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142 lines
6.9 KiB
XML
142 lines
6.9 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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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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<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>
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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
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program. These are described as static libraries, with names of the form
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<filename>libfoo.a</filename> on UNIX-like operating systems.
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On some old operating systems they are the only type available.
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</para>
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<para>
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On almost all Linux platforms there are also <quote>shared</quote>
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(or equivalently <quote>dynamic</quote>)
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libraries (with names of the form <filename>libfoo.so</filename>) –
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one copy of the library is loaded into virtual memory, and shared by
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all the programs which call any of its functions. This is space
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efficient.
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</para>
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<para>
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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
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if shared libraries, such as <filename>libc.so</filename>, became
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damaged (e.g. moved to <filename class="directory">lost+found</filename>
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after <command>fsck</command> following an unclean shutdown). Nowadays,
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most people use an alternative system install or a USB stick if they
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have to recover. Journaling filesystems also reduce the likelihood of
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this sort of problem.
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</para>
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<!-- really?
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<para>
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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.
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</para>
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-->
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<para>
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Within the book, there are various places where configure switches
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such as <parameter>--disable-static</parameter> are employed, and
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other places where the possibility of using system versions of
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libraries instead of the versions included within another package is
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discussed. The main reason for this is to simplify updates of libraries.
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</para>
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<para>
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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 <filename>libfoo.so.2.0</filename> to
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<filename>libfoo.so.2.1</filename>. Going to
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<filename>libfoo.so.3</filename> will require recompilation –
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<command>ldd</command> can be used to find which programs use the old
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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.
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</para>
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<!-- obsolete with /usr merge
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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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-->
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<para>
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One way to identify when a static library is used, is to deal with
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it at the end of the installation of every package. Write a script
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to find all the static libraries in <filename
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class="directory">/usr/lib</filename> or wherever you are installing
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to, and either move them to another directory so that they are no
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longer found by the linker, or rename them so that
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<filename>libfoo.a</filename> becomes
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e.g. <filename>libfoo.a.hidden</filename>. The static library can then
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be temporarily restored if it is ever needed, and the package needing
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it can be identified. This shouldn't be done blindly since many
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libraries only exist in a static version. For example, some libraries
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from the <application>glibc</application> and
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<application>gcc</application> packages should always be
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present on the system (<filename>libc_nonshared.a, libg.a,
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libpthread_nonshared.a, libssp_nonshared.a, libsupc++.a</filename>
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as of glibc-2.36 and gcc-12.2).
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</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
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the package has to be rebuilt to fix a bug or vulnerability, nothing else
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is 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"/> bundle many other libraries in their build tree.
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The version they ship is often older than the version used in the system,
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so it may contain bugs – sometimes developers go to the trouble of
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fixing bugs in their included libraries, other times they do not.</para>
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<para>Sometimes, deciding to use system libraries is an easy decision.
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Other 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'll be lead to use the included library even if
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you usually prefer to use system libraries.</para>
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</sect2>
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</sect1>
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