1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
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3 | "http://www.oasis-open.org/docbook/xml/4.4/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="ch-tools-toolchaintechnotes">
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9 | <?dbhtml filename="toolchaintechnotes.html"?>
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10 |
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11 | <title>Toolchain Technical Notes</title>
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12 |
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13 | <para>This section explains some of the rationale and technical details
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14 | behind the overall build method. It is not essential to immediately
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15 | understand everything in this section. Most of this information will be
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16 | clearer after performing an actual build. This section can be referred
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17 | back to at any time during the process.</para>
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18 |
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19 | <para>The overall goal of <xref linkend="chapter-temporary-tools"/> is to
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20 | provide a temporary environment that can be chrooted into and from which can be
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21 | produced a clean, trouble-free build of the target LFS system in <xref
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22 | linkend="chapter-building-system"/>. Along the way, we separate the new system
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23 | from the host system as much as possible, and in doing so, build a
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24 | self-contained and self-hosted toolchain. It should be noted that the build
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25 | process has been designed to minimize the risks for new readers and provide
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26 | maximum educational value at the same time.</para>
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27 |
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28 | <important>
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29 | <para>Before continuing, be aware of the name of the working platform,
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30 | often referred to as the target triplet. Many times, the target
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31 | triplet will probably be <emphasis>i686-pc-linux-gnu</emphasis>. A
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32 | simple way to determine the name of the target triplet is to run the
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33 | <command>config.guess</command> script that comes with the source for
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34 | many packages. Unpack the Binutils sources and run the script:
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35 | <userinput>./config.guess</userinput> and note the output.</para>
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36 |
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37 | <para>Also be aware of the name of the platform's dynamic linker, often
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38 | referred to as the dynamic loader (not to be confused with the standard
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39 | linker <command>ld</command> that is part of Binutils). The dynamic linker
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40 | provided by Glibc finds and loads the shared libraries needed by a program,
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41 | prepares the program to run, and then runs it. The name of the dynamic
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42 | linker will usually be <filename class="libraryfile">ld-linux.so.2</filename>.
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43 | On platforms that are less prevalent, the name might be <filename
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44 | class="libraryfile">ld.so.1</filename>, and newer 64 bit platforms might
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45 | be named something else entirely. The name of the platform's dynamic linker
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46 | can be determined by looking in the <filename class="directory">/lib</filename>
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47 | directory on the host system. A sure-fire way to determine the name is to
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48 | inspect a random binary from the host system by running:
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49 | <userinput>readelf -l <name of binary> | grep interpreter</userinput>
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50 | and noting the output. The authoritative reference covering all platforms
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51 | is in the <filename>shlib-versions</filename> file in the root of the Glibc
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52 | source tree.</para>
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53 | </important>
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54 |
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55 | <para>Some key technical points of how the <xref
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56 | linkend="chapter-temporary-tools"/> build method works:</para>
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57 |
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58 | <itemizedlist>
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59 | <listitem>
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60 | <para>The process is similar in principle to cross-compiling, whereby
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61 | tools installed in the same prefix work in cooperation, and thus utilize
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62 | a little GNU <quote>magic</quote></para>
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63 | </listitem>
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64 | <listitem>
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65 | <para>Careful manipulation of the standard linker's library search path
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66 | ensures programs are linked only against chosen libraries</para>
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67 | </listitem>
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68 | <listitem>
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69 | <para>Careful manipulation of <command>gcc</command>'s
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70 | <filename>specs</filename> file tells the compiler which target dynamic
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71 | linker will be used</para>
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72 | </listitem>
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73 | </itemizedlist>
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74 |
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75 | <para>Binutils is installed first because the <command>configure</command>
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76 | runs of both GCC and Glibc perform various feature tests on the assembler
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77 | and linker to determine which software features to enable or disable. This
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78 | is more important than one might first realize. An incorrectly configured
