1 | <?xml version="1.0" encoding="UTF-8"?>
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2 | <!DOCTYPE sect1 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="ch-tools-toolchaintechnotes" xreflabel="Toolchain Technical Notes">
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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. Don't try 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. Come back and re-read this chapter
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17 | at any time during the build process.</para>
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18 |
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19 | <para>The overall goal of <xref linkend="chapter-cross-tools"/> and <xref
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20 | linkend="chapter-temporary-tools"/> is to produce a temporary area
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21 | containing a set of tools that are known to be good, and that are isolated from the host system.
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22 | By using the <command>chroot</command> command, the compilations in the remaining chapters
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23 | will be isolated within that environment, ensuring a clean, trouble-free
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24 | build of the target LFS system. The build process has been designed to
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25 | minimize the risks for new readers, and to provide the most educational value
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26 | at the same time.</para>
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27 |
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28 | <para>This build process is based on
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29 | <emphasis>cross-compilation</emphasis>. Cross-compilation is normally used
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30 | to build a compiler and its associated toolchain for a machine different from
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31 | the one that is used for the build. This is not strictly necessary for LFS,
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32 | since the machine where the new system will run is the same as the one
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33 | used for the build. But cross-compilation has one great advantage:
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34 | anything that is cross-compiled cannot depend on the host environment.</para>
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35 |
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36 | <sect2 id="cross-compile" xreflabel="About Cross-Compilation">
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37 |
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38 | <title>About Cross-Compilation</title>
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39 |
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40 | <note>
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41 | <para>
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42 | The LFS book is not (and does not contain) a general tutorial to
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43 | build a cross- (or native) toolchain. Don't use the commands in the
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44 | book for a cross-toolchain for some purpose other
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45 | than building LFS, unless you really understand what you are doing.
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46 | </para>
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47 | </note>
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48 |
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49 | <para>Cross-compilation involves some concepts that deserve a section of
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50 | their own. Although this section may be omitted on a first reading,
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51 | coming back to it later will help you gain a fuller understanding of
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52 | the process.</para>
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53 |
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54 | <para>Let us first define some terms used in this context.</para>
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55 |
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56 | <variablelist>
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57 | <varlistentry><term>The build</term><listitem>
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58 | <para>is the machine where we build programs. Note that this machine
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59 | is also referred to as the <quote>host</quote>.</para></listitem>
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60 | </varlistentry>
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61 |
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62 | <varlistentry><term>The host</term><listitem>
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63 | <para>is the machine/system where the built programs will run. Note
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64 | that this use of <quote>host</quote> is not the same as in other
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65 | sections.</para></listitem>
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66 | </varlistentry>
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67 |
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68 | <varlistentry><term>The target</term><listitem>
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69 | <para>is only used for compilers. It is the machine the compiler
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70 | produces code for. It may be different from both the build and
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71 | the host.</para></listitem>
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72 | </varlistentry>
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73 |
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74 | </variablelist>
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75 |
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76 | <para>As an example, let us imagine the following scenario (sometimes
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77 | referred to as <quote>Canadian Cross</quote>). We have a
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78 | compiler on a slow machine only, let's call it machine A, and the compiler
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79 | ccA. We also have a fast machine (B), but no compiler for (B), and we
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80 | want to produce code for a third, slow machine (C). We will build a
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81 | compiler for machine C in three stages.</para>
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82 |
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83 | <informaltable align="center">
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84 | <tgroup cols="5">
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85 | <colspec colnum="1" align="center"/>
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86 | <colspec colnum="2" align="center"/>
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87 | <colspec colnum="3" align="center"/>
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88 | <colspec colnum="4" align="center"/>
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89 | <colspec colnum="5" align="left"/>
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90 | <thead>
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91 | <row><entry>Stage</entry><entry>Build</entry><entry>Host</entry>
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92 | <entry>Target</entry><entry>Action</entry></row>
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93 | </thead>
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94 | <tbody>
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95 | <row>
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96 | <entry>1</entry><entry>A</entry><entry>A</entry><entry>B</entry>
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97 | <entry>Build cross-compiler cc1 using ccA on machine A.</entry>
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98 | </row>
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99 | <row>
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100 | <entry>2</entry><entry>A</entry><entry>B</entry><entry>C</entry>
