[673b0d8] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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[1375d26] | 2 | <!DOCTYPE section [
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[673b0d8] | 3 | <!ENTITY % general-entities SYSTEM "../general.ent">
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| 4 | %general-entities;
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| 5 | ]>
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[1375d26] | 6 | <section xmlns="http://docbook.org/docbook-ng"
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| 7 | xml:id="ch-tools-toolchaintechnotes">
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| 8 | <info><title>Toolchain technical notes</title></info>
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[673b0d8] | 9 | <?dbhtml filename="toolchaintechnotes.html"?>
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| 10 |
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| 11 | <para>This section attempts to explain some of the rationale and technical
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| 12 | details behind the overall build method. It's not essential that you understand
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| 13 | everything here immediately. Most of it will make sense once you have performed
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| 14 | an actual build. Feel free to refer back here at any time.</para>
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| 15 |
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| 16 | <para>The overall goal of <xref linkend="chapter-temporary-tools"/> is to provide a sane,
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| 17 | temporary environment that we can chroot into, and from which we can produce a
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| 18 | clean, trouble-free build of the target LFS system in
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| 19 | <xref linkend="chapter-building-system"/>. Along the way, we attempt to divorce ourselves
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| 20 | from the host system as much as possible, and in so doing build a
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| 21 | self-contained and self-hosted toolchain. It should be noted that the
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| 22 | build process has been designed to minimize the risks for
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| 23 | new readers and provide maximum educational value at the same time. In other
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| 24 | words, more advanced techniques could be used to build the system.</para>
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| 25 |
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| 26 | <important>
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| 27 | <para>Before continuing, you really should be aware of the name of your working
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| 28 | platform, often also referred to as the <emphasis>target triplet</emphasis>. For
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| 29 | many folks the target triplet will probably be
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| 30 | <emphasis>i686-pc-linux-gnu</emphasis>. A simple way to determine your target
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[574b0ea] | 31 | triplet is to run the <command>config.guess</command> script that comes with
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[673b0d8] | 32 | the source for many packages. Unpack the Binutils sources and run the script:
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| 33 | <userinput>./config.guess</userinput> and note the output.</para>
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| 34 |
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| 35 | <para>You'll also need to be aware of the name of your platform's
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| 36 | <emphasis>dynamic linker</emphasis>, often also referred to as the
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| 37 | <emphasis>dynamic loader</emphasis>, not to be confused with the standard linker
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[574b0ea] | 38 | <command>ld</command> that is part of Binutils. The dynamic linker is provided
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[673b0d8] | 39 | by Glibc and has the job of finding and loading the shared libraries needed by a
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| 40 | program, preparing the program to run and then running it. For most folks the
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[574b0ea] | 41 | name of the dynamic linker will be <filename>ld-linux.so.2</filename>. On
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[673b0d8] | 42 | platforms that are less prevalent, the name might be
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[574b0ea] | 43 | <filename>ld.so.1</filename> and newer 64 bit platforms might even have
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[673b0d8] | 44 | something completely different. You should be able to determine the name
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| 45 | of your platform's dynamic linker by looking in the
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| 46 | <filename class="directory">/lib</filename> directory on your host system. A
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| 47 | sure-fire way is to inspect a random binary from your host system by running:
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| 48 | <userinput>readelf -l <name of binary> | grep interpreter</userinput>
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| 49 | and noting the output. The authoritative reference covering all platforms is in
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| 50 | the <filename>shlib-versions</filename> file in the root of the Glibc source
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| 51 | tree.</para>
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| 52 | </important>
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| 53 |
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| 54 | <para>Some key technical points of how the <xref linkend="chapter-temporary-tools"/> build
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| 55 | method works:</para>
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| 56 |
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| 57 | <itemizedlist>
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| 58 | <listitem><para>Similar in principle to cross compiling whereby tools installed
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| 59 | into the same prefix work in cooperation and thus utilize a little GNU
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| 60 | <quote>magic</quote>.</para></listitem>
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| 61 |
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| 62 | <listitem><para>Careful manipulation of the standard linker's library search
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| 63 | path to ensure programs are linked only against libraries we
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| 64 | choose.</para></listitem>
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| 65 |
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| 66 | <listitem><para>Careful manipulation of <command>gcc</command>'s
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[574b0ea] | 67 | <filename>specs</filename> file to tell the compiler which target dynamic
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[673b0d8] | 68 | linker will be used.</para></listitem>
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| 69 | </itemizedlist>
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| 70 |
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| 71 | <para>Binutils is installed first because the <command>./configure</command> runs of both GCC and Glibc perform various
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| 72 | feature tests on the assembler and linker
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| 73 | to determine which software features to enable
