8.6. Dependencies between the library and other packages

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. These dependencies must be added to the binary package when it is built, since they may change based on which version of a shared library the binary or library was linked with even if there are no changes to the source of the binary (for example, symbol versions change, macros become functions or vice versa, or the binary package may determine at compile-time whether new library interfaces are available and can be called). To allow these dependencies to be constructed, shared libraries must provide either a symbols file or a shlibs file. These provide information on the package dependencies required to ensure the presence of interfaces provided by this library. Any package with binaries or libraries linking to a shared library must use these files to determine the required dependencies when it is built. Other packages which use a shared library (for example using dlopen()) should compute appropriate dependencies using these files at build time as well.

The two mechanisms differ in the degree of detail that they provide. A symbols file documents, for each symbol exported by a library, the minimal version of the package any binary using this symbol will need. This is typically the version of the package in which the symbol was introduced. This information permits detailed analysis of the symbols used by a particular package and construction of an accurate dependency, but it requires the package maintainer to track more information about the shared library.

A shlibs file, in contrast, only documents the last time the library ABI changed in any way. It only provides information about the library as a whole, not individual symbols. When a package is built using a shared library with only a shlibs file, the generated dependency will require a version of the shared library equal to or newer than the version of the last ABI change. This generates unnecessarily restrictive dependencies compared to symbols files if none of the symbols used by the package have changed. This, in turn, may make upgrades needlessly complex and unnecessarily restrict use of the package on systems with older versions of the shared libraries.

shlibs files also only support a limited range of library SONAMEs, making it difficult to use shlibs files in some unusual corner cases. 9

symbols files are therefore recommended for most shared library packages since they provide more accurate dependencies. For most C libraries, the additional detail required by symbols files is not too difficult to maintain. However, maintaining exhaustive symbols information for a C++ library can be quite onerous, so shlibs files may be more appropriate for most C++ libraries. Libraries with a corresponding udeb must also provide a shlibs file, since the udeb infrastructure does not use symbols files.

9

A shlibs file represents an SONAME as a library name and version number, such as libfoo VERSION, instead of recording the actual SONAME. If the SONAME doesn’t match one of the two expected formats (libfoo-VERSION.so or libfoo.so.VERSION), it cannot be represented.

8.6.1. Generating dependencies on shared libraries

When a package that contains any shared libraries or compiled binaries is built, it must run dpkg-shlibdeps on each shared library and compiled binary to determine the libraries used and hence the dependencies needed by the package. 10 To do this, put a call to dpkg-shlibdeps into your debian/rules file in the source package. List all of the compiled binaries, libraries, or loadable modules in your package. 11 dpkg-shlibdeps will use the symbols or shlibs files installed by the shared libraries to generate dependency information. The package must then provide a substitution variable into which the discovered dependency information can be placed.

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 the -tudeb option. 12 If there is no dependency line of type udeb in the shlibs file, dpkg-shlibdeps will fall back to the regular dependency line.

dpkg-shlibdeps puts the dependency information into the debian/substvars file by default, which is then used by dpkg-gencontrol. You will need to place a ${shlibs:Depends} variable in the Depends field in the control file of every binary package built by this source package that contains compiled binaries, libraries, or loadable modules. If you have multiple binary packages, you will need to call dpkg-shlibdeps on each one which contains compiled libraries or binaries. For example, you could use the -T option to the dpkg utilities to specify a different substvars file for each binary package. 13

For more details on dpkg-shlibdeps, see its manual page.

We say that a binary foo directly uses a library libbar if it is explicitly linked with that library (that is, the library is listed in the ELF NEEDED attribute, caused by adding -lbar to the link line when the binary is created). Other libraries that are needed by libbar are linked indirectly to foo, and the dynamic linker will load them automatically when it loads libbar. A package should depend on the libraries it directly uses, but not the libraries it only uses indirectly. The dependencies for the libraries used directly will automatically pull in the indirectly-used libraries. dpkg-shlibdeps will handle this logic automatically, but package maintainers need to be aware of this distinction between directly and indirectly using a library if they have to override its results for some reason. 14

10

dpkg-shlibdeps will use a program like objdump or readelf to find the libraries and the symbols in those libraries directly needed by the binaries or shared libraries in the package.