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79 | GCC or Glibc can result in a subtly broken toolchain, where the impact of
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80 | such breakage might not show up until near the end of the build of an
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81 | entire distribution. A test suite failure will usually highlight this error
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82 | before too much additional work is performed.</para>
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83 |
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84 | <para>Binutils installs its assembler and linker in two locations,
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85 | <filename class="directory">/tools/bin</filename> and <filename
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86 | class="directory">/tools/$TARGET_TRIPLET/bin</filename>. The tools in one
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87 | location are hard linked to the other. An important facet of the linker is
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88 | its library search order. Detailed information can be obtained from
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89 | <command>ld</command> by passing it the <parameter>--verbose</parameter>
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90 | flag. For example, an <userinput>ld --verbose | grep SEARCH</userinput>
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91 | will illustrate the current search paths and their order. It shows which
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92 | files are linked by <command>ld</command> by compiling a dummy program and
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93 | passing the <parameter>--verbose</parameter> switch to the linker. For example,
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94 | <userinput>gcc dummy.c -Wl,--verbose 2>&1 | grep succeeded</userinput>
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95 | will show all the files successfully opened during the linking.</para>
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96 |
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97 | <para>The next package installed is GCC. An example of what can be
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98 | seen during its run of <command>configure</command> is:</para>
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99 |
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100 | <screen><computeroutput>checking what assembler to use...
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101 | /tools/i686-pc-linux-gnu/bin/as
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102 | checking what linker to use... /tools/i686-pc-linux-gnu/bin/ld</computeroutput></screen>
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103 |
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104 | <para>This is important for the reasons mentioned above. It also demonstrates
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105 | that GCC's configure script does not search the PATH directories to find which
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106 | tools to use. However, during the actual operation of <command>gcc</command>
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107 | itself, the same search paths are not necessarily used. To find out which
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108 | standard linker <command>gcc</command> will use, run:
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109 | <userinput>gcc -print-prog-name=ld</userinput>.</para>
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110 |
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111 | <para>Detailed information can be obtained from <command>gcc</command> by
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112 | passing it the <parameter>-v</parameter> command line option while compiling
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113 | a dummy program. For example, <userinput>gcc -v dummy.c</userinput> will show
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114 | detailed information about the preprocessor, compilation, and assembly stages,
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115 | including <command>gcc</command>'s included search paths and their order.</para>
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116 |
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117 | <para>The next package installed is Glibc. The most important considerations
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118 | for building Glibc are the compiler, binary tools, and kernel headers. The
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119 | compiler is generally not an issue since Glibc will always use the
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120 | <command>gcc</command> found in a <envar>PATH</envar> directory. The binary
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121 | tools and kernel headers can be a bit more complicated. Therefore, take no
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122 | risks and use the available configure switches to enforce the correct
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123 | selections. After the run of <command>configure</command>, check the contents
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124 | of the <filename>config.make</filename> file in the <filename
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125 | class="directory">glibc-build</filename> directory for all important details.
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126 | Note the use of <parameter>CC="gcc -B/tools/bin/"</parameter> to control which
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127 | binary tools are used and the use of the <parameter>-nostdinc</parameter>
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128 | and <parameter>-isystem</parameter> flags to control the compiler's include
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129 | search path. These items highlight an important aspect of the Glibc
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130 | package—it is very self-sufficient in terms of its build machinery and
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131 | generally does not rely on toolchain defaults.</para>
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132 |
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133 | <para>After the Glibc installation, make some adjustments to ensure that
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134 | searching and linking take place only within the <filename
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135 | class="directory">/tools</filename> prefix. Install an adjusted
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136 | <command>ld</command>, which has a hard-wired search path limited to
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137 | <filename class="directory">/tools/lib</filename>. Then amend
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138 | <command>gcc</command>'s specs file to point to the new dynamic linker in
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139 | <filename class="directory">/tools/lib</filename>. This last step is vital
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140 | to the whole process. As mentioned above, a hard-wired path to a dynamic
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141 | linker is embedded into every Executable and Link Format (ELF)-shared
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142 | executable. This can be inspected by running:
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143 | <userinput>readelf -l <name of binary> | grep interpreter</userinput>.
