mirror of
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Polish up the prose in "Toolchain Technical Notes". Fix capitalization.
Rough edges remain. For instance, $LFS_TGT-ld is referenced, but not clearly defined. Will need to discuss wirh other editors to resolve.
This commit is contained in:
David Bryant
parent
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commit
562062295e
@ -11,26 +11,26 @@
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<title>Toolchain Technical Notes</title>
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<para>This section explains some of the rationale and technical details
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behind the overall build method. It is not essential to immediately
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behind the overall build method. Don't try to immediately
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understand everything in this section. Most of this information will be
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clearer after performing an actual build. This section can be referred
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to at any time during the process.</para>
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clearer after performing an actual build. Come back and re-read this chapter
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at any time during the build process.</para>
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<para>The overall goal of <xref linkend="chapter-cross-tools"/> and <xref
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linkend="chapter-temporary-tools"/> is to produce a temporary area that
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contains a known-good set of tools that can be isolated from the host system.
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By using <command>chroot</command>, the commands in the remaining chapters
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will be contained within that environment, ensuring a clean, trouble-free
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linkend="chapter-temporary-tools"/> is to produce a temporary area
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containing a set of tools that are known to be good, and that are isolated from the host system.
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By using the <command>chroot</command> command, the compilations in the remaining chapters
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will be isolated within that environment, ensuring a clean, trouble-free
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build of the target LFS system. The build process has been designed to
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minimize the risks for new readers and to provide the most educational value
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minimize the risks for new readers, and to provide the most educational value
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at the same time.</para>
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<para>The build process is based on the process of
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<para>This build process is based on
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<emphasis>cross-compilation</emphasis>. Cross-compilation is normally used
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for building a compiler and its toolchain for a machine different from
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the one that is used for the build. This is not strictly needed for LFS,
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to build a compiler and its associated toolchain for a machine different from
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the one that is used for the build. This is not strictly necessary for LFS,
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since the machine where the new system will run is the same as the one
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used for the build. But cross-compilation has the great advantage that
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used for the build. But cross-compilation has one great advantage:
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anything that is cross-compiled cannot depend on the host environment.</para>
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<sect2 id="cross-compile" xreflabel="About Cross-Compilation">
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@ -39,47 +39,46 @@
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<note>
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<para>
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The LFS book is not, and does not contain a general tutorial to
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build a cross (or native) toolchain. Don't use the command in the
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book for a cross toolchain which will be used for some purpose other
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The LFS book is not (and does not contain) a general tutorial to
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build a cross (or native) toolchain. Don't use the commands in the
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book for a cross toolchain for some purpose other
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than building LFS, unless you really understand what you are doing.
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</para>
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</note>
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<para>Cross-compilation involves some concepts that deserve a section on
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their own. Although this section may be omitted in a first reading,
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coming back to it later will be beneficial to your full understanding of
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<para>Cross-compilation involves some concepts that deserve a section of
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their own. Although this section may be omitted on a first reading,
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coming back to it later will help you gain a fuller understanding of
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the process.</para>
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<para>Let us first define some terms used in this context:</para>
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<para>Let us first define some terms used in this context.</para>
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<variablelist>
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<varlistentry><term>build</term><listitem>
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<varlistentry><term>The build</term><listitem>
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<para>is the machine where we build programs. Note that this machine
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is referred to as the <quote>host</quote> in other
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sections.</para></listitem>
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is also referred to as the <quote>host</quote>.</para></listitem>
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</varlistentry>
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<varlistentry><term>host</term><listitem>
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<varlistentry><term>The host</term><listitem>
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<para>is the machine/system where the built programs will run. Note
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that this use of <quote>host</quote> is not the same as in other
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sections.</para></listitem>
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</varlistentry>
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<varlistentry><term>target</term><listitem>
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<varlistentry><term>The target</term><listitem>
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<para>is only used for compilers. It is the machine the compiler
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produces code for. It may be different from both build and
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host.</para></listitem>
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produces code for. It may be different from both the build and
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the host.</para></listitem>
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</varlistentry>
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</variablelist>