11

The easiest way to call dpkg-shlibdeps correctly is to use a package helper framework such as debhelper. If you are using debhelper, the dh_shlibdeps program will do this work for you. It will also correctly handle multi-binary packages.

12

dh_shlibdeps from the debhelper suite will automatically add this option if it knows it is processing a udeb.

13

Again, dh_shlibdeps and dh_gencontrol will handle everything except the addition of the variable to the control file for you if you’re using debhelper, including generating separate substvars files for each binary package and calling dpkg-gencontrol with the appropriate flags.

14

A good example of where this helps is the following: We could update libimlib with a new version that supports a new revision of a graphics format called dgf (but retaining the same major version number) and depends on a new library package libdgf4 instead of the older libdgf3. If we used ldd to add dependencies for every library directly or indirectly linked with a binary, every package that uses libimlib would need to be recompiled so it would also depend on libdgf4 in order to retire the older libdgf3 package. Since dependencies are only added based on ELF NEEDED attribute, packages using libimlib can rely on libimlib itself having the dependency on an appropriate version of libdgf and do not need rebuilding.

8.6.2. Shared library ABI changes

Maintaining a shared library package using either symbols or shlibs files requires being aware of the exposed ABI of the shared library and any changes to it. Both symbols and shlibs files record every change to the ABI of the shared library; symbols files do so per public symbol, whereas shlibs files record only the last change for the entire library.

There are two types of ABI changes: ones that are backward-compatible and ones that are not. An ABI change is backward-compatible if any reasonable program or library that was linked with the previous version of the shared library will still work correctly with the new version of the shared library. 15 Adding new symbols to the shared library is a backward-compatible change. Removing symbols from the shared library is not. Changing the behavior of a symbol may or may not be backward-compatible depending on the change; for example, changing a function to accept a new enum constant not previously used by the library is generally backward-compatible, but changing the members of a struct that is passed into library functions is generally not unless the library takes special precautions to accept old versions of the data structure.

ABI changes that are not backward-compatible normally require changing the SONAME of the library and therefore the shared library package name, which forces rebuilding all packages using that shared library to update their dependencies and allow them to use the new version of the shared library. For more information, see Run-time shared libraries. The remainder of this section will deal with backward-compatible changes.

Backward-compatible changes require either updating or recording the minimal-version for that symbol in symbols files or updating the version in the dependencies in shlibs files. For more information on how to do this in the two formats, see The symbols File Format and The shlibs File Format. Below are general rules that apply to both files.

The easy case is when a public symbol is added. Simply add the version at which the symbol was introduced (for symbols files) or update the dependency version (for shlibs) files. But special care should be taken to update dependency versions when the behavior of a public symbol changes. This is easy to neglect, since there is no automated method of determining such changes, but failing to update versions in this case may result in binary packages with too-weak dependencies that will fail at runtime, possibly in ways that can cause security vulnerabilities. If the package maintainer believes that a symbol behavior change may have occurred but isn’t sure, it’s safer to update the version rather than leave it unmodified. This may result in unnecessarily strict dependencies, but it ensures that packages whose dependencies are satisfied will work properly.

A common example of when a change to the dependency version is required is a function that takes an enum or struct argument that controls what the function does. For example:

enum library_op { OP_FOO, OP_BAR };
int library_do_operation(enum library_op);

If a new operation, OP_BAZ, is added, the minimal-version of library_do_operation (for symbols files) or the version in the dependency for the shared library (for shlibs files) must be increased to the version at which OP_BAZ was introduced. Otherwise, a binary built against the new version of the library (having detected at compile-time that the library supports OP_BAZ) may be installed with a shared library that doesn’t support OP_BAZ and will fail at runtime when it tries to pass OP_BAZ into this function.