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101 | <entry>Build cross-compiler cc2 using cc1 on machine A.</entry>
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102 | </row>
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103 | <row>
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104 | <entry>3</entry><entry>B</entry><entry>C</entry><entry>C</entry>
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105 | <entry>Build compiler ccC using cc2 on machine B.</entry>
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106 | </row>
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107 | </tbody>
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108 | </tgroup>
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109 | </informaltable>
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110 |
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111 | <para>Then, all the programs needed by machine C can be compiled
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112 | using cc2 on the fast machine B. Note that unless B can run programs
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113 | produced for C, there is no way to test the newly built programs until machine
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114 | C itself is running. For example, to run a test suite on ccC, we may want to add a
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115 | fourth stage:</para>
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116 |
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117 | <informaltable align="center">
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118 | <tgroup cols="5">
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119 | <colspec colnum="1" align="center"/>
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120 | <colspec colnum="2" align="center"/>
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121 | <colspec colnum="3" align="center"/>
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122 | <colspec colnum="4" align="center"/>
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123 | <colspec colnum="5" align="left"/>
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124 | <thead>
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125 | <row><entry>Stage</entry><entry>Build</entry><entry>Host</entry>
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126 | <entry>Target</entry><entry>Action</entry></row>
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127 | </thead>
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128 | <tbody>
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129 | <row>
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130 | <entry>4</entry><entry>C</entry><entry>C</entry><entry>C</entry>
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131 | <entry>Rebuild and test ccC using ccC on machine C.</entry>
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132 | </row>
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133 | </tbody>
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134 | </tgroup>
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135 | </informaltable>
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136 |
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137 | <para>In the example above, only cc1 and cc2 are cross-compilers, that is,
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138 | they produce code for a machine different from the one they are run on.
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139 | The other compilers ccA and ccC produce code for the machine they are run
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140 | on. Such compilers are called <emphasis>native</emphasis> compilers.</para>
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141 |
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142 | </sect2>
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143 |
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144 | <sect2 id="lfs-cross">
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145 | <title>Implementation of Cross-Compilation for LFS</title>
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146 |
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147 | <note>
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148 | <para>All the cross-compiled packages in this book use an
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149 | autoconf-based building system. The autoconf-based building system
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150 | accepts system types in the form cpu-vendor-kernel-os,
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151 | referred to as the system triplet. Since the vendor field is often
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152 | irrelevant, autoconf lets you omit it.</para>
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153 |
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154 | <para>An astute reader may wonder
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155 | why a <quote>triplet</quote> refers to a four component name. The
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156 | kernel field and the os field began as a single
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157 | <quote>system</quote> field. Such a three-field form is still valid
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158 | today for some systems, for example,
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159 | <literal>x86_64-unknown-freebsd</literal>. But
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160 | two systems can share the same kernel and still be too different to
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161 | use the same triplet to describe them. For example, Android running on a
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162 | mobile phone is completely different from Ubuntu running on an ARM64
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163 | server, even though they are both running on the same type of CPU (ARM64) and
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164 | using the same kernel (Linux).</para>
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165 |
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166 | <para>Without an emulation layer, you cannot run an
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167 | executable for a server on a mobile phone or vice versa. So the
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168 | <quote>system</quote> field has been divided into kernel and os fields, to
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169 | designate these systems unambiguously. In our example, the Android
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170 | system is designated <literal>aarch64-unknown-linux-android</literal>,
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171 | and the Ubuntu system is designated
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172 | <literal>aarch64-unknown-linux-gnu</literal>.</para>
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173 |
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174 | <para>The word <quote>triplet</quote> remains embedded in the lexicon. A simple way to determine your
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175 | system triplet is to run the <command>config.guess</command>
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176 | script that comes with the source for many packages. Unpack the binutils
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177 | sources, run the script <userinput>./config.guess</userinput>, and note
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178 | the output. For example, for a 32-bit Intel processor the
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179 | output will be <emphasis>i686-pc-linux-gnu</emphasis>. On a 64-bit
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180 | system it will be <emphasis>x86_64-pc-linux-gnu</emphasis>. On most
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181 | Linux systems the even simpler <command>gcc -dumpmachine</command> command
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182 | will give you similar information.</para>
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183 |
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184 | <para>You should also be aware of the name of the platform's dynamic linker, often
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185 | referred to as the dynamic loader (not to be confused with the standard
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186 | linker <command>ld</command> that is part of binutils). The dynamic linker
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187 | provided by package glibc finds and loads the shared libraries needed by a
