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<!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
to at any time during the process.</para>
<para>The overall goal of <xref linkend="chapter-temporary-tools"/> is to
produce a temporary area that contains a known-good set of tools that can be
isolated from the host system. By using <command>chroot</command>, the
commands in the remaining chapters will be contained within that environment,
ensuring a clean, trouble-free build of the target LFS system. The build
process has been designed to minimize the risks for new readers and to provide
the most educational value at the same time.</para>
<note>
<para>Before continuing, be aware of the name of the working platform,
often referred to as the target triplet. 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. For example, for a 32-bit Intel processor the
output will be <emphasis>i686-pc-linux-gnu</emphasis>. On a 64-bit
system it will be <emphasis>x86_64-pc-linux-gnu</emphasis>.</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 for a 32-bit Intel machine will be <filename
class="libraryfile">ld-linux.so.2</filename> (<filename
class="libraryfile">ld-linux-x86-64.so.2</filename> for 64-bit systems). A
sure-fire way to determine the name of the dynamic linker 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>
</note>
<para>Some key technical points of how the <xref
linkend="chapter-temporary-tools"/> build method works:</para>
<itemizedlist>
<listitem>
<para>Slightly adjusting the name of the working platform, by changing the
"vendor" field target triplet by way of the
<envar>LFS_TGT</envar> variable, ensures that the first build of Binutils
and GCC produces a compatible cross-linker and cross-compiler. Instead of
producing binaries for another architecture, the cross-linker and
cross-compiler will produce binaries compatible with the current
hardware.</para>
</listitem>
<listitem>
<para> The temporary libraries are cross-compiled. Because a
cross-compiler by its nature cannot rely on anything from its host
system, this method removes potential contamination of the target
system by lessening the chance of headers or libraries from the host
being incorporated into the new tools. Cross-compilation also allows for
the possibility of building both 32-bit and 64-bit libraries on 64-bit
capable hardware.</para>
</listitem>
<listitem>
<para>Careful manipulation of the GCC source 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/$LFS_TGT/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-lfs-linux-gnu/bin/as
checking what linker to use... /tools/i686-lfs-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>Next installed are sanitized Linux API headers. These allow the standard
C library (Glibc) to interface with features that the Linux kernel will
provide.</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 compiler
relating to the <parameter>--host</parameter> parameter passed to its
configure script, e.g. in our case,
<command>i686-lfs-linux-gnu-gcc</command>. 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="i686-lfs-gnu-gcc"</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>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.</para>
<para>For the second pass of GCC, its sources also need to be modified to
tell GCC to use the new dynamic linker. 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. 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>, we will perform a quick changeover of the
toolchain defaults, and then proceed in building the rest of the target
LFS system.</para>
</sect1>
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