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123 lines
6.4 KiB
XML
123 lines
6.4 KiB
XML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY % general-entities SYSTEM "../../general.ent">
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%general-entities;
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]>
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<sect1 id="libraries" xreflabel="libraries">
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<?dbhtml filename="libraries.html"?>
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<sect1info>
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<date>$Date$</date>
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</sect1info>
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<title>Libraries: Static or shared?</title>
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<!-- section g : 'Others' in longindex.html -->
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<indexterm zone="libraries">
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<primary sortas="g-libraries">libraries: static or shared</primary>
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</indexterm>
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<sect2 role="package">
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<title>Libraries: Static or shared?</title>
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<para>The original libraries were simply an archive of routines from which
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the required routines were extracted and linked into the executable program.
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These are described as static libraries (libfoo.a). On some old operating
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systems they are the only type available.</para>
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<para>On almost all Linux platforms there are also shared libraries
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(libfoo.so) - one copy of the library is loaded into virtual memory, and
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shared by all the programs which call any of its functions. This is space
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efficient.</para>
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<para>In the past, essential programs such as a shell were often linked
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statically so that some form of minimal recovery system would exist even if
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shared libraries, such as libc.so, became damaged (e.g. moved to
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<filename class="directory">lost+found</filename> after fsck following an
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unclean shutdown). Nowadays, most people use an alternative system install
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or a Live CD if they have to recover. Journaling filesystems also reduce
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the likelihood of this sort of problem.</para>
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<para>Developers, at least while they are developing, often prefer to use
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static versions of the libraries which their code links to.</para>
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<para>Within the book, there are various places where configure switches
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such as <command>--disable-static</command> are employed, and other places
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where the possibility of using system versions of libraries instead of the
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versions included within another package is discussed. The main reason for
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this is to simplify updates of libraries.</para>
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<para>If a package is linked to a dynamic library, updating to a newer
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library version is automatic once the newer library is installed and the
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program is (re)started (provided the library major version is unchanged,
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e.g. going from libfoo.so.2.0 to libfoo.so.2.1. Going to libfoo.so.3
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will require recompilation - <command>ldd</command> can be used to find
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which programs use the old version). If a program is linked to a static
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library, the program always has to be recompiled. If you know which
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programs are linked to a particular static library, this is merely an
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annoyance. But usually you will <emphasis>not</emphasis> know which
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programs to recompile.</para>
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<para>Most libraries are shared, but if you do something unusual, such as
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moving a shared library to <filename class="directory">/lib</filename>
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accidentally breaking the <literal>.so</literal> symlink in
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<filename class="directory">/usr/lib</filename> while keeping the static
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library in <filename class="directory">/lib</filename>, the static library
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will be silently linked into the programs which need it.</para>
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<para>One way to identify when a static library is used, is to deal with it
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at the end of the installation of every package. Write a script to find all
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the static libraries in <filename class="directory">/usr/lib</filename> or
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wherever you are installing to, and either move them to another directory so
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that they are no longer found by the linker, or rename them so that libfoo.a
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becomes e.g. libfoo.a.hidden. The static library can then be temporarily
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restored if it is ever needed, and the package needing it can be
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identified. You may choose to exclude some of the static libraries from
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glibc if you do this (<filename>libc_nonshared.a, libg.a, libieee.a, libm.a,
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libpthread_nonshared.a, librpcsvc.a, libsupc++.a</filename>) to simplify
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compilation.</para>
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<!-- versions hardcoded in this para, it's a comment on those versions -->
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<para>If you use this approach, you may discover that more packages than
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you were expecting use a static library. That was the case with
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<application>nettle-2.4</application> in its default static-only
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configuration: It was required by <application>GnuTLS-3.0.19</application>,
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but also linked into package(s) which used
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<application>GnuTLS</application>, such as
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<application>glib-networking-2.32.3</application>.</para>
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<para>Many packages put some of their common functions into a static
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library which is only used by the programs within the package and,
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crucially, the library is <emphasis>not</emphasis> installed as a
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standalone library. These internal libraries are not a problem - if the
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package has to be rebuilt to fix a bug or vulnerability, nothing else is
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linked to them.</para>
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<para>When BLFS mentions system libraries, it means shared versions of
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libraries. Some packages such as <xref linkend="firefox"/> and
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<xref linkend="gs"/> include many other libraries. When they link to them,
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they link statically so this also makes the programs bigger. The version
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they ship is often older than the version used in the system, so it may
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contain bugs - sometimes developers go to the trouble of fixing bugs in
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their included libraries, other times they do not.</para>
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<para>Sometimes, deciding to use system libraries is an easy decision. Other
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times it may require you to alter the system version (e.g. for
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<xref linkend="libpng"/> if used for <xref linkend="firefox"/>).
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Occasionally, a package ships an old library and can no longer link to
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the current version, but can link to an older version. In this case, BLFS
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will usually just use the shipped version. Sometimes the included library
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is no longer developed separately, or its upstream is now the same as the
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package's upstream and you have no other packages which will use it.
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In those cases, you might decide to use the included static library even if
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you usually prefer to use system libraries.</para>
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<para condition="html" role="usernotes">User Notes:
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<ulink url="&blfs-wiki;/libraries"/></para>
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</sect2>
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</sect1>
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