source: chapter05/toolchaintechnotes.xml@ c863747

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<sect1 id="ch-tools-toolchaintechnotes">
<title>Toolchain Technical Notes</title>
<?dbhtml filename="toolchaintechnotes.html"?>

<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 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. In
other words, more advanced techniques could be used to build the
system.</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>i686-pc-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>ld-linux.so.2</filename>. On platforms that are less
prevalent, the name might be <filename>ld.so.1</filename>, and newer
64 bit platforms might be named something else entirely. 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 tell 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 alert 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 <command>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,
<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-pc-linux-gnu/bin/as
checking what linker to use... /tools/i686-pc-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>. 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 PATH 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>

<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 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, 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 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, 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>