source: introduction/important/libraries.xml@ ccded7e

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<sect1 id="libraries" xreflabel="libraries">
  <?dbhtml filename="libraries.html"?>

  <sect1info>
    <othername>$LastChangedBy$</othername>
    <date>$Date$</date>
  </sect1info>

  <title>Libraries: Static or shared?</title>

  <!-- section g : 'Others' in longindex.html -->
  <indexterm zone="libraries">
    <primary sortas="g-libraries">libraries: static or shared</primary>
  </indexterm>

  <sect2 role="package">
    <title>Libraries: Static or shared?</title>

    <para>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.</para>

    <para>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.</para>

    <para>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
    <filename class="directory">lost+found</filename> 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.</para>

    <para>Developers, at least while they are developing, often prefer to use
    static versions of the libraries which their code links to.</para>

    <para>Within the book, there are various places where configure switches
    such as <command>--disable-static</command> 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.</para>

    <para>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 - <command>ldd</command> 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 <emphasis>not</emphasis> know which
    programs to recompile.</para>

    <para>Most libraries are shared, but if you do something unusual, such as
    moving a shared library to <filename class="directory">/lib</filename>
    accidentally breaking the <literal>.so</literal> symlink in
    <filename class="directory">/usr/lib</filename> while keeping the static
    library in <filename class="directory">/lib</filename>, the static library
    will be silently linked into the programs which need it.</para>

    <para>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 <filename class="directory">/usr/lib</filename> 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 (<filename>libc_nonshared.a, libg.a, libieee.a, libm.a,
    libpthread_nonshared.a, librpcsvc.a, libsupc++.a</filename>) to simplify
    compilation.</para>

<!-- versions hardcoded in this para, it's a comment on those versions  -->
    <para>If you use this approach, you may discover that more packages than
    you were expecting use a static library. That was the case with
    <application>nettle-2.4</application> in its default static-only
    configuration: It was required by <application>GnuTLS-3.0.19</application>,
    but also linked into package(s) which used
    <application>GnuTLS</application>, such as
    <application>glib-networking-2.32.3</application>.</para>

    <para>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 <emphasis>not</emphasis> 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.</para>

    <para>When BLFS mentions system libraries, it means shared versions of
    libraries. Some packages such as <xref linkend="firefox"/> and
    <xref linkend="gs"/> 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.</para>

    <para>Sometimes, deciding to use system libraries is an easy decision. Other
    times it may require you to alter the system version (e.g. for
    <xref linkend="libpng"/> if used for <xref linkend="firefox"/>).
    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&apos;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.</para>

    <para condition="html" role="usernotes">User Notes:
    <ulink url="&blfs-wiki;/libraries"/></para>

  </sect2>

</sect1>