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README: describe how our libraries are linked

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In https://github.com/systemd/systemd/pull/27637#issuecomment-1547517316
we discussed disclaiming warranty when distros do version mixing.
But to make this disclaimer meaningful, we need to document what options are
available.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2023-05-15 22:35:10 +02:00
parent 4bc96dc162
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@ -282,18 +282,54 @@ POLICY FOR SUPPORT OF DISTRIBUTIONS AND ARCHITECTURES:
latest Ubuntu LTS and non-LTS releases, openSUSE Tumbleweed/Leap,
CentOS Stream 8 and 9, up-to-date Arch, etc.) We will generally
attempt to support also other non-EOL versions of various distros.
Features which would break compilation on slightly-older distributions
Features which would break compilation on slightly older distributions
will only be introduced if there are significant reasons for this
(i.e. supporting them interferes with development or requires too many
resources to support). In some cases backports of specific libraries or
tools might be required.
The policy is similar wrt. architecture support. systemd is regularly
The policy is similar for architecture support. systemd is regularly
tested on popular architectures (currently amd64, i386, arm64, ppc64el,
and s390x), but should compile and work also on other architectures, for
which support has been added. systemd will emit warnings when
architecture-specific constants are not defined.
STATIC COMPILATION AND "STANDALONE" BINARIES:
systemd provides a public shared libraries libsystemd.so and
libudev.so. The latter is deprecated, and the sd-device APIs in
libsystemd should be used instead for new code. In addition, systemd is
built with a private shared library, libsystemd-shared-<suffix>.so,
that also includes the libsystemd code, and by default most systemd
binaries are linked to it. Using shared libraries saves disk space and
memory at runtime, because only one copy of the code is needed.
It is possible to build static versions of systemd public shared
libraries (via the configuration options '-Dstatic-libsystemd' and
'-Dstatic-libudev'). This allows the libsystemd and libudev code to be
linked statically into programs. In addition, it is possible to disable
the use of libsystemd-shared-<suffix>.so for various components (via
the configuration options '-Dlink-*-shared'). In this mode, the
libsystemd and libsystemd-shared code is linked statically into
selected binaries. This option is intended for systems where some of
the components are intended to be delivered independently of the main
systemd package. Finally, some binaries can be compiled in a second
version (via the configuration option '-Dstandalone-binaries'). The
version suffixed with ".standalone" has the libsystemd and
libsystemd-shared code linked statically. Those binaries are intended
as replacements to be used in limited installations where the full
systemd is not installed. Yet another option is to rebuild systemd with
a different '-Dshared-lib-tag' setting, allowing different systemd
binaries to be linked to instances of the private shared library that
can be installed in parallel.
Using the default shared linking is recommended. Mixing versions of
systemd components that would normally be built and used together (in
particular various daemons and the manager) is not recommended: we do
not test such combinations upstream and cannot provide support.
Distributors making use of those options are responsible if things do
not work as expected.
USERS AND GROUPS:
Default udev rules use the following standard system group names, which
need to be resolvable by getgrnam() at any time, even in the very early