Dependency versions in either symbols or shlibs files normally should not contain the Debian revision of the package, since the library behavior is normally fixed for a particular upstream version and any Debian packaging of that upstream version will have the same behavior. In the rare case that the library behavior was changed in a particular Debian revision, appending ~ to the end of the version that includes the Debian revision is recommended, since this allows backports of the shared library package using the normal backport versioning convention to satisfy the dependency.

15

An example of an “unreasonable” program is one that uses library interfaces that are documented as internal and unsupported. If the only programs or libraries affected by a change are “unreasonable” ones, other techniques, such as declaring Breaks relationships with affected packages or treating their usage of the library as bugs in those packages, may be appropriate instead of changing the SONAME. However, the default approach is to change the SONAME for any change to the ABI that could break a program.

8.6.3. The symbols system

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

8.6.3.1. The symbols files present on the system

symbols files for a shared library are normally provided by the shared library package as a control file, but there are several override paths that are checked first in case that information is wrong or missing. The following list gives them in the order in which they are read by dpkg-shlibdeps. The first one that contains the required information is used.

debian/*/DEBIAN/symbols

During the package build, if the package itself contains shared libraries with symbols files, they will be generated in these staging directories by dpkg-gensymbols (see Providing a symbols file). symbols files found in the build tree take precedence over symbols files from other binary packages.

These files must exist before dpkg-shlibdeps is run or the dependencies of binaries and libraries from a source package on other libraries from that same source package will not be correct. In practice, this means that dpkg-gensymbols must be run before dpkg-shlibdeps during the package build. 16

/etc/dpkg/symbols/package.symbols.arch and /etc/dpkg/symbols/package.symbols

Per-system overrides of shared library dependencies. These files normally do not exist. They are maintained by the local system administrator and must not be created by any Debian package.

symbols control files for packages installed on the system

The symbols control files for all the packages currently installed on the system are searched last. This will be the most common source of shared library dependency information. These files can be read with dpkg-query --control-show package symbols.

Be aware that if a debian/shlibs.local exists in the source package, it will override any symbols files. This is the only case where a shlibs is used despite symbols files being present. See The shlibs files present on the system and The shlibs system for more information.

16

An example may clarify. Suppose the source package foo generates two binary packages, libfoo2 and foo-runtime. When building the binary packages, the contents of the packages are staged in the directories debian/libfoo2 and debian/foo-runtime respectively. (debian/tmp could be used instead of one of these.) Since libfoo2 provides the libfoo shared library, it will contain a symbols file, which will be installed in debian/libfoo2/DEBIAN/symbols, eventually to be included as a control file in that package. When dpkg-shlibdeps is run on the executable debian/foo-runtime/usr/bin/foo-prog, it will examine the debian/libfoo2/DEBIAN/symbols file to determine whether foo-prog’s library dependencies are satisfied by any of the libraries provided by libfoo2. Since those binaries were linked against the just-built shared library as part of the build process, the symbols file for the newly-built libfoo2 must take precedence over a symbols file for any other libfoo2 package already installed on the system.

8.6.3.2. The symbols File Format

The following documents the format of the symbols control file as included in binary packages. These files are built from template symbols files in the source package by dpkg-gensymbols. The template files support a richer syntax that allows dpkg-gensymbols to do some of the tedious work involved in maintaining symbols files, such as handling C++ symbols or optional symbols that may not exist on particular architectures. When writing symbols files for a shared library package, refer to dpkg-gensymbols(1) for the richer syntax.

