lfs-book/chapter05/toolchaintechnotes.xml

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<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 <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>

  <note>
    <para>Before continuing, be aware of the name of the working platform,
    often referred to as the target triplet. A simple way to determine the
    name of the target 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 modern 32-bit Intel processor the
    output will likely be <emphasis>i686-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>.
    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>Some key technical points of how the <xref
  linkend="chapter-temporary-tools"/> build method works:</para>

  <itemizedlist>
    <listitem>
      <para>Slightly adjusting the name of the working platform, by changing the
      &quot;vendor&quot; field target triplet by way of the
      <envar>LFS_TGT</envar> variable, ensures that the first build of Binutils
      and GCC produces a compatible cross-linker and cross-compiler. Instead of
      producing binaries for another architecture, the cross-linker and
      cross-compiler will produce binaries compatible with the current
      hardware.</para>
    </listitem>
    <listitem>
      <para> The temporary libraries are cross-compiled.  Because a
      cross-compiler by its nature cannot rely on anything from its host
      system, this method removes potential contamination of the target
      system by lessening the chance of headers or libraries from the host
      being incorporated into the new tools.  Cross-compilation also allows for
      the possibility of building both 32-bit and 64-bit libraries on 64-bit
      capable hardware.</para>
    </listitem>
    <listitem>
    <para>Careful manipulation of the GCC source tells the compiler which target
    dynamic linker will be used.</para>
    </listitem>
  </itemizedlist>

  <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">/tools/bin</filename> and <filename
  class="directory">/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, an <userinput>ld --verbose | grep SEARCH</userinput>
  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,
  <userinput>gcc dummy.c -Wl,--verbose 2&gt;&amp;1 | grep succeeded</userinput>
  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... /tools/i686-lfs-linux-gnu/bin/as
checking what linker to use... /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:
  <userinput>gcc -print-prog-name=ld</userinput>.</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, <userinput>gcc -v dummy.c</userinput> 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,
  <command>i686-lfs-linux-gnu-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">glibc-build</filename> directory for all important details.
  Note the use of <parameter>CC="i686-lfs-gnu-gcc"</parameter> 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>During the second pass of Binutils, we are able to utilize the
  <parameter>--with-lib-path</parameter> configure switch to control
  <command>ld</command>'s library search path.</para>

  <para>For the second pass of GCC, its sources also need to be modified to
  tell GCC to use the new dynamic linker. Failure to do so will result in the
  GCC programs themselves having the name of the dynamic linker from the host
  system's <filename class="directory">/lib</filename> directory embedded into
  them, which would defeat the goal of getting away from the host. From this
  point onwards, the core toolchain is self-contained and self-hosted. The
  remainder of the <xref linkend="chapter-temporary-tools"/> packages all build
  against the new Glibc in <filename
  class="directory">/tools</filename>.</para>

  <para>Upon entering the chroot environment in <xref
  linkend="chapter-building-system"/>, the first major package to be
  installed is Glibc, due to its self-sufficient nature mentioned above.
  Once this Glibc is installed into <filename
  class="directory">/usr</filename>, we will perform a quick changeover of the
  toolchain defaults, and then proceed in building the rest of the target
  LFS system.</para>

</sect1>