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144 | Amending gcc's specs file ensures that every program compiled from here
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145 | through the end of this chapter will use the new dynamic linker in
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146 | <filename class="directory">/tools/lib</filename>.</para>
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147 |
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148 | <para>The need to use the new dynamic linker is also the reason why
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149 | the Specs patch is applied for the second pass of GCC. Failure to do
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150 | so will result in the GCC programs themselves having the name of the
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151 | dynamic linker from the host system's <filename
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152 | class="directory">/lib</filename> directory embedded into them, which
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153 | would defeat the goal of getting away from the host.</para>
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154 |
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155 | <para>During the second pass of Binutils, we are able to utilize the
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156 | <parameter>--with-lib-path</parameter> configure switch to control
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157 | <command>ld</command>'s library search path. From this point onwards,
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158 | the core toolchain is self-contained and self-hosted. The remainder of
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159 | the <xref linkend="chapter-temporary-tools"/> packages all build against
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160 | the new Glibc in <filename class="directory">/tools</filename>.</para>
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161 |
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162 | <para>Upon entering the chroot environment in <xref
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163 | linkend="chapter-building-system"/>, the first major package to be
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164 | installed is Glibc, due to its self-sufficient nature mentioned above.
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165 | Once this Glibc is installed into <filename
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166 | class="directory">/usr</filename>, perform a quick changeover of the
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167 | toolchain defaults, then proceed in building the rest of the target
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168 | LFS system.</para>
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169 |
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170 | <!-- FIXME: Removed as part of the fix for bug 1061 - we no longer build pass1
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171 | packages statically, therefore this explanation isn't required
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172 |
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173 | <sect2>
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174 | <title>Notes on Static Linking</title>
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175 |
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176 | <para>Besides their specific task, most programs have to perform many
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177 | common and sometimes trivial operations. These include allocating
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178 | memory, searching directories, reading and writing files, string
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179 | handling, pattern matching, arithmetic, and other tasks. Instead of
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180 | obliging each program to reinvent the wheel, the GNU system provides
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181 | all these basic functions in ready-made libraries. The major library
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182 | on any Linux system is Glibc.</para>
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183 |
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184 | <para>There are two primary ways of linking the functions from a
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185 | library to a program that uses them—statically or dynamically. When
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186 | a program is linked statically, the code of the used functions is
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187 | included in the executable, resulting in a rather bulky program. When
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188 | a program is dynamically linked, it includes a reference to the
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189 | dynamic linker, the name of the library, and the name of the function,
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190 | resulting in a much smaller executable. A third option is to use the
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191 | programming interface of the dynamic linker (see <filename>dlopen(3)</filename>
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192 | for more information).</para>
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193 |
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194 | <para>Dynamic linking is the default on Linux and has three major
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195 | advantages over static linking. First, only one copy of the executable
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196 | library code is needed on the hard disk, instead of having multiple
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197 | copies of the same code included in several programs, thus saving
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198 | disk space. Second, when several programs use the same library
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199 | function at the same time, only one copy of the function's code is
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200 | required in core, thus saving memory space. Third, when a library
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201 | function gets a bug fixed or is otherwise improved, only the one
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202 | library needs to be recompiled instead of recompiling all programs
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203 | that make use of the improved function.</para>
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204 |
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205 | <para>If dynamic linking has several advantages, why then do we
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206 | statically link the first two packages in this chapter? The reasons
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207 | are threefold—historical, educational, and technical. The
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208 | historical reason is that earlier versions of LFS statically linked
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209 | every program in this chapter. Educationally, knowing the difference
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210 | between static and dynamic linking is useful. The technical benefit is
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211 | a gained element of independence from the host, meaning that those
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212 | programs can be used independently of the host system. However, it is
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213 | worth noting that an overall successful LFS build can still be
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214 | achieved when the first two packages are built dynamically.</para>
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215 |
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216 | </sect2>-->
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217 |
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218 | </sect1>
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