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188 | program, prepares the program to run, and then runs it. The name of the
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189 | dynamic linker for a 32-bit Intel machine is <filename
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190 | class="libraryfile">ld-linux.so.2</filename>; it's <filename
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191 | class="libraryfile">ld-linux-x86-64.so.2</filename> on 64-bit systems. A
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192 | sure-fire way to determine the name of the dynamic linker is to inspect a
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193 | random binary from the host system by running: <userinput>readelf -l
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194 | <name of binary> | grep interpreter</userinput> and noting the
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195 | output. The authoritative reference covering all platforms is in the
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196 | <filename>shlib-versions</filename> file in the root of the glibc source
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197 | tree.</para>
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198 | </note>
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199 |
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200 | <para>In order to fake a cross-compilation in LFS, the name of the host triplet
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201 | is slightly adjusted by changing the "vendor" field in the
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202 | <envar>LFS_TGT</envar> variable so it says "lfs". We also use the
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203 | <parameter>--with-sysroot</parameter> option when building the cross-linker and
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204 | cross-compiler, to tell them where to find the needed host files. This
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205 | ensures that none of the other programs built in <xref
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206 | linkend="chapter-temporary-tools"/> can link to libraries on the build
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207 | machine. Only two stages are mandatory, plus one more for tests.</para>
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208 |
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209 | <informaltable align="center">
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210 | <tgroup cols="5">
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211 | <colspec colnum="1" align="center"/>
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212 | <colspec colnum="2" align="center"/>
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213 | <colspec colnum="3" align="center"/>
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214 | <colspec colnum="4" align="center"/>
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215 | <colspec colnum="5" align="left"/>
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216 | <thead>
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217 | <row><entry>Stage</entry><entry>Build</entry><entry>Host</entry>
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218 | <entry>Target</entry><entry>Action</entry></row>
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219 | </thead>
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220 | <tbody>
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221 | <row>
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222 | <entry>1</entry><entry>pc</entry><entry>pc</entry><entry>lfs</entry>
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223 | <entry>Build cross-compiler cc1 using cc-pc on pc.</entry>
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224 | </row>
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225 | <row>
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226 | <entry>2</entry><entry>pc</entry><entry>lfs</entry><entry>lfs</entry>
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227 | <entry>Build compiler cc-lfs using cc1 on pc.</entry>
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228 | </row>
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229 | <row>
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230 | <entry>3</entry><entry>lfs</entry><entry>lfs</entry><entry>lfs</entry>
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231 | <entry>Rebuild and test cc-lfs using cc-lfs on lfs.</entry>
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232 | </row>
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233 | </tbody>
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234 | </tgroup>
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235 | </informaltable>
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236 |
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237 | <para>In the preceding table, <quote>on pc</quote> means the commands are run
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238 | on a machine using the already installed distribution. <quote>On
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239 | lfs</quote> means the commands are run in a chrooted environment.</para>
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240 |
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241 | <para>This is not yet the end of the story. The C language is not
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242 | merely a compiler; it also defines a standard library. In this book, the
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243 | GNU C library, named glibc, is used (there is an alternative, "musl"). This library must
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244 | be compiled for the LFS machine; that is, using the cross-compiler cc1.
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245 | But the compiler itself uses an internal library providing complex
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246 | subroutines for functions not available in the assembler instruction set. This
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247 | internal library is named libgcc, and it must be linked to the glibc
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248 | library to be fully functional. Furthermore, the standard library for
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249 | C++ (libstdc++) must also be linked with glibc. The solution to this
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250 | chicken and egg problem is first to build a degraded cc1-based libgcc,
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251 | lacking some functionalities such as threads and exception handling, and then
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252 | to build glibc using this degraded compiler (glibc itself is not
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253 | degraded), and also to build libstdc++. This last library will lack some of the
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254 | functionality of libgcc.</para>
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255 |
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256 | <para>The upshot of the preceding paragraph is that cc1 is unable to
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257 | build a fully functional libstdc++ with the degraded libgcc, but cc1
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258 | is the only compiler available for building the C/C++ libraries
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259 | during stage 2. There are two reasons we don't immediately use the
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260 | compiler built in stage 2, cc-lfs, to build those libraries.</para>
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261 |
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262 | <itemizedlist>
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263 | <listitem>
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264 | <para>
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265 | Generally speaking, cc-lfs cannot run on pc (the host system). Even though the
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266 | triplets for pc and lfs are compatible with each other, an executable
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267 | for lfs must depend on glibc-&glibc-version;; the host distro
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268 | may utilize either a different implementation of libc (for example, musl), or
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269 | a previous release of glibc (for example, glibc-2.13).
