1 | <sect1 id="ch05-toolchaintechnotes">
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2 | <title>Toolchain technical notes</title>
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3 | <?dbhtml filename="toolchaintechnotes.html" dir="chapter05"?>
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4 |
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5 | <para>This section attempts to explain some of the rationale and technical
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6 | details behind the overall build method. It's not essential that you understand
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7 | everything here immediately. Most of it will make sense once you have performed
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8 | an actual build. Feel free to refer back here at any time.</para>
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9 |
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10 | <para>The overall goal of <xref linkend="chapter05"/> is to provide a sane,
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11 | temporary environment that we can chroot into, and from which we can produce a
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12 | clean, trouble-free build of the target LFS system in
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13 | <xref linkend="chapter06"/>. Along the way, we attempt to divorce ourselves
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14 | from the host system as much as possible, and in so doing build a
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15 | self-contained and self-hosted toolchain. It should be noted that the
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16 | build process has been designed in such a way so as to minimize the risks for
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17 | new readers and provide maximum educational value at the same time. In other
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18 | words, more advanced techniques could be used to build the system.</para>
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19 |
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20 | <important>
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21 | <para>Before continuing, you really should be aware of the name of your working
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22 | platform, often also referred to as the <emphasis>target triplet</emphasis>. For
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23 | many folks the target triplet will be, for example:
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24 | <emphasis>i686-pc-linux-gnu</emphasis>. A simple way to determine your target
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25 | triplet is to run the <filename>config.guess</filename> script that comes with
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26 | the source for many packages. Unpack the Binutils sources and run the script:
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27 | <userinput>./config.guess</userinput> and note the output.</para>
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28 |
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29 | <para>You'll also need to be aware of the name of your platform's
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30 | <emphasis>dynamic linker</emphasis>, often also referred to as the
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31 | <emphasis>dynamic loader</emphasis>, not to be confused with the standard linker
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32 | <emphasis>ld</emphasis> that is part of Binutils. The dynamic linker is provided
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33 | by Glibc and has the job of finding and loading the shared libraries needed by a
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34 | program, preparing the program to run and then running it. For most folks, the
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35 | name of the dynamic linker will be <emphasis>ld-linux.so.2</emphasis>. On
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36 | platforms that are less prevalent, the name might be
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37 | <emphasis>ld.so.1</emphasis> and newer 64 bit platforms might even have
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38 | something completely different. You should be able to determine the name
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39 | of your platform's dynamic linker by looking in the
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40 | <filename class="directory">/lib</filename> directory on your host system. A
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41 | surefire way is to inspect a random binary from your host system by running:
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42 | <userinput>'readelf -l <name of binary> | grep interpreter'</userinput>
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43 | and noting the output. The authoritative reference covering all platforms is in
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44 | the <filename>shlib-versions</filename> file in the root of the Glibc source
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45 | tree.</para>
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46 | </important>
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47 |
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48 | <para>Some key technical points of how the <xref linkend="chapter05"/> build
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49 | method works:</para>
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50 |
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51 | <itemizedlist>
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52 | <listitem><para>Similar in principle to cross compiling whereby tools installed
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53 | into the same prefix work in cooperation and thus utilize a little GNU
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54 | "magic".</para></listitem>
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55 |
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56 | <listitem><para>Careful manipulation of the standard linker's library search
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57 | path to ensure programs are linked only against libraries we
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58 | choose.</para></listitem>
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59 |
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60 | <listitem><para>Careful manipulation of <userinput>gcc</userinput>'s
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61 | <emphasis>specs</emphasis> file to tell the compiler which target dynamic
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62 | linker will be used.</para></listitem>
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63 | </itemizedlist>
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64 |
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65 | <para>Binutils is installed first because both GCC and Glibc perform various
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66 | feature tests on the assembler and linker during their respective runs of
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67 | <userinput>./configure</userinput> to determine which software features to enable
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68 | or disable. This is more important than one might first realize. An incorrectly
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69 | configured GCC or Glibc can result in a subtly broken toolchain where the impact