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| 74 | or disable. This is more important than one might first realize. An incorrectly
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| 75 | configured GCC or Glibc can result in a subtly broken toolchain where the impact
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| 76 | of such breakage might not show up until near the end of the build of a whole
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| 77 | distribution. Thankfully, a test suite failure will usually alert us before too
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| 78 | much time is wasted.</para>
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| 79 |
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| 80 | <para>Binutils installs its assembler and linker into two locations,
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| 81 | <filename class="directory">/tools/bin</filename> and
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| 82 | <filename class="directory">/tools/$TARGET_TRIPLET/bin</filename>. In reality,
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| 83 | the tools in one location are hard linked to the other. An important facet of
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| 84 | the linker is its library search order. Detailed information can be obtained
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[574b0ea] | 85 | from <command>ld</command> by passing it the <parameter>--verbose</parameter>
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[673b0d8] | 86 | flag. For example: <command>ld --verbose | grep SEARCH</command> will
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| 87 | show you the current search paths and their order. You can see what files are
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| 88 | actually linked by <command>ld</command> by compiling a dummy program and
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[574b0ea] | 89 | passing the <parameter>--verbose</parameter> switch to the linker. For example:
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| 90 | <userinput>gcc dummy.c -Wl,--verbose 2>&1 | grep succeeded</userinput>
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[673b0d8] | 91 | will show you all the files successfully opened during the linking.</para>
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| 92 |
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| 93 | <para>The next package installed is GCC and during its run of
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| 94 | <command>./configure</command> you'll see, for example:</para>
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| 95 |
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[574b0ea] | 96 | <blockquote><screen><computeroutput>checking what assembler to use... /tools/i686-pc-linux-gnu/bin/as
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| 97 | checking what linker to use... /tools/i686-pc-linux-gnu/bin/ld</computeroutput></screen></blockquote>
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[673b0d8] | 98 |
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| 99 | <para>This is important for the reasons mentioned above. It also demonstrates
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| 100 | that GCC's configure script does not search the PATH directories to find which
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| 101 | tools to use. However, during the actual operation of <command>gcc</command>
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| 102 | itself, the same search paths are not necessarily used. You can find out which
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| 103 | standard linker <command>gcc</command> will use by running:
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[574b0ea] | 104 | <userinput>gcc -print-prog-name=ld</userinput>.
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[673b0d8] | 105 | Detailed information can be obtained from <command>gcc</command> by passing
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[574b0ea] | 106 | it the <parameter>-v</parameter> flag while compiling a dummy program. For
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| 107 | example: <userinput>gcc -v dummy.c</userinput> will show you detailed
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[673b0d8] | 108 | information about the preprocessor, compilation and assembly stages, including
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| 109 | <command>gcc</command>'s include search paths and their order.</para>
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| 110 |
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| 111 | <para>The next package installed is Glibc. The most important considerations for
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| 112 | building Glibc are the compiler, binary tools and kernel headers. The compiler
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| 113 | is generally no problem as Glibc will always use the <command>gcc</command>
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| 114 | found in a PATH directory. The binary tools and kernel headers can be a little
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| 115 | more troublesome. Therefore we take no risks and use the available configure
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| 116 | switches to enforce the correct selections. After the run of
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| 117 | <command>./configure</command> you can check the contents of the
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| 118 | <filename>config.make</filename> file in the
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| 119 | <filename class="directory">glibc-build</filename> directory for all the
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| 120 | important details. You'll note some interesting items like the use of
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[574b0ea] | 121 | <parameter>CC="gcc -B/tools/bin/"</parameter> to control which binary tools are
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| 122 | used, and also the use of the <parameter>-nostdinc</parameter> and
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| 123 | <parameter>-isystem</parameter> flags to control the compiler's include search
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[673b0d8] | 124 | path. These items help to highlight an important aspect of the Glibc package:
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| 125 | it is very self-sufficient in terms of its build machinery and generally does
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| 126 | not rely on toolchain defaults.</para>
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| 127 |
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| 128 | <para>After the Glibc installation, we make some adjustments to ensure that
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[574b0ea] | 129 | searching and linking take place only within our <filename class="directory">/tools</filename>
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[673b0d8] | 130 | prefix. We install an adjusted <command>ld</command>, which has a hard-wired
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| 131 | search path limited to <filename class="directory">/tools/lib</filename>. Then
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| 132 | we amend <command>gcc</command>'s specs file to point to our new dynamic
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| 133 | linker in <filename class="directory">/tools/lib</filename>. This last step is
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| 134 | <emphasis>vital</emphasis> to the whole process. As mentioned above, a
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| 135 | hard-wired path to a dynamic linker is embedded into every ELF shared
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| 136 | executable. You can inspect this by running:
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[574b0ea] | 137 | <userinput>readelf -l <name of binary> | grep interpreter</userinput>.
