/info/. Let's determine the package responsible:

     $ dpkg -S /usr/lib/libbar.so.1
     bar1: /usr/lib/libbar.so.1
     $ dpkg -s bar1 | grep Version
     Version: 1.0-1

This tells us that the bar1 package, version 1.0-1, is the one we are using. Now we can file a bug against the bar1 package and create our own debian/shlibs.local to locally fix the problem. Including the following line into your debian/shlibs.local file:

     libbar 1 bar1 (>= 1.0-1)

should allow the package build to work.

As soon as the maintainer of bar1 provides a correct shlibs file, you should remove this line from your debian/shlibs.local file. (You should probably also then have a versioned Build-Depends on bar1 to help ensure that others do not have the same problem building your package.)


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Ubuntu Policy Manual

version 3.8.2.0ubuntu1, 2009-06-19

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