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+<?xml version="1.0" encoding="ISO-8859-1"?>
+<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
+  <!ENTITY % general-entities SYSTEM "../general.ent">
+  %general-entities;
+]>
+
+<sect1 id="ch-tools-toolchaintechnotes">
+  <?dbhtml filename="toolchaintechnotes.html"?>
+
+  <title>Toolchain Technical Notes</title>
+
+  <para>This section explains some of the rationale and technical details
+  behind the overall build method. It is not essential to immediately
+  understand everything in this section. Most of this information will be
+  clearer after performing an actual build. This section can be referred
+  to at any time during the process.</para>
+
+  <para>The overall goal of this chapter and <xref
+  linkend="chapter-temporary-tools"/> is to produce a temporary area that
+  contains a known-good set of tools that can be isolated from the host system.
+  By using <command>chroot</command>, the commands in the remaining chapters
+  will be contained within that environment, ensuring a clean, trouble-free
+  build of the target LFS system. The build process has been designed to
+  minimize the risks for new readers and to provide the most educational value
+  at the same time.</para>
+
+  <para>The build process is based on the process of
+  <emphasis>cross-compilation</emphasis>. Cross-compilation is normally used
+  for building a compiler and its toolchain for a machine different from
+  the one that is used for the build. This is not strictly needed for LFS,
+  since the machine where the new system will run is the same as the one
+  used for the build. But cross-compilation has the great advantage that
+  anything that is cross-compiled cannot depend on the host environment.</para>
+
+  <sect2 id="cross-compile" xreflabel="About Cross-Compilation">
+
+    <title>About Cross-Compilation</title>
+
+    <para>Cross-compilation involves some concepts that deserve a section on
+    their own. Although this section may be omitted in a first reading, it
+    is strongly suggested to come back to it later in order to get a full
+    grasp of the build process.</para>
+
+    <para>Let us first define some terms used in this context:</para>
+
+    <variablelist>
+      <varlistentry><term>build</term><listitem>
+        <para>is the machine where we build programs. Note that this machine
+        is referred to as the <quote>host</quote> in other
+        sections.</para></listitem>
+      </varlistentry>
+
+      <varlistentry><term>host</term><listitem>
+        <para>is the machine/system where the built programs will run. Note
+        that this use of <quote>host</quote> is not the same as in other
+        sections.</para></listitem>
+      </varlistentry>
+
+      <varlistentry><term>target</term><listitem>
+        <para>is only used for compilers. It is the machine the compiler
+        produces code for. It may be different from both build and
+        host.</para></listitem>
+      </varlistentry>
+
+    </variablelist>
+
+    <para>As an example, let us imagine the following scenario: we may have a
+    compiler on a slow machine only, let's call the machine A, and the compiler
+    ccA. We may have also a fast machine (B), but with no compiler, and we may
+    want to produce code for a another slow machine (C). Then, to build a
+    compiler for machine C, we would have three stages:</para>
+
+    <informaltable align="center">
+      <tgroup cols="5">
+        <colspec colnum="1" align="center"/>
+        <colspec colnum="2" align="center"/>
+        <colspec colnum="3" align="center"/>
+        <colspec colnum="4" align="center"/>
+        <colspec colnum="5" align="left"/>
+        <thead>
+          <row><entry>Stage</entry><entry>Build</entry><entry>Host</entry>
+               <entry>Target</entry><entry>Action</entry></row>
+        </thead>
+        <tbody>
+          <row>
+            <entry>1</entry><entry>A</entry><entry>A</entry><entry>B</entry>
+            <entry>build cross-compiler cc1 using ccA on machine A</entry>
+          </row>
+          <row>
+            <entry>2</entry><entry>A</entry><entry>B</entry><entry>B</entry>
+            <entry>build cross-compiler cc2 using cc1 on machine A</entry>
+          </row>
+          <row>
+            <entry>3</entry><entry>B</entry><entry>C</entry><entry>C</entry>
