1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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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">
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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 | 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 | produce a temporary area that contains a known-good set of tools that can be
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21 | isolated from the host system. By using <command>chroot</command>, the
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22 | commands in the remaining chapters will be contained within that environment,
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23 | ensuring a clean, trouble-free build of the target LFS system. The build
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24 | process has been designed to minimize the risks for new readers and to provide
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25 | the most educational value at the same time.</para>
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26 |
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27 | <note>
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28 | <para>Before continuing, be aware of the name of the working platform,
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29 | often referred to as the target triplet. A simple way to determine the
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30 | name of the target triplet is to run the <command>config.guess</command>
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31 | script that comes with the source for many packages. Unpack the Binutils
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32 | sources and run the script: <userinput>./config.guess</userinput> and note
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33 | the output. For example, for a modern 32-bit Intel processor the
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34 | output will likely be <emphasis>i686-pc-linux-gnu</emphasis>.</para>
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35 |
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36 | <para>Also be aware of the name of the platform's dynamic linker, often
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37 | referred to as the dynamic loader (not to be confused with the standard
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38 | linker <command>ld</command> that is part of Binutils). The dynamic linker
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39 | provided by Glibc finds and loads the shared libraries needed by a program,
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40 | prepares the program to run, and then runs it. The name of the dynamic
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41 | linker for a 32-bit Intel machine will be
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42 | <filename class="libraryfile">ld-linux.so.2</filename>.
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43 | A sure-fire way to determine the name of the dynamic linker is to
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44 | inspect a random binary from the host system by running:
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45 | <userinput>readelf -l <name of binary> | grep interpreter</userinput>
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46 | and noting the output. The authoritative reference covering all platforms
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47 | is in the <filename>shlib-versions</filename> file in the root of the Glibc
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48 | source tree.</para>
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49 | </note>
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50 |
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51 | <para>Some key technical points of how the <xref
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52 | linkend="chapter-temporary-tools"/> build method works:</para>
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53 |
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54 | <itemizedlist>
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55 | <listitem>
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56 | <para>Slightly adjusting the name of the working platform, by changing the
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57 | "vendor" field target triplet by way of the
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58 | <envar>LFS_TGT</envar> variable, ensures that the first build of Binutils
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59 | and GCC produces a compatible cross-linker and cross-compiler. Instead of
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60 | producing binaries for another architecture, the cross-linker and
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61 | cross-compiler will produce binaries compatible with the current
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62 | hardware.</para>
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63 | </listitem>
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64 | <listitem>
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65 | <para> The temporary libraries are cross-compiled. Because a
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66 | cross-compiler by its nature cannot rely on anything from its host
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67 | system, this method removes potential contamination of the target
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68 | system by lessening the chance of headers or libraries from the host
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69 | being incorporated into the new tools. Cross-compilation also allows for
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70 | the possibility of building both 32-bit and 64-bit libraries on 64-bit
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71 | capable hardware.</para>
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72 | </listitem>
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73 | <listitem>
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74 | <para>Careful manipulation of the GCC source tells the compiler which target
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75 | dynamic linker will be used.</para>
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76 | </listitem>
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77 | </itemizedlist>
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78 |
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79 | <para>Binutils is installed first because the <command>configure</command>
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80 | runs of both GCC and Glibc perform various feature tests on the assembler
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81 | and linker to determine which software features to enable or disable. This
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82 | is more important than one might first realize. An incorrectly configured
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83 | GCC or Glibc can result in a subtly broken toolchain, where the impact of
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84 | such breakage might not show up until near the end of the build of an
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85 | entire distribution. A test suite failure will usually highlight this error
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86 | before too much additional work is performed.</para>
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87 |
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88 | <para>Binutils installs its assembler and linker in two locations,
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89 | <filename class="directory">/tools/bin</filename> and <filename
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90 | class="directory">/tools/$LFS_TGT/bin</filename>. The tools in one
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91 | location are hard linked to the other. An important facet of the linker is
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92 | its library search order. Detailed information can be obtained from
