[673b0d8] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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[1770019] | 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd" [
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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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| 6 | <sect1 id="ch-tools-toolchaintechnotes">
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[bce08ef] | 7 | <title>Toolchain Technical Notes</title>
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[673b0d8] | 8 | <?dbhtml filename="toolchaintechnotes.html"?>
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| 9 |
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[81fd230] | 10 | <para>This section explains some of the rationale and technical
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| 11 | details behind the overall build method. It is not essential to
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| 12 | immediately understand everything in this section. Most of this
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| 13 | information will be clearer after performing an actual build. This
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| 14 | section can be referred back to at any time during the process.</para>
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| 15 |
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[782446b] | 16 | <para>The overall goal of <xref linkend="chapter-temporary-tools"/> is to
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| 17 | provide a temporary environment that can be chrooted into and from which can be
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| 18 | produced a clean, trouble-free build of the target LFS system in <xref
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| 19 | linkend="chapter-building-system"/>. Along the way, we separate the new system
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| 20 | from the host system as much as possible, and in doing so, build a
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| 21 | self-contained and self-hosted toolchain. It should be noted that the build
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| 22 | process has been designed to minimize the risks for new readers and provide
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| 23 | maximum educational value at the same time.</para>
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[81fd230] | 24 |
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| 25 | <important>
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| 26 | <para>Before continuing, be aware of the name of the working platform,
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| 27 | often referred to as the target triplet. Many times, the target
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| 28 | triplet will probably be <emphasis>i686-pc-linux-gnu</emphasis>. A
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| 29 | simple way to determine the name of the target triplet is to run the
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| 30 | <command>config.guess</command> script that comes with the source for
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| 31 | many packages. Unpack the Binutils sources and run the script:
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| 32 | <userinput>./config.guess</userinput> and note the output.</para>
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| 33 |
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| 34 | <para>Also be aware of the name of the platform's dynamic linker,
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| 35 | often referred to as the dynamic loader (not to be confused with the
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| 36 | standard linker <command>ld</command> that is part of Binutils). The
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| 37 | dynamic linker provided by Glibc finds and loads the shared libraries
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| 38 | needed by a program, prepares the program to run, and then runs it.
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| 39 | The name of the dynamic linker will usually be
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| 40 | <filename class="libraryfile">ld-linux.so.2</filename>. On platforms that are less
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| 41 | prevalent, the name might be <filename class="libraryfile">ld.so.1</filename>,
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| 42 | and newer 64 bit platforms might be named something else entirely. The name of
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| 43 | the platform's dynamic linker can be determined by looking in the
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| 44 | <filename class="directory">/lib</filename> directory on the host
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| 45 | system. A sure-fire way to determine the name is to inspect a random
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| 46 | binary from the host system by running: <userinput>readelf -l <name
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| 47 | of binary> | grep interpreter</userinput> and noting the output.
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| 48 | The authoritative reference covering all platforms is in the
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| 49 | <filename>shlib-versions</filename> file in the root of the Glibc
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| 50 | source tree.</para>
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| 51 | </important>
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| 52 |
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| 53 | <para>Some key technical points of how the <xref linkend="chapter-temporary-tools"/> build
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| 54 | method works:</para>
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| 55 |
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| 56 | <itemizedlist>
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| 57 | <listitem><para>The process is similar in principle to
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| 58 | cross-compiling, whereby tools installed in the same prefix work in
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| 59 | 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
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| 63 | search path ensures programs are linked only against chosen
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| 64 | libraries</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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[afbe6d9] | 67 | <filename>specs</filename> file tells the compiler which target dynamic
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[81fd230] | 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
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| 72 | <command>./configure</command> runs of both GCC and Glibc perform
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| 73 | various feature tests on the assembler and linker to determine which
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| 74 | software features to enable or disable. This is more important than
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| 75 | one might first realize. An incorrectly configured GCC or Glibc can
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| 76 | result in a subtly broken toolchain, where the impact of such breakage
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| 77 | might not show up until near the end of the build of an entire
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[afbe6d9] | 78 | distribution. A test suite failure will usually highlight this error
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[81fd230] | 79 | before too much additional work is performed.</para>
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| 80 |
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[0b5096ca] | 81 | <beginpage/>
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| 82 |
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[81fd230] | 83 | <para>Binutils installs its assembler and linker in two locations,
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| 84 | <filename class="directory">/tools/bin</filename> and <filename
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| 85 | class="directory">/tools/$TARGET_TRIPLET/bin</filename>. The tools in
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| 86 | one location are hard linked to the other. An important facet of the
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| 87 | linker is its library search order. Detailed information can be
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| 88 | obtained from <command>ld</command> by passing it the
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| 89 | <parameter>--verbose</parameter> flag. For example, an <userinput>ld
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| 90 | --verbose | grep SEARCH</userinput> will illustrate the current search
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| 91 | paths and their order. It shows which files are linked by
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| 92 | <command>ld</command> by compiling a dummy program and passing the
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| 93 | <parameter>--verbose</parameter> switch to the linker. For example,
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| 94 | <userinput>gcc dummy.c -Wl,--verbose 2>&1 | grep
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| 95 | succeeded</userinput> will show all the files successfully opened
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| 96 | during the linking.</para>
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| 97 |
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| 98 | <para>The next package installed is GCC. An example of what can be
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| 99 | seen during its run of <command>./configure</command> is:</para>
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| 100 |
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| 101 | <screen><computeroutput>checking what assembler to use...
