| 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
|
|---|
| 400 | operation of the toolchain are performed. From this point onwards, the
|
|---|
| 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
|
|---|
| 403 | packages needed for a fully functional system are built, tested, and
|
|---|
| 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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