+            <entry>build compiler ccC using cc2 on machine B</entry>
+          </row>
+        </tbody>
+      </tgroup>
+    </informaltable>
+
+    <para>Then, all the other programs needed by machine C can be compiled
+    using cc2 on the fast machine B. Note that unless B can run programs
+    produced for C, there is no way to test the built programs until machine
+    C itself is running. For example, for testing ccC, we may want to add a
+    fourth stage:</para>
+
+    <informaltable align="center">
+      <tgroup cols="5">
+        <colspec colnum="1" align="center"/>
+        <colspec colnum="2" align="center"/>
+        <colspec colnum="3" align="center"/>
+        <colspec colnum="4" align="center"/>
+        <colspec colnum="5" align="left"/>
+        <thead>
+          <row><entry>Stage</entry><entry>Build</entry><entry>Host</entry>
+               <entry>Target</entry><entry>Action</entry></row>
+        </thead>
+        <tbody>
+          <row>
+            <entry>4</entry><entry>C</entry><entry>C</entry><entry>C</entry>
+            <entry>rebuild  and test ccC using itself on machine C</entry>
+          </row>
+        </tbody>
+      </tgroup>
+    </informaltable>
+
+    <para>In the example above, only cc1 and cc2 are cross-compilers, that is,
+    they produce code for a machine different from the one they are run on.
+    The other compilers ccA and ccC produce code for the machine they are run
+    on. Such compilers are called <emphasis>native</emphasis> compilers.</para>
+
+  </sect2>
+
+  <sect2 id="lfs-cross">
+    <title>Implementation of Cross-Compilation for LFS</title>
+
+    <note>
+      <para>Almost all the build systems use names of the form
+      cpu-vendor-kernel-os referred to as the machine triplet. An astute
+      reader may wonder why a <quote>triplet</quote> refers to a four component
+      name. The reason is history: initially, three component names were enough
+      to designate unambiguously a machine, but with new machines and systems
+      appearing, that proved insufficient. The word <quote>triplet</quote>
+      remained. A simple way to determine your machine triplet is to run
+      the <command>config.guess</command>
+      script that comes with the source for many packages. Unpack the binutils
+      sources and run the script: <userinput>./config.guess</userinput> and note
+      the output. For example, for a 32-bit Intel processor the
+      output will be <emphasis>i686-pc-linux-gnu</emphasis>. On a 64-bit
+      system it will be <emphasis>x86_64-pc-linux-gnu</emphasis>.</para>
+
+      <para>Also be aware of the name of the platform's dynamic linker, often
+      referred to as the dynamic loader (not to be confused with the standard
+      linker <command>ld</command> that is part of binutils). The dynamic linker
+      provided by Glibc finds and loads the shared libraries needed by a
+      program, prepares the program to run, and then runs it. The name of the
+      dynamic linker for a 32-bit Intel machine will be <filename
+      class="libraryfile">ld-linux.so.2</filename> (<filename
+      class="libraryfile">ld-linux-x86-64.so.2</filename> for 64-bit systems). A
+      sure-fire way to determine the name of the dynamic linker is to inspect a
+      random binary from the host system by running: <userinput>readelf -l
+      &lt;name of binary&gt; | grep interpreter</userinput> and noting the
+      output. The authoritative reference covering all platforms is in the
+      <filename>shlib-versions</filename> file in the root of the Glibc source
+      tree.</para>
+    </note>
+
+    <para>In order to fake a cross compilation, the name of the host triplet
+    is slightly adjusted by changing the &quot;vendor&quot; field in the
+    <envar>LFS_TGT</envar> variable. We also use the
+    <parameter>--with-sysroot</parameter> option when building the cross linker and
+    cross compiler to tell them where to find the needed host files. This
+    ensures that none of the other programs built in <xref
+    linkend="chapter-temporary-tools"/> can link to libraries on the build
+    machine. Only two stages are mandatory, and one more for tests:</para>
+
+    <informaltable align="center">
+      <tgroup cols="5">
+        <colspec colnum="1" align="center"/>
+        <colspec colnum="2" align="center"/>
+        <colspec colnum="3" align="center"/>
