(debian-policy.info)Shared libraries

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8 Shared libraries
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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.  Note: Run-time shared libraries. describes how to
do this.  (1)

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.  Note:
Libraries. should be read in conjunction with this section and
contains additional rules for the files contained in the shared library
packages.

Run-time shared libraries

Shared library support files

Static libraries

Development files

Dependencies between the packages of the same library

Dependencies between the library and other packages

   ---------- Footnotes ----------

   (1) This is a convention of shared library versioning, but not a
requirement.  Some libraries use the ‘SONAME’ as the full library file
name instead and therefore do not need a symlink.  Most, however, encode
additional information about backwards-compatible revisions as a minor
version number in the file name.  The ‘SONAME’ itself only changes when
binaries linked with the earlier version of the shared library may no
longer work, but the filename may change with each release of the
library.  See Note: Run-time shared libraries. for more information.