A symbols may contain one or more entries, one for each shared library contained in the package corresponding to that symbols. Each entry has the following format:

library-soname main-dependency-template
 [| alternative-dependency-template]
 [...]
 [* field-name: field-value]
 [...]
 symbol minimal-version[ id-of-dependency-template]

To explain this format, we’ll use the zlib1g package as an example, which (at the time of writing) installs the shared library /usr/lib/libz.so.1.2.3.4. Mandatory lines will be described first, followed by optional lines.

library-soname must contain exactly the value of the ELF SONAME attribute of the shared library. In our example, this is libz.so.1. 17

main-dependency-template has the same syntax as a dependency field in a binary package control file, except that the string #MINVER# is replaced by a version restriction like (>= version) or by nothing if an unversioned dependency is deemed sufficient. The version restriction will be based on which symbols from the shared library are referenced and the version at which they were introduced (see below). In nearly all cases, main-dependency-template will be package #MINVER#, where package is the name of the binary package containing the shared library. This adds a simple, possibly-versioned dependency on the shared library package. In some rare cases, such as when multiple packages provide the same shared library ABI, the dependency template may need to be more complex.

In our example, the first line of the zlib1g symbols file would be:

libz.so.1 zlib1g #MINVER#

Each public symbol exported by the shared library must have a corresponding symbol line, indented by one space. symbol is the exported symbol (which, for C++, means the mangled symbol) followed by @ and the symbol version, or the string Base if there is no symbol version. minimal-version is the most recent version of the shared library that changed the behavior of that symbol, whether by adding it, changing its function signature (the parameters, their types, or the return type), or changing its behavior in a way that is visible to a caller. id-of-dependency-template is an optional field that references an alternative-dependency-template; see below for a full description.

For example, libz.so.1 contains the symbols compress and compressBound. compress has no symbol version and last changed its behavior in upstream version 1:1.1.4. compressBound has the symbol version ZLIB_1.2.0, was introduced in upstream version 1:1.2.0, and has not changed its behavior. Its symbols file therefore contains the lines:

compress@Base 1:1.1.4
compressBound@ZLIB_1.2.0 1:1.2.0

Packages using only compress would then get a dependency on zlib1g (>= 1:1.1.4), but packages using compressBound would get a dependency on zlib1g (>= 1:1.2.0).

One or more alternative-dependency-template lines may be provided. These are used in cases where some symbols in the shared library should use one dependency template while others should use a different template. The alternative dependency templates are used only if a symbol line contains the id-of-dependency-template field. The first alternative dependency template is numbered 1, the second 2, and so forth. 18

Finally, the entry for the library may contain one or more metadata fields. Currently, the only supported field-name is Build-Depends-Package, whose value lists the library development package on which packages using this shared library declare a build dependency. If this field is present, dpkg-shlibdeps uses it to ensure that the resulting binary package dependency on the shared library is at least as strict as the source package dependency on the shared library development package. 19 For our example, the zlib1g symbols file would contain:

* Build-Depends-Package: zlib1g-dev

Also see deb-symbols(5).

17

This can be determined by using the command

readelf -d /usr/lib/libz.so.1.2.3.4 | grep SONAME

18

An example of where this may be needed is with a library that implements the libGL interface. All GL implementations provide the same set of base interfaces, and then may provide some additional interfaces only used by programs that require that specific GL implementation. So, for example, libgl1-mesa-glx may use the following symbols file:

libGL.so.1 libgl1
 | libgl1-mesa-glx #MINVER#
 publicGlSymbol@Base 6.3-1 [...]
 implementationSpecificSymbol@Base 6.5.2-7 1
 [...]

Binaries or shared libraries using only publicGlSymbol would depend only on libgl1 (which may be provided by multiple packages), but ones using implementationSpecificSymbol would get a dependency on libgl1-mesa-glx (>= 6.5.2-7).

19

This field should normally not be necessary, since if the behavior of any symbol has changed, the corresponding symbol minimal-version should have been increased. But including it makes the symbols system more robust by tightening the dependency in cases where the package using the shared library specifically requires at least a particular version of the shared library development package for some reason.