+        <colspec colnum="4" align="center"/>
+        <colspec colnum="5" align="left"/>
+        <thead>
+          <row><entry>Stage</entry><entry>Build</entry><entry>Host</entry>
+               <entry>Target</entry><entry>Action</entry></row>
+        </thead>
+        <tbody>
+          <row>
+            <entry>1</entry><entry>pc</entry><entry>pc</entry><entry>lfs</entry>
+            <entry>build cross-compiler cc1 using cc-pc on pc</entry>
+          </row>
+          <row>
+            <entry>2</entry><entry>pc</entry><entry>lfs</entry><entry>lfs</entry>
+            <entry>build compiler cc-lfs using cc1 on pc</entry>
+          </row>
+          <row>
+            <entry>3</entry><entry>lfs</entry><entry>lfs</entry><entry>lfs</entry>
+            <entry>rebuild and test cc-lfs using itself on lfs</entry>
+          </row>
+        </tbody>
+      </tgroup>
+    </informaltable>
+
+    <para>In the above table, <quote>on pc</quote> means the commands are run
+    on a machine using the already installed distribution. <quote>On
+    lfs</quote> means the commands are run in a chrooted environment.</para>
+
+    <para>Now, there is more about cross-compiling: the C language is not
+    just a compiler, but also defines a standard library. In this book, the
+    GNU C library, named glibc, is used. This library must
+    be compiled for the lfs machine, that is, using the cross compiler cc1. 
+    But the compiler itself uses an internal library implementing complex
+    instructions not available in the assembler instruction set. This
+    internal library is named libgcc, and must be linked to the glibc
+    library to be fully functional! Furthermore, the standard library for
+    C++ (libstdc++) also needs being linked to glibc. The solution
+    to this chicken and egg problem is to first build a degraded cc1 based libgcc,
+    lacking some fuctionalities such as threads and exception handling, then
+    build glibc using this degraded compiler (glibc itself is not
+    degraded), then build libstdc++. But this last library will lack the
+    same functionalities as libgcc.</para>
+
+    <para>This is not the end of the story: the conclusion of the preceding
+    paragraph is that cc1 is unable to build a fully functional libstdc++, but
+    this is the only compiler available for building the C/C++ libraries
+    during stage 2! Of course, the compiler built during stage 2, cc-lfs,
+    would be able to build those libraries, but (1) the build system of
+    GCC does not know that it is usable on pc, and (2) using it on pc
+    would be at risk of linking to the pc libraries, since cc-lfs is a native
+    compiler. So we have to build libstdc++ later, in chroot.</para>
+
+  </sect2>
+
+  <sect2 id="other-details">
+
+    <title>Other procedural details</title>
+
+    <para>The cross-compiler will be installed in a separate <filename
+    class="directory">$LFS/tools</filename> directory, since it will not
+    be part of the final system.</para>
+
+    <para>Binutils is installed first because the <command>configure</command>
+    runs of both GCC and Glibc perform various feature tests on the assembler
+    and linker to determine which software features to enable or disable. This
+    is more important than one might first realize. An incorrectly configured
+    GCC or Glibc can result in a subtly broken toolchain, where the impact of
+    such breakage might not show up until near the end of the build of an
+    entire distribution. A test suite failure will usually highlight this error
+    before too much additional work is performed.</para>
+
+    <para>Binutils installs its assembler and linker in two locations,
+    <filename class="directory">$LFS/tools/bin</filename> and <filename
+    class="directory">$LFS/tools/$LFS_TGT/bin</filename>. The tools in one
+    location are hard linked to the other. An important facet of the linker is
+    its library search order. Detailed information can be obtained from
+    <command>ld</command> by passing it the <parameter>--verbose</parameter>
+    flag. For example, <command>$LFS_TGT-ld --verbose | grep SEARCH</command>
+    will illustrate the current search paths and their order. It shows which
+    files are linked by <command>ld</command> by compiling a dummy program and
+    passing the <parameter>--verbose</parameter> switch to the linker. For
