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+<sect1 id="ch05-toolchaintechnotes">
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+<title>Toolchain technical notes</title>
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+<?dbhtml filename="toolchaintechnotes.html" dir="chapter05"?>
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+
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+<para>This section attempts to explain some of the rationale and technical
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+details behind the overall build method. It's not essential that you understand
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+everything here immediately. Most of it will make sense once you have performed
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+an actual build. Feel free to refer back here at any time.</para>
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+
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+<para>The overall goal of Chapter 5 is to provide a sane, temporary environment
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+that we can chroot into, and from which we can produce a clean, trouble-free
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+build of the target LFS system in Chapter 6. Along the way, we attempt to
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+divorce ourselves from the host system as much as possible, and in so doing
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+build a self-contained and self-hosted toolchain. It should be noted that the
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+build process has been designed in such a way so as to minimize the risks for
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+new readers and also provide maximum educational value at the same time. In
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+other words, more advanced techniques could be used to achieve the same
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+goals.</para>
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+
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+<important>
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+<para>Before continuing, you really should be aware of the name of your working
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+platform, often also referred to as the <emphasis>target triplet</emphasis>. For
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+many folks the target triplet will be, for example:
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+<emphasis>i686-pc-linux-gnu</emphasis>. A simple way to determine your target
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+triplet is to run the <filename>config.guess</filename> script that comes with
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+the source for many packages. Unpack the Binutils sources and run the script:
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+<userinput>./config.guess</userinput> and note the output.</para>
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+
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+<para>You'll also need to be aware of the name of your platform's
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+<emphasis>dynamic linker</emphasis>, often also referred to as the
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+<emphasis>dynamic loader</emphasis>, not to be confused with the standard linker
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+<emphasis>ld</emphasis> that is part of Binutils. The dynamic linker is provided
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+by Glibc and has the job of finding and loading the shared libraries needed by a
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+program, preparing the program to run and then running it. For most folks, the
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+name of the dynamic linker will be <emphasis>ld-linux.so.2</emphasis>. On
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+platforms that are less prevalent, the name might be
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+<emphasis>ld.so.1</emphasis> and newer 64 bit platforms might even have
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+something completely different. You should be able to determine the name
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+of your platform's dynamic linker by looking in the
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+<filename class="directory">/lib</filename> directory on your host system. A
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+surefire way is to inspect a random binary from your host system by running:
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+<userinput>`readelf -l <name of binary> | grep interpreter`</userinput>
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+and noting the output. The authoritative reference covering all platforms is in
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+the <filename>shlib-versions</filename> file in the root of the Glibc source
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+tree.</para>
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+</important>
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+
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+<para>Some key technical points of how the Chapter 5 build method works:</para>
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+
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+<itemizedlist>
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+<listitem><para>Similar in principle to cross compiling whereby tools installed
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+into the same prefix work in cooperation and thus utilize a little GNU
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+"magic".</para></listitem>
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+
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+<listitem><para>Careful manipulation of the standard linker's library search
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+path to ensure programs are linked only against libraries we
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+choose.</para></listitem>
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+
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+<listitem><para>Careful manipulation of GCC's <emphasis>specs</emphasis> file to
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+tell GCC which target dynamic linker will be used.</para></listitem>
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+</itemizedlist>
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+
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+<para>Binutils is installed first because both GCC and Glibc perform various
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+feature tests on the assembler and linker during their respective runs of
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+<filename>./configure</filename> to determine which software features to enable
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+or disable. This is more important than one might first realize. An incorrectly
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+configured GCC or Glibc can result in a subtly broken toolchain where the impact
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+of such breakage might not show up until near the end of a build of a whole
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+distribution. Thankfully, a test suite failure will usually alert us before too
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+much harm is done.</para>
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+
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+<para>Binutils installs its assembler and linker into two locations,
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+<filename class="directory">/tools/bin</filename> and
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+<filename class="directory">/tools/$TARGET_TRIPLET/bin</filename>. In reality,
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+the tools in one location are hard linked to the other. An important facet of ld
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+is its library search order. Detailed information can be obtained from ld by
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+passing it the <emphasis>--verbose</emphasis> flag. For example:
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+<userinput>`ld --verbose | grep SEARCH`</userinput> will show you the current
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+search paths and order. You can see what files are actually linked by ld by
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+compiling a dummy program and passing the --verbose switch. For example:
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+<userinput>`gcc dummy.c -Wl,--verbose 2>&1 | grep succeeded`</userinput>
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+will show you all the files successfully opened during the link.</para>
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+
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+<para>The next package installed is GCC and during its run of
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+<filename>./configure</filename> you'll see, for example:</para>
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+
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+<blockquote><screen>checking what assembler to use... /tools/i686-pc-linux-gnu/bin/as
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+checking what linker to use... /tools/i686-pc-linux-gnu/bin/ld</screen></blockquote>
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+
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+<para>This is important for the reasons mentioned above. It also demonstrates
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+that GCC's configure script does not search the $PATH directories to find which
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+tools to use. However, during the actual operation of GCC itself, the same
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+search paths are not necessarily used. You can find out which standard linker
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+GCC will use by running: <userinput>`gcc -print-prog-name=ld`</userinput>.
