source: chapter05/toolchaintechnotes.xml@ d22a5031

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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
  back to at any time during the process.</para>

  <para>The overall goal of <xref linkend="chapter-temporary-tools"/> is to
  provide a temporary environment that can be chrooted into and from which can be
  produced a clean, trouble-free build of the target LFS system in <xref
  linkend="chapter-building-system"/>. Along the way, we separate the new system
  from the host system as much as possible, and in doing so, build a
  self-contained and self-hosted toolchain. It should be noted that the build
  process has been designed to minimize the risks for new readers and provide
  maximum educational value at the same time.</para>

  <important>
    <para>Before continuing, be aware of the name of the working platform,
    often referred to as the target triplet. Many times, the target
    triplet will probably be <emphasis>x86_64-unknown-linux-gnu</emphasis>. 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.</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 will usually be <filename class="libraryfile">ld-linux-x86-64.so.2</filename>.
    The name of the platform's dynamic linker can be determined by looking in the
    <filename class="directory">/lib</filename>
    directory on the host system. A sure-fire way to determine the name 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>
  </important>

  <para>Some key technical points of how the <xref
  linkend="chapter-temporary-tools"/> build method works:</para>

  <itemizedlist>
    <listitem>
      <para>The process is similar in principle to cross-compiling, whereby
      tools installed in the same prefix work in cooperation, and thus utilize
      a little GNU <quote>magic</quote></para>
    </listitem>
    <listitem>
      <para>Careful manipulation of the standard linker's library search path
      ensures programs are linked only against chosen libraries</para>
    </listitem>
    <listitem>
      <para>Careful manipulation of <command>gcc</command>'s
      <filename>specs</filename> file 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/$TARGET_TRIPLET/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/x86_64-unknown-linux-gnu/bin/as
checking what linker to use... /tools/x86_64-unknown-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>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
  <command>gcc</command> found in a <envar>PATH</envar> directory. 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="gcc -B/tools/bin/"</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>After the Glibc installation, make some adjustments to ensure that
  searching and linking take place only within the <filename
  class="directory">/tools</filename> prefix.  Install an adjusted
  <command>ld</command>, which has a hard-wired search path limited to
  <filename class="directory">/tools/lib</filename>. Then amend
  <command>gcc</command>'s specs file to point to the new dynamic linker in
  <filename class="directory">/tools/lib</filename>. This last step is vital
  to the whole process. As mentioned above, a hard-wired path to a dynamic
  linker is embedded into every Executable and Link Format (ELF)-shared
  executable.  This can be inspected by running:
  <userinput>readelf -l &lt;name of binary&gt; | grep interpreter</userinput>.
  Amending gcc's specs file ensures that every program compiled from here
  through the end of this chapter will use the new dynamic linker in
  <filename class="directory">/tools/lib</filename>.</para>

  <para>The need to use the new dynamic linker is also the reason why
  the Specs patch is applied for the second pass of GCC. 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.</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.  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>, perform a quick changeover of the
  toolchain defaults, then proceed in building the rest of the target
  LFS system.</para>

  <!-- FIXME: Removed as part of the fix for bug 1061 - we no longer build pass1
      packages statically, therefore this explanation isn't required

  <sect2>
  <title>Notes on Static Linking</title>

  <para>Besides their specific task, most programs have to perform many
  common and sometimes trivial operations. These include allocating
  memory, searching directories, reading and writing files, string
  handling, pattern matching, arithmetic, and other tasks. Instead of
  obliging each program to reinvent the wheel, the GNU system provides
  all these basic functions in ready-made libraries. The major library
  on any Linux system is Glibc.</para>

  <para>There are two primary ways of linking the functions from a
  library to a program that uses them&mdash;statically or dynamically. When
  a program is linked statically, the code of the used functions is
  included in the executable, resulting in a rather bulky program. When
  a program is dynamically linked, it includes a reference to the
  dynamic linker, the name of the library, and the name of the function,
  resulting in a much smaller executable. A third option is to use the
  programming interface of the dynamic linker (see <filename>dlopen(3)</filename>
  for more information).</para>

  <para>Dynamic linking is the default on Linux and has three major
  advantages over static linking. First, only one copy of the executable
  library code is needed on the hard disk, instead of having multiple
  copies of the same code included in several programs, thus saving
  disk space. Second, when several programs use the same library
  function at the same time, only one copy of the function's code is
  required in core, thus saving memory space. Third, when a library
  function gets a bug fixed or is otherwise improved, only the one
  library needs to be recompiled instead of recompiling all programs
  that make use of the improved function.</para>

  <para>If dynamic linking has several advantages, why then do we
  statically link the first two packages in this chapter? The reasons
  are threefold&mdash;historical, educational, and technical. The
  historical reason is that earlier versions of LFS statically linked
  every program in this chapter. Educationally, knowing the difference
  between static and dynamic linking is useful. The technical benefit is
  a gained element of independence from the host, meaning that those
  programs can be used independently of the host system. However, it is
  worth noting that an overall successful LFS build can still be
  achieved when the first two packages are built dynamically.</para>

  </sect2>-->

</sect1>