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<para>As an example, let us imagine the following scenario (sometimes
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referred to as <quote>Canadian Cross</quote>): we may have a
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referred to as <quote>Canadian Cross</quote>): we have a
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compiler on a slow machine only, let's call it machine A, and the compiler
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ccA. We may have also a fast machine (B), but with no compiler, and we may
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want to produce code for another slow machine (C). To build a
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compiler for machine C, we would have three stages:</para>
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ccA. We also have a fast machine (B), but no compiler for (B), and we
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want to produce code for a third, slow machine (C). We will build a
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compiler for machine C in three stages.</para>
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<informaltable align="center">
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<tgroup cols="5">
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@ -95,24 +94,24 @@
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<tbody>
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<row>
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<entry>1</entry><entry>A</entry><entry>A</entry><entry>B</entry>
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<entry>build cross-compiler cc1 using ccA on machine A</entry>
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<entry>Build cross-compiler cc1 using ccA on machine A.</entry>
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</row>
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<row>
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<entry>2</entry><entry>A</entry><entry>B</entry><entry>C</entry>
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<entry>build cross-compiler cc2 using cc1 on machine A</entry>
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<entry>Build cross-compiler cc2 using cc1 on machine A.</entry>
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</row>
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<row>
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<entry>3</entry><entry>B</entry><entry>C</entry><entry>C</entry>
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<entry>build compiler ccC using cc2 on machine B</entry>
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<entry>Build compiler ccC using cc2 on machine B.</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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<para>Then, all the other programs needed by machine C can be compiled
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<para>Then, all the programs needed by machine C can be compiled
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using cc2 on the fast machine B. Note that unless B can run programs
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produced for C, there is no way to test the built programs until machine
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C itself is running. For example, for testing ccC, we may want to add a
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produced for C, there is no way to test the newly built programs until machine
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C itself is running. For example, to run a test suite on ccC, we may want to add a
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fourth stage:</para>
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<informaltable align="center">
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@ -129,7 +128,7 @@
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<tbody>
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<row>
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<entry>4</entry><entry>C</entry><entry>C</entry><entry>C</entry>
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<entry>rebuild and test ccC using itself on machine C</entry>
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<entry>Rebuild and test ccC using ccC on machine C.</entry>
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</row>
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</tbody>
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</tgroup>
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@ -147,43 +146,45 @@
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<note>
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<para>Almost all the build systems use names of the form
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cpu-vendor-kernel-os referred to as the machine triplet. An astute
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cpu-vendor-kernel-os, referred to as the machine triplet. An astute
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reader may wonder why a <quote>triplet</quote> refers to a four component
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name. The reason is history: initially, three component names were enough
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to designate a machine unambiguously, but with new machines and systems
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appearing, that proved insufficient. The word <quote>triplet</quote>
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name. The reason is historical: initially, three component names were enough
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to designate a machine unambiguously, but as new machines and systems
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proliferated, that proved insufficient. The word <quote>triplet</quote>
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remained. A simple way to determine your machine triplet is to run
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the <command>config.guess</command>
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script that comes with the source for many packages. Unpack the binutils
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sources and run the script: <userinput>./config.guess</userinput> and note
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the output. For example, for a 32-bit Intel processor the
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output will be <emphasis>i686-pc-linux-gnu</emphasis>. On a 64-bit
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system it will be <emphasis>x86_64-pc-linux-gnu</emphasis>.</para>
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system it will be <emphasis>x86_64-pc-linux-gnu</emphasis>. On most
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Linux systems the even simpler <command>gcc -dumpmachine</command> command
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will give you the same information.</para>
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<para>Also be aware of the name of the platform's dynamic linker, often
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<para>You should also be aware of the name of the platform's dynamic linker, often
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referred to as the dynamic loader (not to be confused with the standard
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linker <command>ld</command> that is part of binutils). The dynamic linker
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provided by Glibc finds and loads the shared libraries needed by a
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provided by package glibc finds and loads the shared libraries needed by a
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program, prepares the program to run, and then runs it. The name of the
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dynamic linker for a 32-bit Intel machine is <filename
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class="libraryfile">ld-linux.so.2</filename> and is <filename
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class="libraryfile">ld-linux-x86-64.so.2</filename> for 64-bit systems. A
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class="libraryfile">ld-linux.so.2</filename>; it's <filename
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class="libraryfile">ld-linux-x86-64.so.2</filename> on 64-bit systems. A
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sure-fire way to determine the name of the dynamic linker is to inspect a
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random binary from the host system by running: <userinput>readelf -l
