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Debian 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).

This section deals only with public shared libraries: shared libraries that are placed in directories searched by the dynamic linker by default or which are intended to be linked against normally and possibly used by other, independent packages. Shared libraries that are internal to a particular package or that are only loaded as dynamic modules are not covered by this section and are not subject to its requirements.

A shared library is identified by the SONAME attribute stored in its dynamic section. When a binary is linked against a shared library, the SONAME of the shared library is recorded in the binary's NEEDED section so that the dynamic linker knows that library must be loaded at runtime. The shared library file's full name (which usually contains additional version information not needed in the SONAME) is therefore normally not referenced directly. Instead, the shared library is loaded by its SONAME, which exists on the file system as a symlink pointing to the full name of the shared library. This symlink must be provided by the package. Run-time shared libraries, Section 8.1 describes how to do this. [58]

When linking a binary or another shared library against a shared library, the SONAME for that shared library is not yet known. Instead, the shared library is found by looking for a file matching the library name with .so appended. This file exists on the file system as a symlink pointing to the shared library.

Shared libraries are normally split into several binary packages. The SONAME symlink is installed by the runtime shared library package, and the bare .so symlink is installed in the development package since it's only used when linking binaries or shared libraries. However, there are some exceptions for unusual shared libraries or for shared libraries that are also loaded as dynamic modules by other programs.

This section is primarily concerned with how the separation of shared libraries into multiple packages should be done and how dependencies on and between shared library binary packages are managed in Debian. Libraries, Section 10.2 should be read in conjunction with this section and contains additional rules for the files contained in the shared library packages.


8.1 Run-time shared libraries

The run-time shared library must be placed in a package whose name changes whenever the SONAME of the shared library changes. This allows several versions of the shared library to be installed at the same time, allowing installation of the new version of the shared library without immediately breaking binaries that depend on the old version. Normally, the run-time shared library and its SONAME symlink should be placed in a package named librarynamesoversion, where soversion is the version number in the SONAME of the shared library. Alternatively, if it would be confusing to directly append soversion to libraryname (if, for example, libraryname itself ends in a number), you should use libraryname-soversion instead.

To determine the soversion, look at the SONAME of the library, stored in the ELF SONAME attribute. It is usually of the form name.so.major-version (for example, libz.so.1). The version part is the part which comes after .so., so in that example it is 1. The soname may instead be of the form name-major-version.so, such as libdb-5.1.so, in which case the name would be libdb and the version would be 5.1.

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 all of their SONAMEs will always change together. Be aware that this is not normally the case, and if the SONAMEs do not change together, upgrading such a merged shared library package will be unnecessarily difficult because of file conflicts with the old version of the package. When in doubt, always split shared library packages so that each binary package installs a single shared library.

Every time the shared library ABI changes in a way that may break binaries linked against older versions of the shared library, the SONAME of the library and the corresponding name for the binary package containing the runtime shared library should change. Normally, this means the SONAME should change any time an interface is removed from the shared library or the signature of an interface (the number of parameters or the types of parameters that it takes, for example) is changed. This practice is vital to allowing clean upgrades from older versions of the package and clean transitions between the old ABI and new ABI without having to upgrade every affected package simultaneously.

The SONAME and binary package name need not, and indeed normally should not, change if new interfaces are added but none are removed or changed, since this will not break binaries linked against the old shared library. Correct versioning of dependencies on the newer shared library by binaries that use the new interfaces is handled via the symbols or shlibs system.

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 for the SONAME 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.[59]


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[60] must use ldconfig to update the shared library system.

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

[61]


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.[62]


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

If there are development files associated with a shared library, the source package needs to generate a binary development package named librarynamesoversion-dev, or if you prefer only to support one development version at a time, libraryname-dev. Installing the development package must result in installation of all the development files necessary for compiling programs against that shared library.[63]

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 unpacked).

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.

If the package provides Ada Library Information (*.ali) files for use with GNAT, these files must be installed read-only (mode 0444) so that GNAT will not attempt to recompile them. This overrides the normal file mode requirements given in Permissions and owners, Section 10.9.


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. [64]


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.[65]

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.


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.[66] 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.[67] 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[68]. 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.[69]

For more details on dpkg-shlibdeps, see dpkg-shlibdeps(1).

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. [70]


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.[71] 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, Section 8.1. 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, Section 8.6.3.2 and The shlibs File Format, Section 8.6.4.2. 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.


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.

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, Section 8.6.4.1 and The shlibs system, Section 8.6.4 for more information.


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 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.[73]

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.[74]

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.[75] For our example, the zlib1g symbols file would contain:

     		* Build-Depends-Package: zlib1g-dev

Also see deb-symbols(5).


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.[76]

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, Section 8.6.2), 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, Section 8.1 for more information on SONAMEs.


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.)

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, Section 8.1 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, Section 7.1 for details on the syntax, and Shared library ABI changes, Section 8.6.2 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[77].

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.


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Debian Policy Manual

version 3.9.6.1, 2014-11-22

The Debian Policy Mailing List