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270 | </para>
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271 | </listitem>
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272 | <listitem>
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273 | <para>
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274 | Even if cc-lfs can run on pc, using it on pc would create
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275 | a risk of linking to the pc libraries, since cc-lfs is a native
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276 | compiler.
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277 | </para>
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278 | </listitem>
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279 | </itemizedlist>
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280 |
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281 | <para>So when we build gcc stage 2, we instruct the building system to
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282 | rebuild libgcc and libstdc++ with cc1, but we link libstdc++ to the newly
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283 | rebuilt libgcc instead of the old, degraded build. This makes the rebuilt
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284 | libstdc++ fully functional.</para>
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285 |
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286 | <para>In &ch-final; (or <quote>stage 3</quote>), all the packages needed for
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287 | the LFS system are built. Even if a package has already been installed into
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288 | the LFS system in a previous chapter, we still rebuild the package. The main reason for
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289 | rebuilding these packages is to make them stable: if we reinstall an LFS
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290 | package on a completed LFS system, the reinstalled content of the package
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291 | should be the same as the content of the same package when first installed in
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292 | &ch-final;. The temporary packages installed in &ch-tmp-cross; or
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293 | &ch-tmp-chroot; cannot satisfy this requirement, because some of them
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294 | are built without optional dependencies, and autoconf cannot
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295 | perform some feature checks in &ch-tmp-cross; because of cross-compilation,
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296 | causing the temporary packages to lack optional features,
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297 | or use suboptimal code routines. Additionally, a minor reason for
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298 | rebuilding the packages is to run the test suites.</para>
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299 |
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300 | </sect2>
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301 |
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302 | <sect2 id="other-details">
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303 |
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304 | <title>Other Procedural Details</title>
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305 |
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306 | <para>The cross-compiler will be installed in a separate <filename
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307 | class="directory">$LFS/tools</filename> directory, since it will not
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308 | be part of the final system.</para>
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309 |
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310 | <para>Binutils is installed first because the <command>configure</command>
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311 | runs of both gcc and glibc perform various feature tests on the assembler
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312 | and linker to determine which software features to enable or disable. This
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313 | is more important than one might realize at first. An incorrectly configured
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314 | gcc or glibc can result in a subtly broken toolchain, where the impact of
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315 | such breakage might not show up until near the end of the build of an
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316 | entire distribution. A test suite failure will usually highlight this error
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317 | before too much additional work is performed.</para>
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318 |
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319 | <para>Binutils installs its assembler and linker in two locations,
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320 | <filename class="directory">$LFS/tools/bin</filename> and <filename
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321 | class="directory">$LFS/tools/$LFS_TGT/bin</filename>. The tools in one
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322 | location are hard linked to the other. An important facet of the linker is
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323 | its library search order. Detailed information can be obtained from
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324 | <command>ld</command> by passing it the <parameter>--verbose</parameter>
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325 | flag. For example, <command>$LFS_TGT-ld --verbose | grep SEARCH</command>
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326 | will illustrate the current search paths and their order. (Note that this
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327 | example can be run as shown only while logged in as user
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328 | <systemitem class="username">lfs</systemitem>. If you come back to this
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329 | page later, replace <command>$LFS_TGT-ld</command> with
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330 | <command>ld</command>).</para>
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331 |
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332 | <para>The next package installed is gcc. An example of what can be
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333 | seen during its run of <command>configure</command> is:</para>
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334 |
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335 | <screen><computeroutput>checking what assembler to use... /mnt/lfs/tools/i686-lfs-linux-gnu/bin/as
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336 | checking what linker to use... /mnt/lfs/tools/i686-lfs-linux-gnu/bin/ld</computeroutput></screen>
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337 |
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338 | <para>This is important for the reasons mentioned above. It also
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339 | demonstrates that gcc's configure script does not search the PATH
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340 | directories to find which tools to use. However, during the actual