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70 | of such breakage might not show up until near the end of the build of a whole
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71 | distribution. Thankfully, a test suite failure will usually alert us before too
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72 | much time is wasted.</para>
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73 |
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74 | <para>Binutils installs its assembler and linker into two locations,
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75 | <filename class="directory">/tools/bin</filename> and
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76 | <filename class="directory">/tools/$TARGET_TRIPLET/bin</filename>. In reality,
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77 | the tools in one location are hard linked to the other. An important facet of
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78 | the linker is its library search order. Detailed information can be obtained
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79 | from <userinput>ld</userinput> by passing it the <emphasis>--verbose</emphasis>
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80 | flag. For example: <userinput>'ld --verbose | grep SEARCH'</userinput> will
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81 | show you the current search paths and their order. You can see what files are
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82 | actually linked by <userinput>ld</userinput> by compiling a dummy program and
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83 | passing the <emphasis>--verbose</emphasis> switch. For example:
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84 | <userinput>'gcc dummy.c -Wl,--verbose 2>&1 | grep succeeded'</userinput>
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85 | will show you all the files successfully opened during the link.</para>
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86 |
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87 | <para>The next package installed is GCC and during its run of
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88 | <userinput>./configure</userinput> you'll see, for example:</para>
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89 |
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90 | <blockquote><screen>checking what assembler to use... /tools/i686-pc-linux-gnu/bin/as
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91 | checking what linker to use... /tools/i686-pc-linux-gnu/bin/ld</screen></blockquote>
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92 |
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93 | <para>This is important for the reasons mentioned above. It also demonstrates
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94 | that GCC's configure script does not search the $PATH directories to find which
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95 | tools to use. However, during the actual operation of <userinput>gcc</userinput>
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96 | itself, the same search paths are not necessarily used. You can find out which
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97 | standard linker <userinput>gcc</userinput> will use by running:
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98 | <userinput>'gcc -print-prog-name=ld'</userinput>.
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99 | Detailed information can be obtained from <userinput>gcc</userinput> by passing
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100 | it the <emphasis>-v</emphasis> flag while compiling a dummy program. For
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101 | example: <userinput>'gcc -v dummy.c'</userinput> will show you detailed
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102 | information about the preprocessor, compilation and assembly stages, including
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103 | <userinput>gcc</userinput>'s include search paths and their order.</para>
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104 |
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105 | <para>The next package installed is Glibc. The most important considerations for
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106 | building Glibc are the compiler, binary tools and kernel headers. The compiler
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107 | is generally no problem as Glibc will always use the <userinput>gcc</userinput>
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108 | found in a $PATH directory. The binary tools and kernel headers can be a little
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109 | more troublesome. Therefore we take no risks and use the available configure
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110 | switches to enforce the correct selections. After the run of
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111 | <userinput>./configure</userinput> you can check the contents of the
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112 | <filename>config.make</filename> file in the
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113 | <filename class="directory">glibc-build</filename> directory for all the
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114 | important details. You'll note some interesting items like the use of
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115 | <userinput>CC="gcc -B/tools/bin/"</userinput> to control which binary tools are
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116 | used, and also the use of the <emphasis>-nostdinc</emphasis> and
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117 | <emphasis>-isystem</emphasis> flags to control the compiler's include search
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118 | path. These items help to highlight an important aspect of the Glibc package:
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119 | it is very self-sufficient in terms of its build machinery and generally does
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120 | not rely on toolchain defaults.</para>
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121 |
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122 | <para>After the Glibc installation, we make some adjustments to ensure that
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123 | searching and linking take place only within our <filename>/tools</filename>
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124 | prefix. We install an adjusted <userinput>ld</userinput>, which has a hard-wired
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125 | search path limited to <filename class="directory">/tools/lib</filename>. Then
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126 | we amend <userinput>gcc</userinput>'s specs file to point to our new dynamic
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127 | linker in <filename class="directory">/tools/lib</filename>. This last step is
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128 | <emphasis>vital</emphasis> to the whole process. As mentioned above, a
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129 | hard-wired path to a dynamic linker is embedded into every ELF shared
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130 | executable. You can inspect this by running:
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131 | <userinput>'readelf -l <name of binary> | grep interpreter'</userinput>.
