Chapter 5: Add new section "Toolchain technical notes". Integrate and scale back the old "Why we use static linking" section. Closes Bug 658.

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@2927 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

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