source: chapter05/toolchaintechnotes.xml@ e1c3882

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Last change on this file since e1c3882 was e1c3882, checked in by Bruce Dubbs <bdubbs@…>, 6 years ago

Fix a leftover reference to glibc-build
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1<?xml version="1.0" encoding="ISO-8859-1"?>
2<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
3 "" [
4 <!ENTITY % general-entities SYSTEM "../general.ent">
5 %general-entities;
8<sect1 id="ch-tools-toolchaintechnotes">
9 <?dbhtml filename="toolchaintechnotes.html"?>
11 <title>Toolchain Technical Notes</title>
13 <para>This section explains some of the rationale and technical details
14 behind the overall build method. It is not essential to immediately
15 understand everything in this section. Most of this information will be
16 clearer after performing an actual build. This section can be referred
17 to at any time during the process.</para>
19 <para>The overall goal of <xref linkend="chapter-temporary-tools"/> is to
20 produce a temporary area that contains a known-good set of tools that can be
21 isolated from the host system. By using <command>chroot</command>, the
22 commands in the remaining chapters will be contained within that environment,
23 ensuring a clean, trouble-free build of the target LFS system. The build
24 process has been designed to minimize the risks for new readers and to provide
25 the most educational value at the same time.</para>
27 <note>
28 <para>Before continuing, be aware of the name of the working platform,
29 often referred to as the target triplet. A simple way to determine the
30 name of the target triplet is to run the <command>config.guess</command>
31 script that comes with the source for many packages. Unpack the Binutils
32 sources and run the script: <userinput>./config.guess</userinput> and note
33 the output. For example, for a 32-bit Intel processor the
34 output will be <emphasis>i686-pc-linux-gnu</emphasis>. On a 64-bit
35 system it will be <emphasis>x86_64-pc-linux-gnu</emphasis>.</para>
37 <para>Also be aware of the name of the platform's dynamic linker, often
38 referred to as the dynamic loader (not to be confused with the standard
39 linker <command>ld</command> that is part of Binutils). The dynamic linker
40 provided by Glibc finds and loads the shared libraries needed by a program,
41 prepares the program to run, and then runs it. The name of the dynamic
42 linker for a 32-bit Intel machine will be <filename
43 class="libraryfile"></filename> (<filename
44 class="libraryfile"></filename> for 64-bit systems). A
45 sure-fire way to determine the name of the dynamic linker is to inspect a
46 random binary from the host system by running: <userinput>readelf -l
47 &lt;name of binary&gt; | grep interpreter</userinput> and noting the
48 output. The authoritative reference covering all platforms is in the
49 <filename>shlib-versions</filename> file in the root of the Glibc source
50 tree.</para>
51 </note>
53 <para>Some key technical points of how the <xref
54 linkend="chapter-temporary-tools"/> build method works:</para>
56 <itemizedlist>
57 <listitem>
58 <para>Slightly adjusting the name of the working platform, by changing the
59 &quot;vendor&quot; field target triplet by way of the
60 <envar>LFS_TGT</envar> variable, ensures that the first build of Binutils
61 and GCC produces a compatible cross-linker and cross-compiler. Instead of
62 producing binaries for another architecture, the cross-linker and
63 cross-compiler will produce binaries compatible with the current
64 hardware.</para>
65 </listitem>
66 <listitem>
67 <para> The temporary libraries are cross-compiled. Because a
68 cross-compiler by its nature cannot rely on anything from its host
69 system, this method removes potential contamination of the target
70 system by lessening the chance of headers or libraries from the host
71 being incorporated into the new tools. Cross-compilation also allows for
72 the possibility of building both 32-bit and 64-bit libraries on 64-bit
73 capable hardware.</para>
74 </listitem>
75 <listitem>
76 <para>Careful manipulation of the GCC source tells the compiler which target
77 dynamic linker will be used.</para>
78 </listitem>
79 </itemizedlist>
81 <para>Binutils is installed first because the <command>configure</command>
82 runs of both GCC and Glibc perform various feature tests on the assembler
83 and linker to determine which software features to enable or disable. This
84 is more important than one might first realize. An incorrectly configured
85 GCC or Glibc can result in a subtly broken toolchain, where the impact of
86 such breakage might not show up until near the end of the build of an
87 entire distribution. A test suite failure will usually highlight this error
88 before too much additional work is performed.</para>