+    example,
+    <command>$LFS_TGT-gcc dummy.c -Wl,--verbose 2&gt;&amp;1 | grep succeeded</command>
+    will show all the files successfully opened during the linking.</para>
+
+    <para>The next package installed is GCC. An example of what can be
+    seen during its run of <command>configure</command> is:</para>
+
+<screen><computeroutput>checking what assembler to use... /mnt/lfs/tools/i686-lfs-linux-gnu/bin/as
+checking what linker to use... /mnt/lfs/tools/i686-lfs-linux-gnu/bin/ld</computeroutput></screen>
+
+    <para>This is important for the reasons mentioned above. It also
+    demonstrates that GCC's configure script does not search the PATH
+    directories to find which tools to use. However, during the actual
+    operation of <command>gcc</command> itself, the same search paths are not
+    necessarily used. To find out which standard linker <command>gcc</command>
+    will use, run: <command>$LFS_TGT-gcc -print-prog-name=ld</command>.</para>
+
+    <para>Detailed information can be obtained from <command>gcc</command> by
+    passing it the <parameter>-v</parameter> command line option while compiling
+    a dummy program. For example, <command>gcc -v dummy.c</command> will show
+    detailed information about the preprocessor, compilation, and assembly
+    stages, including <command>gcc</command>'s included search paths and their
+    order.</para>
+
+    <para>Next installed are sanitized Linux API headers. These allow the
+    standard C library (Glibc) to interface with features that the Linux
+    kernel will provide.</para>
+
+    <para>The next package installed is Glibc. The most important
+    considerations for building Glibc are the compiler, binary tools, and
+    kernel headers. The compiler is generally not an issue since Glibc will
+    always use the compiler relating to the <parameter>--host</parameter>
+    parameter passed to its configure script; e.g. in our case, the compiler
+    will be <command>$LFS_TGT-gcc</command>. The binary tools and kernel
+    headers can be a bit more complicated. Therefore, take no risks and use
+    the available configure switches to enforce the correct selections. After
+    the run of <command>configure</command>, check the contents of the
+    <filename>config.make</filename> file in the <filename
+    class="directory">build</filename> directory for all important details.
+    Note the use of <parameter>CC="$LFS_TGT-gcc"</parameter> (with
+    <envar>$LFS_TGT</envar> expanded) to control which binary tools are used
+    and the use of the <parameter>-nostdinc</parameter> and
+    <parameter>-isystem</parameter> flags to control the compiler's include
+    search path. These items highlight an important aspect of the Glibc
+    package&mdash;it is very self-sufficient in terms of its build machinery
+    and generally does not rely on toolchain defaults.</para>
+
+    <para>As said above, the standard C++ library is compiled next, followed in
+    Chapter 6 by all the programs that need themselves to be built. The install
+    step of libstdc++ uses the <envar>DESTDIR</envar> variable to have the
+    programs land into the LFS filesystem.</para>
+
+    <para>In Chapter 7 the native lfs compiler is built. First binutils-pass2,
+    with the same <envar>DESTDIR</envar> install as the other programs is
+    built, and then the second pass of GCC is constructed, omitting libstdc++
+    and other non-important libraries.  Due to some weird logic in GCC's
+    configure script, <envar>CC_FOR_TARGET</envar> ends up as
+    <command>cc</command> when the host is the same as the target, but is
+    different from the build system. This is why
+    <parameter>CC_FOR_TARGET=$LFS_TGT-gcc</parameter> is put explicitely into
+    the configure options.</para>
+
+    <para>Upon entering the chroot environment in <xref
+    linkend="chapter-chroot-temporary-tools"/>, the first task is to install
+    libstdc++. Then temporary installations of programs needed for the proper
+    operation of the toolchain are performed. Programs needed for testing
+    other programs are also built. From this point onwards, the
+    core toolchain is self-contained and self-hosted.  In 
+    <xref linkend="chapter-building-system"/>, final versions of all the
+    packages needed for a fully functional system are built, tested and
+    installed.</para>
+
+  </sect2>
+
+</sect1>