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93 | <command>ld</command> by passing it the <parameter>--verbose</parameter>
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94 | flag. For example, an <userinput>ld --verbose | grep SEARCH</userinput>
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95 | will illustrate the current search paths and their order. It shows which
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96 | files are linked by <command>ld</command> by compiling a dummy program and
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97 | passing the <parameter>--verbose</parameter> switch to the linker. For example,
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98 | <userinput>gcc dummy.c -Wl,--verbose 2>&1 | grep succeeded</userinput>
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99 | will show all the files successfully opened during the linking.</para>
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100 |
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101 | <para>The next package installed is GCC. An example of what can be
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102 | seen during its run of <command>configure</command> is:</para>
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103 |
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104 | <screen><computeroutput>checking what assembler to use... /tools/i686-lfs-linux-gnu/bin/as
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105 | checking what linker to use... /tools/i686-lfs-linux-gnu/bin/ld</computeroutput></screen>
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106 |
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107 | <para>This is important for the reasons mentioned above. It also demonstrates
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108 | that GCC's configure script does not search the PATH directories to find which
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109 | tools to use. However, during the actual operation of <command>gcc</command>
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110 | itself, the same search paths are not necessarily used. To find out which
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111 | standard linker <command>gcc</command> will use, run:
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112 | <userinput>gcc -print-prog-name=ld</userinput>.</para>
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113 |
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114 | <para>Detailed information can be obtained from <command>gcc</command> by
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115 | passing it the <parameter>-v</parameter> command line option while compiling
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116 | a dummy program. For example, <userinput>gcc -v dummy.c</userinput> will show
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117 | detailed information about the preprocessor, compilation, and assembly stages,
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118 | including <command>gcc</command>'s included search paths and their order.</para>
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119 |
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120 | <para>Next installed are sanitized Linux API headers. These allow the standard
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121 | C library (Glibc) to interface with features that the Linux kernel will
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122 | provide.</para>
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123 |
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124 | <para>The next package installed is Glibc. The most important considerations
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125 | for building Glibc are the compiler, binary tools, and kernel headers. The
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126 | compiler is generally not an issue since Glibc will always use the compiler
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127 | relating to the <parameter>--host</parameter> parameter passed to its
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128 | configure script, e.g. in our case,
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129 | <command>i686-lfs-linux-gnu-gcc</command>. The binary tools and kernel
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130 | headers can be a bit more complicated. Therefore, take no risks and use the
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131 | available configure switches to enforce the correct selections. After the run
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132 | of <command>configure</command>, check the contents of the
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133 | <filename>config.make</filename> file in the <filename
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134 | class="directory">glibc-build</filename> directory for all important details.
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135 | Note the use of <parameter>CC="i686-lfs-gnu-gcc"</parameter> to control which
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136 | binary tools are used and the use of the <parameter>-nostdinc</parameter> and
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137 | <parameter>-isystem</parameter> flags to control the compiler's include
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138 | search path. These items highlight an important aspect of the Glibc
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139 | package—it is very self-sufficient in terms of its build machinery and
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140 | generally does not rely on toolchain defaults.</para>
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141 |
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142 | <para>During the second pass of Binutils, we are able to utilize the
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143 | <parameter>--with-lib-path</parameter> configure switch to control
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144 | <command>ld</command>'s library search path.</para>
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145 |
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146 | <para>For the second pass of GCC, its sources also need to be modified to
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147 | tell GCC to use the new dynamic linker. Failure to do so will result in the
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148 | GCC programs themselves having the name of the dynamic linker from the host
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149 | system's <filename class="directory">/lib</filename> directory embedded into
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150 | them, which would defeat the goal of getting away from the host. From this
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151 | point onwards, the core toolchain is self-contained and self-hosted. The
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152 | remainder of the <xref linkend="chapter-temporary-tools"/> packages all build
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153 | against the new Glibc in <filename
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154 | class="directory">/tools</filename>.</para>
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155 |
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156 | <para>Upon entering the chroot environment in <xref
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157 | linkend="chapter-building-system"/>, the first major package to be
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158 | installed is Glibc, due to its self-sufficient nature mentioned above.
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159 | Once this Glibc is installed into <filename
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160 | class="directory">/usr</filename>, we will perform a quick changeover of the
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161 | toolchain defaults, and then proceed in building the rest of the target
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162 | LFS system.</para>
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163 |
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164 | </sect1>
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