mirror of
https://git.linuxfromscratch.org/lfs.git
synced 2025-03-06 06:14:47 +00:00
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
This commit is contained in:
Greg Schafer
parent
076ddfe40d
commit
98c95f5392
@ -95,6 +95,10 @@
|
||||
</itemizedlist>
|
||||
</para></listitem>
|
||||
|
||||
<listitem><para>October 5th, 2003 [greg]: Chapter 5: Add new section "Toolchain
|
||||
technical notes". Integrate and scale back the old "Why we use static linking"
|
||||
section. Closes Bug 658.</para></listitem>
|
||||
|
||||
<listitem><para>October 4th, 2003 [greg]: Chapter 5 - Binutils Pass 1: Add
|
||||
extra LDFLAGS to ensure static rebuild of ld.</para></listitem>
|
||||
|
||||
|
||||
@ -3,7 +3,7 @@
|
||||
<?dbhtml filename="chapter05.html" dir="chapter05"?>
|
||||
|
||||
&c5-introduction;
|
||||
&c5-whystatic;
|
||||
&c5-toolchaintechnotes;
|
||||
&c5-creatingtoolsdir;
|
||||
&c5-addinguser;
|
||||
&c5-settingenviron;
|
||||
|
||||
192
chapter05/toolchaintechnotes.xml
Normal file
192
chapter05/toolchaintechnotes.xml
Normal file
@ -0,0 +1,192 @@
|
||||
<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 <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 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>&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 <name of binary> | 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>
|
||||
|
||||
@ -1,61 +0,0 @@
|
||||
<sect1 id="ch05-whystatic">
|
||||
<title>Why we use static linking</title>
|
||||
<?dbhtml filename="whystatic.html" dir="chapter05"?>
|
||||
|
||||
<para>Most programs have to perform, beside their specific task, many rather
|
||||
common and trivial operations, such as allocating memory, searching
|
||||
directories, opening and closing files, reading and writing them, string
|
||||
handling, pattern matching, arithmetic, and so on. Instead of obliging each
|
||||
program to reinvent the wheel, the GNU system provides all these basic
|
||||
functions ready-made in libraries. The major library on any Linux system is
|
||||
<filename>glibc</filename>. To get an idea of what it contains, have a look at
|
||||
<filename>glibc/index.html</filename> somewhere on your host system.</para>
|
||||
|
||||
<para>There are two 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 linker, the name of the library, and
|
||||
the name of the function, resulting in a much smaller executable. Under
|
||||
certain circumstances, this executable can have the disadvantage of being
|
||||
somewhat slower than a statically linked one, as the linking at run time takes
|
||||
a few moments. It should be noted, however, that under normal circumstances on
|
||||
today's hardware, a dynamically linked executable will be faster than a
|
||||
statically linked one as the library function being called by the dynamically
|
||||
linked executable has a good chance of already being loaded in your system's
|
||||
RAM.</para>
|
||||
|
||||
<para>Aside from this small drawback, dynamic linking has two 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.</para>
|
||||
|
||||
<para>Nowadays saving a few megabytes of space may not seem like much, but
|
||||
many moons ago, when disks were measured in megabytes and core in kilobytes,
|
||||
such savings were essential. It meant being able to keep several programs in
|
||||
core at the same time and to contain an entire Unix system on just a few disk
|
||||
volumes.</para>
|
||||
|
||||
<para>A third but minor advantage of dynamic linking is that 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>In summary we can say that dynamic linking trades run time against
|
||||
memory space, disk space, and recompile time.</para>
|
||||
|
||||
<para>But if dynamic linking saves so much space, why then are we linking
|
||||
the first two packages in this chapter statically? The reason is to make them
|
||||
independent from the libraries on your host system. The advantage is that, if
|
||||
you are pressed for time, you could skip the second passes over GCC and
|
||||
Binutils, and just use the static versions to compile the rest of this chapter
|
||||
and the first few packages in the next. In the next chapter we will be
|
||||
chrooted to the LFS partition and once inside the chroot environment, the host
|
||||
system's Glibc won't be available, thus the programs from GCC and Binutils
|
||||
will need to be self-contained, i.e. statically linked. However, we strongly
|
||||
advise <emphasis>against</emphasis> skipping the second passes.</para>
|
||||
|
||||
</sect1>
|
||||
|
||||
@ -1,6 +1,6 @@
|
||||
<!ENTITY chapter05 SYSTEM "../chapter05/chapter05.xml">
|
||||
<!ENTITY c5-introduction SYSTEM "../chapter05/introduction.xml">
|
||||
<!ENTITY c5-whystatic SYSTEM "../chapter05/whystatic.xml">
|
||||
<!ENTITY c5-toolchaintechnotes SYSTEM "../chapter05/toolchaintechnotes.xml">
|
||||
<!ENTITY c5-creatingtoolsdir SYSTEM "../chapter05/creatingstage1dir.xml">
|
||||
<!ENTITY c5-addinguser SYSTEM "../chapter05/adding-user.xml">
|
||||
<!ENTITY c5-settingenviron SYSTEM "../chapter05/setting-environment.xml">
|
||||
|
||||
Loading…
Reference in New Issue
Block a user