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| 102 | /tools/i686-pc-linux-gnu/bin/as
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| 103 | checking what linker to use... /tools/i686-pc-linux-gnu/bin/ld</computeroutput></screen>
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| 104 |
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| 105 | <para>This is important for the reasons mentioned above. It also
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| 106 | demonstrates that GCC's configure script does not search the PATH
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| 107 | directories to find which tools to use. However, during the actual
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| 108 | operation of <command>gcc</command> itself, the same
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| 109 | search paths are not necessarily used. To find out which standard
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| 110 | linker <command>gcc</command> will use, run: <userinput>gcc
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| 111 | -print-prog-name=ld</userinput>.</para>
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| 112 |
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| 113 | <para>Detailed information can be obtained from <command>gcc</command>
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| 114 | by passing it the <parameter>-v</parameter> command line option while
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| 115 | compiling a dummy program. For example, <userinput>gcc -v
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| 116 | dummy.c</userinput> will show detailed information about the
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| 117 | preprocessor, compilation, and assembly stages, including
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| 118 | <command>gcc</command>'s included search paths and their order.</para>
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| 119 |
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| 120 | <para>The next package installed is Glibc. The most important
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| 121 | considerations for building Glibc are the compiler, binary tools, and
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| 122 | kernel headers. The compiler is generally not an issue since Glibc
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| 123 | will always use the <command>gcc</command> found in a
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| 124 | <envar>PATH</envar> directory.
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| 125 | The binary tools and kernel headers can be a bit more complicated.
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| 126 | Therefore, take no risks and use the available configure switches to
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| 127 | enforce the correct selections. After the run of
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| 128 | <command>./configure</command>, check the contents of the
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| 129 | <filename>config.make</filename> file in the <filename
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| 130 | class="directory">glibc-build</filename> directory for all important
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| 131 | details. Note the use of <parameter>CC="gcc -B/tools/bin/"</parameter>
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| 132 | to control which binary tools are used and the use of the
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| 133 | <parameter>-nostdinc</parameter> and <parameter>-isystem</parameter>
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| 134 | flags to control the compiler's include search path. These items
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| 135 | highlight an important aspect of the Glibc package—it is very
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| 136 | self-sufficient in terms of its build machinery and generally does not
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| 137 | rely on toolchain defaults.</para>
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| 138 |
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| 139 | <para>After the Glibc installation, make some adjustments to ensure
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| 140 | that searching and linking take place only within the <filename
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| 141 | class="directory">/tools</filename> prefix. Install an adjusted
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| 142 | <command>ld</command>, which has a hard-wired search path limited to
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| 143 | <filename class="directory">/tools/lib</filename>. Then amend
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| 144 | <command>gcc</command>'s specs file to point to the new dynamic linker
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| 145 | in <filename class="directory">/tools/lib</filename>. This last step
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| 146 | is vital to the whole process. As mentioned above, a hard-wired path
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| 147 | to a dynamic linker is embedded into every Executable and Link Format
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| 148 | (ELF)-shared executable. This can be inspected by running:
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| 149 | <userinput>readelf -l <name of binary> | grep
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| 150 | interpreter</userinput>. Amending gcc's specs file
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| 151 | ensures that every program compiled from here through the end of this
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| 152 | chapter will use the new dynamic linker in <filename
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| 153 | class="directory">/tools/lib</filename>.</para>
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| 154 |
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| 155 | <para>The need to use the new dynamic linker is also the reason why
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| 156 | the Specs patch is applied for the second pass of GCC. Failure to do
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| 157 | so will result in the GCC programs themselves having the name of the
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| 158 | dynamic linker from the host system's <filename
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| 159 | class="directory">/lib</filename> directory embedded into them, which
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| 160 | would defeat the goal of getting away from the host.</para>
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| 161 |
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| 162 | <para>During the second pass of Binutils, we are able to utilize the
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| 163 | <parameter>--with-lib-path</parameter> configure switch to control
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| 164 | <command>ld</command>'s library search path. From this point onwards,
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| 165 | the core toolchain is self-contained and self-hosted. The remainder of
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| 166 | the <xref linkend="chapter-temporary-tools"/> packages all build
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| 167 | against the new Glibc in <filename
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| 168 | class="directory">/tools</filename>.</para>
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| 169 |
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[0b5096ca] | 170 | <beginpage/>
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| 171 |
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[81fd230] | 172 | <para>Upon entering the chroot environment in <xref
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| 173 | linkend="chapter-building-system"/>, the first major package to be
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| 174 | installed is Glibc, due to its self-sufficient nature mentioned above.