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341 | operation of <command>gcc</command> itself, the same search paths are not
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342 | necessarily used. To find out which standard linker <command>gcc</command>
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343 | will use, run: <command>$LFS_TGT-gcc -print-prog-name=ld</command>. (Again,
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344 | remove the <command>$LFS_TGT-</command> prefix if coming back to this
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345 | later.)</para>
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346 |
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347 | <para>Detailed information can be obtained from <command>gcc</command> by
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348 | passing it the <parameter>-v</parameter> command line option while compiling
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349 | a program. For example, <command>$LFS_TGT-gcc -v
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350 | <replaceable>example.c</replaceable></command> (or without <command>
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351 | $LFS_TGT-</command> if coming back later) will show
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352 | detailed information about the preprocessor, compilation, and assembly
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353 | stages, including <command>gcc</command>'s search paths for included
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354 | headers and their order.</para>
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355 |
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356 | <para>Next up: sanitized Linux API headers. These allow the
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357 | standard C library (glibc) to interface with features that the Linux
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358 | kernel will provide.</para>
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359 |
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360 | <para>Next comes glibc. The most important
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361 | considerations for building glibc are the compiler, binary tools, and
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362 | kernel headers. The compiler is generally not an issue since glibc will
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363 | always use the compiler relating to the <parameter>--host</parameter>
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364 | parameter passed to its configure script; e.g., in our case, the compiler
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365 | will be <command>$LFS_TGT-gcc</command>. The binary tools and kernel
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366 | headers can be a bit more complicated. Therefore, we take no risks and use
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367 | the available configure switches to enforce the correct selections. After
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368 | the run of <command>configure</command>, check the contents of the
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369 | <filename>config.make</filename> file in the <filename
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370 | class="directory">build</filename> directory for all important details.
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371 | Note the use of <parameter>CC="$LFS_TGT-gcc"</parameter> (with
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372 | <envar>$LFS_TGT</envar> expanded) to control which binary tools are used
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373 | and the use of the <parameter>-nostdinc</parameter> and
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374 | <parameter>-isystem</parameter> flags to control the compiler's include
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375 | search path. These items highlight an important aspect of the glibc
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376 | package—it is very self-sufficient in terms of its build machinery,
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377 | and generally does not rely on toolchain defaults.</para>
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378 |
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379 | <para>As mentioned above, the standard C++ library is compiled next, followed in
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380 | <xref linkend="chapter-temporary-tools"/> by other programs that must
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381 | be cross-compiled to break circular dependencies at build time.
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382 | The install step of all those packages uses the
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383 | <envar>DESTDIR</envar> variable to force installation
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384 | in the LFS filesystem.</para>
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385 |
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386 | <para>At the end of <xref linkend="chapter-temporary-tools"/> the native
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387 | LFS compiler is installed. First binutils-pass2 is built,
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388 | in the same <envar>DESTDIR</envar> directory as the other programs,
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389 | then the second pass of gcc is constructed, omitting some
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390 | non-critical libraries. Due to some weird logic in gcc's
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391 | configure script, <envar>CC_FOR_TARGET</envar> ends up as
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392 | <command>cc</command> when the host is the same as the target, but
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393 | different from the build system. This is why
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394 | <parameter>CC_FOR_TARGET=$LFS_TGT-gcc</parameter> is declared explicitly
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395 | as one of the configuration options.</para>
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396 |
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397 | <para>Upon entering the chroot environment in <xref
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398 | linkend="chapter-chroot-temporary-tools"/>,
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399 | the temporary installations of programs needed for the proper
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400 | operation of the toolchain are performed. From this point onwards, the
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401 | core toolchain is self-contained and self-hosted. In
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402 | <xref linkend="chapter-building-system"/>, final versions of all the
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403 | packages needed for a fully functional system are built, tested, and
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404 | installed.</para>
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405 |
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406 | </sect2>
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407 |
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408 | </sect1>
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