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132 | By amending <userinput>gcc</userinput>'s specs file, we are ensuring that every
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133 | program compiled from here through the end of <xref linkend="chapter05"/> will
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134 | use our new dynamic linker in
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135 | <filename class="directory">/tools/lib</filename>.</para>
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136 |
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137 | <para>The need to use the new dynamic linker is also the reason why we apply the
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138 | Specs patch for the second pass of GCC. Failure to do so will result in the GCC
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139 | programs themselves having the name of the dynamic linker from the host system's
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140 | <filename class="directory">/lib</filename> directory embedded into them, which
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141 | would defeat our goal of getting away from the host.</para>
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142 |
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143 | <para>During the second pass of Binutils, we are able to utilize the
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144 | <emphasis>--with-lib-path</emphasis> configure switch to control
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145 | <userinput>ld</userinput>'s library search path. From this point onwards, the
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146 | core toolchain is self-contained and self-hosted. The remainder of the
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147 | <xref linkend="chapter05"/> packages all build against the new Glibc in
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148 | <filename class="directory">/tools</filename> and all is well.</para>
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149 |
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150 | <para>Upon entering the chroot environment in <xref linkend="chapter06"/>, the
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151 | first major package we install is Glibc, due to its self-sufficient nature that
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152 | we mentioned above. Once this Glibc is installed into
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153 | <filename class="directory">/usr</filename>, we perform a quick changeover of
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154 | the toolchain defaults, then proceed for real in building the rest of the
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155 | target <xref linkend="chapter06"/> LFS system.</para>
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156 |
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157 | <sect2>
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158 | <title>Notes on static linking</title>
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159 |
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160 | <para>Most programs have to perform, beside their specific task, many rather
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161 | common and sometimes trivial operations. These include allocating memory,
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162 | searching directories, reading and writing files, string handling, pattern
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163 | matching, arithmetic and many other tasks. Instead of obliging each program to
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164 | reinvent the wheel, the GNU system provides all these basic functions in
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165 | ready-made libraries. The major library on any Linux system is
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166 | <emphasis>Glibc</emphasis>.</para>
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167 |
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168 | <para>There are two primary ways of linking the functions from a library to a
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169 | program that uses them: statically or dynamically. When a program is linked
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170 | statically, the code of the used functions is included in the executable,
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171 | resulting in a rather bulky program. When a program is dynamically linked, what
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172 | is included is a reference to the dynamic linker, the name of the library, and
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173 | the name of the function, resulting in a much smaller executable. (A third way
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174 | is to use the programming interface of the dynamic linker. See the
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175 | <emphasis>dlopen</emphasis> man page for more information.)</para>
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176 |
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177 | <para>Dynamic linking is the default on Linux and has three major advantages
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178 | over static linking. First, you need only one copy of the executable library
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179 | code on your hard disk, instead of having many copies of the same code included
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180 | into a whole bunch of programs -- thus saving disk space. Second, when several
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181 | programs use the same library function at the same time, only one copy of the
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182 | function's code is required in core -- thus saving memory space. Third, when a
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183 | library function gets a bug fixed or is otherwise improved, you only need to
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184 | recompile this one library, instead of having to recompile all the programs that
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185 | make use of the improved function.</para>
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186 |
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187 | <para>If dynamic linking has several advantages, why then do we statically link
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188 | the first two packages in this chapter? The reasonsare threefold: historical,
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189 | educational, and technical. Historical, because earlier versions of LFS
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190 | statically linked every program in this chapter. Educational, because knowing
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191 | the difference is useful. Technical, because we gain an element of independence
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192 | from the host in doing so, meaning that those programs can be used
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193 | independently of the host system. However, it's worth noting that an overall
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194 | successful LFS build can still be achieved when the first two packages are
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195 | built dynamically.</para>
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196 |
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197 | </sect2>
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198 |
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199 | </sect1>
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200 |
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