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<name of binary> | grep interpreter</userinput> and noting the
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output. The authoritative reference covering all platforms is in the
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<filename>shlib-versions</filename> file in the root of the Glibc source
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<filename>shlib-versions</filename> file in the root of the glibc source
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tree.</para>
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</note>
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<para>In order to fake a cross compilation in LFS, the name of the host triplet
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is slightly adjusted by changing the "vendor" field in the
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<envar>LFS_TGT</envar> variable. We also use the
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<envar>LFS_TGT</envar> variable so it says "lfs". We also use the
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<parameter>--with-sysroot</parameter> option when building the cross linker and
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cross compiler to tell them where to find the needed host files. This
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ensures that none of the other programs built in <xref
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linkend="chapter-temporary-tools"/> can link to libraries on the build
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machine. Only two stages are mandatory, and one more for tests:</para>
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machine. Only two stages are mandatory, plus one more for tests.</para>
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<informaltable align="center">
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<tgroup cols="5">
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@ -199,47 +200,47 @@
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<tbody>
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<row>
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<entry>1</entry><entry>pc</entry><entry>pc</entry><entry>lfs</entry>
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<entry>build cross-compiler cc1 using cc-pc on pc</entry>
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<entry>Build cross-compiler cc1 using cc-pc on pc.</entry>
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</row>
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<row>
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<entry>2</entry><entry>pc</entry><entry>lfs</entry><entry>lfs</entry>
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<entry>build compiler cc-lfs using cc1 on pc</entry>
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<entry>Build compiler cc-lfs using cc1 on pc.</entry>
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</row>
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<row>
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<entry>3</entry><entry>lfs</entry><entry>lfs</entry><entry>lfs</entry>
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<entry>rebuild and test cc-lfs using itself on lfs</entry>
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<entry>Rebuild and test cc-lfs using cc-lfs on lfs.</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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<para>In the above table, <quote>on pc</quote> means the commands are run
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<para>In the preceding table, <quote>on pc</quote> means the commands are run
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on a machine using the already installed distribution. <quote>On
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lfs</quote> means the commands are run in a chrooted environment.</para>
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<para>Now, there is more about cross-compiling: the C language is not
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just a compiler, but also defines a standard library. In this book, the
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GNU C library, named glibc, is used. This library must
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be compiled for the lfs machine, that is, using the cross compiler cc1.
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GNU C library, named glibc, is used (there is an alternative, "musl"). This library must
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be compiled for the LFS machine; that is, using the cross compiler cc1.
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But the compiler itself uses an internal library implementing complex
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instructions not available in the assembler instruction set. This
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internal library is named libgcc, and must be linked to the glibc
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subroutines for functions not available in the assembler instruction set. This
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internal library is named libgcc, and it must be linked to the glibc
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library to be fully functional! Furthermore, the standard library for
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C++ (libstdc++) also needs being linked to glibc. The solution to this
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chicken and egg problem is to first build a degraded cc1 based libgcc,
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lacking some functionalities such as threads and exception handling, then
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build glibc using this degraded compiler (glibc itself is not
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degraded), then build libstdc++. But this last library will lack the
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same functionalities as libgcc.</para>
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C++ (libstdc++) must also be linked with glibc. The solution to this
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chicken and egg problem is first to build a degraded cc1-based libgcc,
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lacking some functionalities such as threads and exception handling, and then
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to build glibc using this degraded compiler (glibc itself is not
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degraded), and also to build libstdc++. This last library will lack some of the
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functionality of libgcc.</para>
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<para>This is not the end of the story: the conclusion of the preceding
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<para>This is not the end of the story: the upshot of the preceding
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paragraph is that cc1 is unable to build a fully functional libstdc++, but
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this is the only compiler available for building the C/C++ libraries
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during stage 2! Of course, the compiler built during stage 2, cc-lfs,
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would be able to build those libraries, but (1) the build system of
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GCC does not know that it is usable on pc, and (2) using it on pc
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would be at risk of linking to the pc libraries, since cc-lfs is a native
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compiler. So we have to build libstdc++ later, in chroot.</para>
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gcc does not know that it is usable on pc, and (2) using it on pc
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would create a risk of linking to the pc libraries, since cc-lfs is a native
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compiler. So we have to re-build libstdc++ later, in the chroot environment.</para>
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</sect2>
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@ -252,10 +253,10 @@
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be part of the final system.</para>
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<para>Binutils is installed first because the <command>configure</command>
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runs of both GCC and Glibc perform various feature tests on the assembler
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runs of both gcc and glibc perform various feature tests on the assembler