90 <para>Binutils installs its assembler and linker in two locations,
91 <filename class="directory">/tools/bin</filename> and <filename
92 class="directory">/tools/$LFS_TGT/bin</filename>. The tools in one
93 location are hard linked to the other. An important facet of the linker is
94 its library search order. Detailed information can be obtained from
95 <command>ld</command> by passing it the <parameter>--verbose</parameter>
96 flag. For example, an <userinput>ld --verbose | grep SEARCH</userinput>
97 will illustrate the current search paths and their order. It shows which
98 files are linked by <command>ld</command> by compiling a dummy program and
99 passing the <parameter>--verbose</parameter> switch to the linker. For example,
100 <userinput>gcc dummy.c -Wl,--verbose 2&gt;&amp;1 | grep succeeded</userinput>
101 will show all the files successfully opened during the linking.</para>
103 <para>The next package installed is GCC. An example of what can be
104 seen during its run of <command>configure</command> is:</para>
106<screen><computeroutput>checking what assembler to use... /tools/i686-lfs-linux-gnu/bin/as
107checking what linker to use... /tools/i686-lfs-linux-gnu/bin/ld</computeroutput></screen>
109 <para>This is important for the reasons mentioned above. It also demonstrates
110 that GCC's configure script does not search the PATH directories to find which
111 tools to use. However, during the actual operation of <command>gcc</command>
112 itself, the same search paths are not necessarily used. To find out which
113 standard linker <command>gcc</command> will use, run:
114 <userinput>gcc -print-prog-name=ld</userinput>.</para>
116 <para>Detailed information can be obtained from <command>gcc</command> by
117 passing it the <parameter>-v</parameter> command line option while compiling
118 a dummy program. For example, <userinput>gcc -v dummy.c</userinput> will show
119 detailed information about the preprocessor, compilation, and assembly stages,
120 including <command>gcc</command>'s included search paths and their order.</para>
122 <para>Next installed are sanitized Linux API headers. These allow the standard
123 C library (Glibc) to interface with features that the Linux kernel will
124 provide.</para>
126 <para>The next package installed is Glibc. The most important considerations
127 for building Glibc are the compiler, binary tools, and kernel headers. The
128 compiler is generally not an issue since Glibc will always use the compiler
129 relating to the <parameter>--host</parameter> parameter passed to its
130 configure script, e.g. in our case,
131 <command>i686-lfs-linux-gnu-gcc</command>. The binary tools and kernel
132 headers can be a bit more complicated. Therefore, take no risks and use the
133 available configure switches to enforce the correct selections. After the run
134 of <command>configure</command>, check the contents of the
135 <filename>config.make</filename> file in the <filename
136 class="directory">glibc-build</filename> directory for all important details.
137 Note the use of <parameter>CC="i686-lfs-gnu-gcc"</parameter> to control which
138 binary tools are used and the use of the <parameter>-nostdinc</parameter> and
139 <parameter>-isystem</parameter> flags to control the compiler's include
140 search path. These items highlight an important aspect of the Glibc
141 package&mdash;it is very self-sufficient in terms of its build machinery and
142 generally does not rely on toolchain defaults.</para>
144 <para>During the second pass of Binutils, we are able to utilize the
145 <parameter>--with-lib-path</parameter> configure switch to control
146 <command>ld</command>'s library search path.</para>
148 <para>For the second pass of GCC, its sources also need to be modified to
149 tell GCC to use the new dynamic linker. Failure to do so will result in the
150 GCC programs themselves having the name of the dynamic linker from the host
151 system's <filename class="directory">/lib</filename> directory embedded into
152 them, which would defeat the goal of getting away from the host. From this
153 point onwards, the core toolchain is self-contained and self-hosted. The
154 remainder of the <xref linkend="chapter-temporary-tools"/> packages all build
155 against the new Glibc in <filename
156 class="directory">/tools</filename>.</para>
158 <para>Upon entering the chroot environment in <xref
159 linkend="chapter-building-system"/>, the first major package to be
160 installed is Glibc, due to its self-sufficient nature mentioned above.
161 Once this Glibc is installed into <filename
162 class="directory">/usr</filename>, we will perform a quick changeover of the
163 toolchain defaults, and then proceed in building the rest of the target
164 LFS system.</para>
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