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[673b0d8] | 138 | By amending <command>gcc</command>'s specs file, we are ensuring that every
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| 139 | program compiled from here through the end of this chapter will use our new
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| 140 | dynamic linker in <filename class="directory">/tools/lib</filename>.</para>
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| 141 |
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| 142 | <para>The need to use the new dynamic linker is also the reason why we apply the
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| 143 | Specs patch for the second pass of GCC. Failure to do so will result in the GCC
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| 144 | programs themselves having the name of the dynamic linker from the host system's
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| 145 | <filename class="directory">/lib</filename> directory embedded into them, which
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| 146 | would defeat our goal of getting away from the host.</para>
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| 147 |
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| 148 | <para>During the second pass of Binutils, we are able to utilize the
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[574b0ea] | 149 | <parameter>--with-lib-path</parameter> configure switch to control
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[673b0d8] | 150 | <command>ld</command>'s library search path. From this point onwards, the
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| 151 | core toolchain is self-contained and self-hosted. The remainder of the
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| 152 | <xref linkend="chapter-temporary-tools"/> packages all build against the new Glibc in
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| 153 | <filename class="directory">/tools</filename> and all is well.</para>
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| 154 |
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| 155 | <para>Upon entering the chroot environment in <xref linkend="chapter-building-system"/>, the
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| 156 | first major package we install is Glibc, due to its self-sufficient nature that
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| 157 | we mentioned above. Once this Glibc is installed into
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| 158 | <filename class="directory">/usr</filename>, we perform a quick changeover of
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| 159 | the toolchain defaults, then proceed for real in building the rest of the
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| 160 | target LFS system.</para>
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| 161 |
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[1375d26] | 162 | <section>
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[673b0d8] | 163 | <title>Notes on static linking</title>
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| 164 |
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| 165 | <para>Most programs have to perform, beside their specific task, many rather
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| 166 | common and sometimes trivial operations. These include allocating memory,
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| 167 | searching directories, reading and writing files, string handling, pattern
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| 168 | matching, arithmetic and many other tasks. Instead of obliging each program to
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| 169 | reinvent the wheel, the GNU system provides all these basic functions in
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| 170 | ready-made libraries. The major library on any Linux system is
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| 171 | <emphasis>Glibc</emphasis>.</para>
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| 172 |
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| 173 | <para>There are two primary ways of linking the functions from a library to a
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| 174 | program that uses them: statically or dynamically. When a program is linked
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| 175 | statically, the code of the used functions is included in the executable,
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| 176 | resulting in a rather bulky program. When a program is dynamically linked, what
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| 177 | is included is a reference to the dynamic linker, the name of the library, and
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| 178 | the name of the function, resulting in a much smaller executable. (A third way
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| 179 | is to use the programming interface of the dynamic linker. See the
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| 180 | <emphasis>dlopen</emphasis> man page for more information.)</para>
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| 181 |
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| 182 | <para>Dynamic linking is the default on Linux and has three major advantages
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| 183 | over static linking. First, you need only one copy of the executable library
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| 184 | code on your hard disk, instead of having many copies of the same code included
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| 185 | into a whole bunch of programs -- thus saving disk space. Second, when several
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| 186 | programs use the same library function at the same time, only one copy of the
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| 187 | function's code is required in core -- thus saving memory space. Third, when a
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| 188 | library function gets a bug fixed or is otherwise improved, you only need to
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| 189 | recompile this one library, instead of having to recompile all the programs that
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| 190 | make use of the improved function.</para>
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| 191 |
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| 192 | <para>If dynamic linking has several advantages, why then do we statically link
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| 193 | the first two packages in this chapter? The reasons are threefold: historical,
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| 194 | educational, and technical. Historical, because earlier versions of LFS
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| 195 | statically linked every program in this chapter. Educational, because knowing
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| 196 | the difference is useful. Technical, because we gain an element of independence
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| 197 | from the host in doing so, meaning that those programs can be used
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| 198 | independently of the host system. However, it's worth noting that an overall
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| 199 | successful LFS build can still be achieved when the first two packages are
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| 200 | built dynamically.</para>
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| 201 |
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[1375d26] | 202 | </section>
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[673b0d8] | 203 |
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[1375d26] | 204 | </section>
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