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+Detailed information can be obtained from GCC by passing it the
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+<emphasis>-v</emphasis> flag while compiling a dummy program. For example:
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+<userinput>`gcc -v dummy.c`</userinput> will show you detailed information about
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+the preprocessor, compilation and assembly stages, including GCC's include
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+search paths and order.</para>
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+
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+<para>The next package installed is Glibc. The most important considerations for
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+building Glibc are the compiler, binary tools and kernel headers. The compiler
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+is generally no problem as it will always use the GCC found in a $PATH
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+directory. The binary tools and kernel headers can be a little more troublesome.
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+Therefore we take no risks and we use the available configure switches to
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+enforce the correct selections. After the run of
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+<filename>./configure</filename> you can check the contents of the
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+<filename>config.make</filename> file in the
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+<filename class="directory">glibc-build</filename> directory for all the
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+important details. You'll note some interesting items like the use of
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+<userinput>CC="gcc -B/tools/bin/"</userinput> to control which binary tools are
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+used and also the use of the <emphasis>-nostdinc</emphasis> and
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+<emphasis>-isystem</emphasis> flags to control the GCC include search path.
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+These items help to highlight an important aspect of the Glibc package: it is
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+very self sufficient in terms of its build machinery and generally does not rely
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+on toolchain defaults.</para>
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+
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+<para>After the Glibc installation, we make some adjustments to ensure that
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+searching and linking take place only within our /tools prefix. We install an
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+adjusted ld, which has a hard-wired search path limited to
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+<filename class="directory">/tools/lib</filename>. Then we amend GCC's specs
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+file to point to our new dynamic linker in
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+<filename class="directory">/tools/lib</filename>. This last step is
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+<emphasis>vital</emphasis> to the whole process. As mentioned above, a
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+hard-wired path to a dynamic linker is embedded into every executable binary.
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+You can inspect this by running:
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+<userinput>`readelf -l <name of binary> | grep interpreter`</userinput>.
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+By amending the GCC specs file, we are ensuring that every program compiled from
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+here through the end of Chapter 5 will use our new dynamic linker in
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+<filename class="directory">/tools/lib</filename>.</para>
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+
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+<para>The need to use the new dynamic linker is also the reason why we apply the
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+specs patch for the second pass of GCC. Failure to do so will result in the GCC
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+programs themselves having the dynamic linker from the host system's
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+<filename class="directory">/lib</filename> directory embedded into them, which
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+would defeat our goal of getting away from the host system.</para>
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+
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+<para>During the second pass of Binutils, we are able to utilize the
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+<userinput>--with-lib-path</userinput> configure switch to control ld's library
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+search path. From this point onwards, the core toolchain is self-contained and
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+self-hosted. The remainder of the Chapter 5 packages all build against the new
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+Glibc in <filename class="directory">/tools</filename> and all is well.</para>
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+
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+<para>Upon entering the chroot environment in Chapter 6, the first major package
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+we install is Glibc, due to its self sufficient nature that we mentioned above.
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+Once this Glibc is installed into <filename class="directory">/usr</filename>,
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+we perform a quick changeover of the toolchain defaults, then proceed for real
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+in building the rest of the target Chapter 6 LFS system.</para>
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+
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+<sect2>
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+<title>Notes on static linking</title>
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+
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+<para>Most programs have to perform, beside their specific task, many rather
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+common and sometimes trivial operations, These include allocating memory,
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+searching directories, reading and writing files, string handling, pattern
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+matching, arithmetic, and many other tasks. Instead of obliging each program to
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+reinvent the wheel, the GNU system provides all these basic functions in
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+ready-made libraries. The major library on any Linux system is
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+<emphasis>Glibc</emphasis>.</para>
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+
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+<para>There are two primary ways of linking the functions from a library to a
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+program that uses them: statically or dynamically. When a program is linked
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+statically, the code of the used functions is included in the executable,
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+resulting in a rather bulky program. When a program is dynamically linked, what
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+is included is a reference to the dynamic linker, the name of the library, and
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+the name of the function, resulting in a much smaller executable. A third way is
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+to use the programming interface of the dynamic linker. See the
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+<emphasis>dlopen</emphasis> man page for more information.</para>
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+
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+<para>Dynamic linking is the default on Linux and has three major advantages
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+over static linking. First, you need only one copy of the executable library
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+code on your hard disk, instead of having many copies of the same code included
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+into a whole bunch of programs -- thus saving disk space. Second, when several
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+programs use the same library function at the same time, only one copy of the
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+function's code is required in core -- thus saving memory space. Third, when a
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+library function gets a bug fixed or is otherwise improved, you only need to
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+recompile this one library, instead of having to recompile all the programs that
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+make use of the improved function.</para>
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+
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+<para>Why do we statically link the first two packages in Chapter 5? The reasons
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+are threefold: historical, educational and technical. Historical because earlier
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+versions of LFS statically linked every program in Chapter 5. Educational
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+because knowing the difference is useful. Technical because we gain an element
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+of independence from the host in doing so, i.e. those programs can be used
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+independently of the host system. However, it's worth noting that an overall
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+successful LFS build can still be achieved when the first two packages are built
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+dynamically.</para>
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+
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+</sect2>
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+
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+</sect1>
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+
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Greg Schafer