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| 175 | Once this Glibc is installed into <filename
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| 176 | class="directory">/usr</filename>, perform a quick changeover of the
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| 177 | toolchain defaults, then proceed in building the rest of the target
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| 178 | LFS system.</para>
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| 179 |
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[516b668] | 180 | <!-- Removed as part of the fix for bug 1061 - we no longer build pass1
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| 181 | packages statically, therefore this explanation isn't required -->
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| 182 |
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| 183 | <!--<sect2>
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[81fd230] | 184 | <title>Notes on Static Linking</title>
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| 185 |
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| 186 | <para>Besides their specific task, most programs have to perform many
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| 187 | common and sometimes trivial operations. These include allocating
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| 188 | memory, searching directories, reading and writing files, string
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| 189 | handling, pattern matching, arithmetic, and other tasks. Instead of
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| 190 | obliging each program to reinvent the wheel, the GNU system provides
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| 191 | all these basic functions in ready-made libraries. The major library
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| 192 | on any Linux system is Glibc.</para>
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| 193 |
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| 194 | <para>There are two primary ways of linking the functions from a
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| 195 | library to a program that uses them—statically or dynamically. When
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| 196 | a program is linked statically, the code of the used functions is
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| 197 | included in the executable, resulting in a rather bulky program. When
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| 198 | a program is dynamically linked, it includes a reference to the
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| 199 | dynamic linker, the name of the library, and the name of the function,
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| 200 | resulting in a much smaller executable. A third option is to use the
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[37428334] | 201 | programming interface of the dynamic linker (see <filename>dlopen(3)</filename>
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| 202 | for more information).</para>
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[81fd230] | 203 |
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| 204 | <para>Dynamic linking is the default on Linux and has three major
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| 205 | advantages over static linking. First, only one copy of the executable
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| 206 | library code is needed on the hard disk, instead of having multiple
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| 207 | copies of the same code included in several programs, thus saving
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| 208 | disk space. Second, when several programs use the same library
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| 209 | function at the same time, only one copy of the function's code is
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| 210 | required in core, thus saving memory space. Third, when a library
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| 211 | function gets a bug fixed or is otherwise improved, only the one
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| 212 | library needs to be recompiled instead of recompiling all programs
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| 213 | that make use of the improved function.</para>
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| 214 |
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| 215 | <para>If dynamic linking has several advantages, why then do we
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| 216 | statically link the first two packages in this chapter? The reasons
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| 217 | are threefold—historical, educational, and technical. The
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| 218 | historical reason is that earlier versions of LFS statically linked
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| 219 | every program in this chapter. Educationally, knowing the difference
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| 220 | between static and dynamic linking is useful. The technical benefit is
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| 221 | a gained element of independence from the host, meaning that those
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| 222 | programs can be used independently of the host system. However, it is
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| 223 | worth noting that an overall successful LFS build can still be
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| 224 | achieved when the first two packages are built dynamically.</para>
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| 225 |
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[516b668] | 226 | </sect2>-->
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[673b0d8] | 227 |
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| 228 | </sect1>
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[81fd230] | 229 |
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