1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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2 | <!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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4 | <!ENTITY % general-entities SYSTEM "../../general.ent">
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5 | %general-entities;
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6 | ]>
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7 |
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8 | <sect1 id="libraries" xreflabel="libraries">
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9 | <?dbhtml filename="libraries.html"?>
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10 |
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11 | <sect1info>
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12 | <date>$Date$</date>
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13 | </sect1info>
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14 |
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15 | <title>Libraries: Static or shared?</title>
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16 |
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17 | <!-- section g : 'Others' in longindex.html -->
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18 | <indexterm zone="libraries">
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19 | <primary sortas="g-libraries">libraries: static or shared</primary>
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20 | </indexterm>
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21 |
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22 | <sect2 role="package">
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23 | <title>Libraries: Static or shared?</title>
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24 |
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25 | <para>The original libraries were simply an archive of routines from which
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26 | the required routines were extracted and linked into the executable program.
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27 | These are described as static libraries (libfoo.a). On some old operating
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28 | systems they are the only type available.</para>
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29 |
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30 | <para>On almost all Linux platforms there are also shared libraries
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31 | (libfoo.so) - one copy of the library is loaded into virtual memory, and
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32 | shared by all the programs which call any of its functions. This is space
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33 | efficient.</para>
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34 |
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35 | <para>In the past, essential programs such as a shell were often linked
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36 | statically so that some form of minimal recovery system would exist even if
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37 | shared libraries, such as libc.so, became damaged (e.g. moved to
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38 | <filename class="directory">lost+found</filename> after fsck following an
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39 | unclean shutdown). Nowadays, most people use an alternative system install
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40 | or a Live CD if they have to recover. Journaling filesystems also reduce
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41 | the likelihood of this sort of problem.</para>
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42 |
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43 | <para>Developers, at least while they are developing, often prefer to use
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44 | static versions of the libraries which their code links to.</para>
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45 |
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46 | <para>Within the book, there are various places where configure switches
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47 | such as <command>--disable-static</command> are employed, and other places
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48 | where the possibility of using system versions of libraries instead of the
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49 | versions included within another package is discussed. The main reason for
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50 | this is to simplify updates of libraries.</para>
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51 |
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52 | <para>If a package is linked to a dynamic library, updating to a newer
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53 | library version is automatic once the newer library is installed and the
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54 | program is (re)started (provided the library major version is unchanged,
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55 | e.g. going from libfoo.so.2.0 to libfoo.so.2.1. Going to libfoo.so.3
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56 | will require recompilation - <command>ldd</command> can be used to find
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57 | which programs use the old version). If a program is linked to a static
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58 | library, the program always has to be recompiled. If you know which
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59 | programs are linked to a particular static library, this is merely an
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60 | annoyance. But usually you will <emphasis>not</emphasis> know which
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61 | programs to recompile.</para>
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62 |
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63 | <para>Most libraries are shared, but if you do something unusual, such as
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64 | moving a shared library to <filename class="directory">/lib</filename>
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65 | accidentally breaking the <literal>.so</literal> symlink in
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66 | <filename class="directory">/usr/lib</filename> while keeping the static
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67 | library in <filename class="directory">/lib</filename>, the static library
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68 | will be silently linked into the programs which need it.</para>
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69 |
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70 | <para>One way to identify when a static library is used, is to deal with it
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71 | at the end of the installation of every package. Write a script to find all
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72 | the static libraries in <filename class="directory">/usr/lib</filename> or
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73 | wherever you are installing to, and either move them to another directory so
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74 | that they are no longer found by the linker, or rename them so that libfoo.a
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75 | becomes e.g. libfoo.a.hidden. The static library can then be temporarily
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76 | restored if it is ever needed, and the package needing it can be
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77 | identified. You may choose to exclude some of the static libraries from
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78 | glibc if you do this (<filename>libc_nonshared.a, libg.a, libieee.a, libm.a,
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79 | libpthread_nonshared.a, librpcsvc.a, libsupc++.a</filename>) to simplify
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80 | compilation.</para>
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81 |
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82 | <!-- versions hardcoded in this para, it's a comment on those versions -->
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83 | <para>If you use this approach, you may discover that more packages than
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84 | you were expecting use a static library. That was the case with
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85 | <application>nettle-2.4</application> in its default static-only
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86 | configuration: It was required by <application>GnuTLS-3.0.19</application>,
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87 | but also linked into package(s) which used
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88 | <application>GnuTLS</application>, such as
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89 | <application>glib-networking-2.32.3</application>.</para>
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90 |
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91 | <para>Many packages put some of their common functions into a static
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92 | library which is only used by the programs within the package and,
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93 | crucially, the library is <emphasis>not</emphasis> installed as a
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94 | standalone library. These internal libraries are not a problem - if the
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95 | package has to be rebuilt to fix a bug or vulnerability, nothing else is
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96 | linked to them.</para>
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97 |
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98 | <para>When BLFS mentions system libraries, it means shared versions of
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99 | libraries. Some packages such as <xref linkend="firefox"/> and
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100 | <xref linkend="gs"/> include many other libraries. When they link to them,
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101 | they link statically so this also makes the programs bigger. The version
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102 | they ship is often older than the version used in the system, so it may
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103 | contain bugs - sometimes developers go to the trouble of fixing bugs in
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104 | their included libraries, other times they do not.</para>
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105 |
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106 | <para>Sometimes, deciding to use system libraries is an easy decision. Other
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107 | times it may require you to alter the system version (e.g. for
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108 | <xref linkend="libpng"/> if used for <xref linkend="firefox"/>).
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109 | Occasionally, a package ships an old library and can no longer link to
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110 | the current version, but can link to an older version. In this case, BLFS
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111 | will usually just use the shipped version. Sometimes the included library
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112 | is no longer developed separately, or its upstream is now the same as the
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113 | package's upstream and you have no other packages which will use it.
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114 | In those cases, you might decide to use the included static library even if
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115 | you usually prefer to use system libraries.</para>
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116 |
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117 | <para condition="html" role="usernotes">User Notes:
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118 | <ulink url="&blfs-wiki;/libraries"/></para>
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119 |
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120 | </sect2>
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121 |
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122 | </sect1>
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