The original libraries were simply an archive of routines from which the required routines were extracted and linked into the executable program. These are described as static libraries (libfoo.a). On some old operating systems they are the only type available.
On almost all Linux platforms there are also shared libraries (libfoo.so) - one copy of the library is loaded into virtual memory, and shared by all the programs which call any of its functions. This is space efficient.
In the past, essential programs such as a shell were often linked
statically so that some form of minimal recovery system would exist
even if shared libraries, such as libc.so, became damaged (e.g.
moved to lost+found after fsck
following an unclean shutdown). Nowadays, most people use an
alternative system install or a Live CD if they have to recover.
Journaling filesystems also reduce the likelihood of this sort of
problem.
Developers, at least while they are developing, often prefer to use static versions of the libraries which their code links to.
Within the book, there are various places where configure switches such as --disable-static are employed, and other places where the possibility of using system versions of libraries instead of the versions included within another package is discussed. The main reason for this is to simplify updates of libraries.
If a package is linked to a dynamic library, updating to a newer library version is automatic once the newer library is installed and the program is (re)started (provided the library major version is unchanged, e.g. going from libfoo.so.2.0 to libfoo.so.2.1. Going to libfoo.so.3 will require recompilation - ldd can be used to find which programs use the old version). If a program is linked to a static library, the program always has to be recompiled. If you know which programs are linked to a particular static library, this is merely an annoyance. But usually you will not know which programs to recompile.
Most libraries are shared, but if you do something unusual, such as
moving a shared library to /lib
accidentally breaking the .so symlink
in /usr/lib while keeping the static
library in /lib, the static library
will be silently linked into the programs which need it.
One way to identify when a static library is used, is to deal with
it at the end of the installation of every package. Write a script
to find all the static libraries in /usr/lib or wherever you are installing to, and
either move them to another directory so that they are no longer
found by the linker, or rename them so that libfoo.a becomes e.g.
libfoo.a.hidden. The static library can then be temporarily
restored if it is ever needed, and the package needing it can be
identified. You may choose to exclude some of the static libraries
from glibc if you do this (libc_nonshared.a,
libg.a, libieee.a, libm.a, libpthread_nonshared.a, librpcsvc.a,
libsupc++.a) to simplify compilation.
If you use this approach, you may discover that more packages than you were expecting use a static library. That was the case with nettle-2.4 in its default static-only configuration: It was required by GnuTLS-3.0.19, but also linked into package(s) which used GnuTLS, such as glib-networking-2.32.3.
Many packages put some of their common functions into a static library which is only used by the programs within the package and, crucially, the library is not installed as a standalone library. These internal libraries are not a problem - if the package has to be rebuilt to fix a bug or vulnerability, nothing else is linked to them.
When BLFS mentions system libraries, it means shared versions of libraries. Some packages such as Firefox-91.0.1 and ghostscript-9.54.0 include many other libraries. When they link to them, they link statically so this also makes the programs bigger. The version they ship is often older than the version used in the system, so it may contain bugs - sometimes developers go to the trouble of fixing bugs in their included libraries, other times they do not.
Sometimes, deciding to use system libraries is an easy decision. Other times it may require you to alter the system version (e.g. for libpng-1.6.37 if used for Firefox-91.0.1). Occasionally, a package ships an old library and can no longer link to the current version, but can link to an older version. In this case, BLFS will usually just use the shipped version. Sometimes the included library is no longer developed separately, or its upstream is now the same as the package's upstream and you have no other packages which will use it. In those cases, you might decide to use the included static library even if you usually prefer to use system libraries.
User Notes: https://wiki.linuxfromscratch.org/blfs/wiki/libraries
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