8.6.3.3. Providing a symbols file

If your package provides a shared library, you should arrange to include a symbols control file following the format described above in that package. You must include either a symbols control file or a shlibs control file.

Normally, this is done by creating a symbols in the source package named debian/package.symbols or debian/symbols, possibly with .arch appended if the symbols information varies by architecture. This file may use the extended syntax documented in dpkg-gensymbols(1). Then, call dpkg-gensymbols as part of the package build process. It will create symbols files in the package staging area based on the binaries and libraries in the package staging area and the symbols files in the source package. 20

Packages that provide symbols files must keep them up-to-date to ensure correct dependencies in packages that use the shared libraries. This means updating the symbols file whenever a new public symbol is added, changing the minimal-version field whenever a symbol changes behavior or signature in a backward-compatible way (see Shared library ABI changes), and changing the library-soname and main-dependency-template, and probably all of the minimal-version fields, when the library changes SONAME. Removing a public symbol from the symbols file because it’s no longer provided by the library normally requires changing the SONAME of the library. See Run-time shared libraries for more information on SONAMEs.

20

If you are using debhelper, dh_makeshlibs will take care of calling either dpkg-gensymbols or generating a shlibs file as appropriate.

8.6.4. The shlibs system

The shlibs system is a simpler alternative to the symbols system for declaring dependencies for shared libraries. It may be more appropriate for C++ libraries and other cases where tracking individual symbols is too difficult. It predated the symbols system and is therefore frequently seen in older packages. It is also required for udebs, which do not support symbols.

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.

8.6.4.1. 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.)

debian/shlibs.local

This lists overrides for this package. This file should normally not be used, but may be needed temporarily in unusual situations to work around bugs in other packages, or in unusual cases where the normally declared dependency information in the installed shlibs file for a library cannot be used. This file overrides information obtained from any other source.

/etc/dpkg/shlibs.override

This lists global overrides. This list is normally empty. It is maintained by the local system administrator.

DEBIAN/shlibs files in the “build directory”

These files are generated as part of the package build process and staged for inclusion as control files in the binary packages being built. They provide details of any shared libraries included in the same package.

shlibs control files for packages installed on the system

The shlibs control files for all the packages currently installed on the system. These files can be read using dpkg-query --control-show package shlibs.

/etc/dpkg/shlibs.default

This file lists any shared libraries whose packages have failed to provide correct shlibs files. It was used when the shlibs setup was first introduced, but it is now normally empty. It is maintained by the dpkg maintainer.

If a symbols file for a shared library package is available, dpkg-shlibdeps will always use it in preference to a shlibs, with the exception of debian/shlibs.local. The latter overrides any other shlibs or symbols files.

8.6.4.2. 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.1.2.3.4.

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 ELF SONAME attribute of the library, determined the same way that the soversion component of the recommended shared library package name is determined. See Run-time shared libraries for the details.

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 for details on the syntax, and Shared library ABI changes for details on how to maintain the dependency version constraint.

In our example, if the last change to the zlib1g package that could change behavior for a client of that library was in version 1:1.2.3.3.dfsg-1, then the shlibs entry for this library could say:

libz 1 zlib1g (>= 1:1.2.3.3.dfsg)

This version restriction must be new enough that any binary built against the current version of the library will work with any version of the shared library that satisfies that dependency.

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

udeb: libz 1 zlib1g-udeb (>= 1:1.2.3.3.dfsg)

8.6.4.3. Providing a shlibs file

To provide a shlibs file for a shared library binary package, create a shlibs file following the format described above and place it in the DEBIAN directory for that package during the build. It will then be included as a control file for that package. 21

Since 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.

21

This is what dh_makeshlibs in the debhelper suite does. If your package also has a udeb that provides a shared library, dh_makeshlibs can automatically generate the udeb: lines if you specify the name of the udeb with the --add-udeb option.