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and linker to determine which software features to enable or disable. This
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is more important than one might first realize. An incorrectly configured
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GCC or Glibc can result in a subtly broken toolchain, where the impact of
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is more important than one might realize at first. An incorrectly configured
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gcc or glibc can result in a subtly broken toolchain, where the impact of
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such breakage might not show up until near the end of the build of an
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entire distribution. A test suite failure will usually highlight this error
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before too much additional work is performed.</para>
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@ -274,14 +275,14 @@
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<command>$LFS_TGT-gcc dummy.c -Wl,--verbose 2>&1 | grep succeeded</command>
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will show all the files successfully opened during the linking.</para>
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<para>The next package installed is GCC. An example of what can be
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<para>The next package installed is gcc. An example of what can be
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seen during its run of <command>configure</command> is:</para>
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<screen><computeroutput>checking what assembler to use... /mnt/lfs/tools/i686-lfs-linux-gnu/bin/as
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checking what linker to use... /mnt/lfs/tools/i686-lfs-linux-gnu/bin/ld</computeroutput></screen>
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<para>This is important for the reasons mentioned above. It also
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demonstrates that GCC's configure script does not search the PATH
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demonstrates that gcc's configure script does not search the PATH
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directories to find which tools to use. However, during the actual
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operation of <command>gcc</command> itself, the same search paths are not
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necessarily used. To find out which standard linker <command>gcc</command>
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@ -295,12 +296,12 @@ checking what linker to use... /mnt/lfs/tools/i686-lfs-linux-gnu/bin/ld</compute
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order.</para>
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<para>Next installed are sanitized Linux API headers. These allow the
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standard C library (Glibc) to interface with features that the Linux
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standard C library (glibc) to interface with features that the Linux
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kernel will provide.</para>
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<para>The next package installed is Glibc. The most important
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considerations for building Glibc are the compiler, binary tools, and
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kernel headers. The compiler is generally not an issue since Glibc will
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<para>The next package installed is glibc. The most important
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considerations for building glibc are the compiler, binary tools, and
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kernel headers. The compiler is generally not an issue since glibc will
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always use the compiler relating to the <parameter>--host</parameter>
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parameter passed to its configure script; e.g. in our case, the compiler
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will be <command>$LFS_TGT-gcc</command>. The binary tools and kernel
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@ -313,26 +314,26 @@ checking what linker to use... /mnt/lfs/tools/i686-lfs-linux-gnu/bin/ld</compute
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<envar>$LFS_TGT</envar> expanded) to control which binary tools are used
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and the use of the <parameter>-nostdinc</parameter> and
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<parameter>-isystem</parameter> flags to control the compiler's include
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search path. These items highlight an important aspect of the Glibc
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search path. These items highlight an important aspect of the glibc
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package—it is very self-sufficient in terms of its build machinery
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and generally does not rely on toolchain defaults.</para>
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<para>As said above, the standard C++ library is compiled next, followed in
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<xref linkend="chapter-temporary-tools"/> by all the programs that need
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themselves to be built. The install step of all those packages uses the
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<envar>DESTDIR</envar> variable to have the
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programs land into the LFS filesystem.</para>
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<para>As mentioned above, the standard C++ library is compiled next, followed in
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<xref linkend="chapter-temporary-tools"/> by all the remaining programs that need
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to be cross compiled. The install step of all those packages uses the
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<envar>DESTDIR</envar> variable to force installation
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in the LFS filesystem.</para>
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<para>At the end of <xref linkend="chapter-temporary-tools"/> the native
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lfs compiler is installed. First binutils-pass2 is built,
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with the same <envar>DESTDIR</envar> install as the other programs,
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then the second pass of GCC is constructed, omitting libstdc++
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and other non-important libraries. Due to some weird logic in GCC's
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LFS compiler is installed. First binutils-pass2 is built,
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in the same <envar>DESTDIR</envar> directory as the other programs,
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then the second version of gcc is constructed, omitting libstdc++
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and other non-critical libraries. Due to some weird logic in gcc's
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configure script, <envar>CC_FOR_TARGET</envar> ends up as
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<command>cc</command> when the host is the same as the target, but is
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<command>cc</command> when the host is the same as the target, but
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different from the build system. This is why
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<parameter>CC_FOR_TARGET=$LFS_TGT-gcc</parameter> is put explicitly into
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the configure options.</para>
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<parameter>CC_FOR_TARGET=$LFS_TGT-gcc</parameter> is declared explicitly
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as one of the configuration options.</para>
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<para>Upon entering the chroot environment in <xref
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linkend="chapter-chroot-temporary-tools"/>, the first task is to install
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