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<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY % general-entities SYSTEM "../general.ent">
%general-entities;
]>
<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>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 class="libraryfile">ld-linux.so.2</filename>.
On platforms that are less prevalent, the name might be <filename
class="libraryfile">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 <name of binary> | 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>&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>.</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—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 <name of binary> | 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—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—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>
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