1 | <?xml version="1.0" encoding="ISO-8859-1"?>
|
---|
2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd" [
|
---|
3 | <!ENTITY % general-entities SYSTEM "../general.ent">
|
---|
4 | %general-entities;
|
---|
5 | ]>
|
---|
6 | <sect1 id="ch-tools-toolchaintechnotes">
|
---|
7 | <title>Toolchain Technical Notes</title>
|
---|
8 | <?dbhtml filename="toolchaintechnotes.html"?>
|
---|
9 |
|
---|
10 | <para>This section explains some of the rationale and technical
|
---|
11 | details behind the overall build method. It is not essential to
|
---|
12 | immediately understand everything in this section. Most of this
|
---|
13 | information will be clearer after performing an actual build. This
|
---|
14 | section can be referred back to at any time during the process.</para>
|
---|
15 |
|
---|
16 | <para>The overall goal of <xref linkend="chapter-temporary-tools"/> is to
|
---|
17 | provide a temporary environment that can be chrooted into and from which can be
|
---|
18 | produced a clean, trouble-free build of the target LFS system in <xref
|
---|
19 | linkend="chapter-building-system"/>. Along the way, we separate the new system
|
---|
20 | from the host system as much as possible, and in doing so, build a
|
---|
21 | self-contained and self-hosted toolchain. It should be noted that the build
|
---|
22 | process has been designed to minimize the risks for new readers and provide
|
---|
23 | maximum educational value at the same time.</para>
|
---|
24 |
|
---|
25 | <important>
|
---|
26 | <para>Before continuing, be aware of the name of the working platform,
|
---|
27 | often referred to as the target triplet. Many times, the target
|
---|
28 | triplet will probably be <emphasis>i686-pc-linux-gnu</emphasis>. A
|
---|
29 | simple way to determine the name of the target triplet is to run the
|
---|
30 | <command>config.guess</command> script that comes with the source for
|
---|
31 | many packages. Unpack the Binutils sources and run the script:
|
---|
32 | <userinput>./config.guess</userinput> and note the output.</para>
|
---|
33 |
|
---|
34 | <para>Also be aware of the name of the platform's dynamic linker,
|
---|
35 | often referred to as the dynamic loader (not to be confused with the
|
---|
36 | standard linker <command>ld</command> that is part of Binutils). The
|
---|
37 | dynamic linker provided by Glibc finds and loads the shared libraries
|
---|
38 | needed by a program, prepares the program to run, and then runs it.
|
---|
39 | The name of the dynamic linker will usually be
|
---|
40 | <filename class="libraryfile">ld-linux.so.2</filename>. On platforms that are less
|
---|
41 | prevalent, the name might be <filename class="libraryfile">ld.so.1</filename>,
|
---|
42 | and newer 64 bit platforms might be named something else entirely. The name of
|
---|
43 | the platform's dynamic linker can be determined by looking in the
|
---|
44 | <filename class="directory">/lib</filename> directory on the host
|
---|
45 | system. A sure-fire way to determine the name is to inspect a random
|
---|
46 | binary from the host system by running: <userinput>readelf -l <name
|
---|
47 | of binary> | grep interpreter</userinput> and noting the output.
|
---|
48 | The authoritative reference covering all platforms is in the
|
---|
49 | <filename>shlib-versions</filename> file in the root of the Glibc
|
---|
50 | source tree.</para>
|
---|
51 | </important>
|
---|
52 |
|
---|
53 | <para>Some key technical points of how the <xref linkend="chapter-temporary-tools"/> build
|
---|
54 | method works:</para>
|
---|
55 |
|
---|
56 | <itemizedlist>
|
---|
57 | <listitem><para>The process is similar in principle to
|
---|
58 | cross-compiling, whereby tools installed in the same prefix work in
|
---|
59 | cooperation, and thus utilize a little GNU
|
---|
60 | <quote>magic</quote></para></listitem>
|
---|
61 |
|
---|
62 | <listitem><para>Careful manipulation of the standard linker's library
|
---|
63 | search path ensures programs are linked only against chosen
|
---|
64 | libraries</para></listitem>
|
---|
65 |
|
---|
66 | <listitem><para>Careful manipulation of <command>gcc</command>'s
|
---|
67 | <filename>specs</filename> file tells the compiler which target dynamic
|
---|
68 | linker will be used</para></listitem>
|
---|
69 | </itemizedlist>
|
---|
70 |
|
---|
71 | <para>Binutils is installed first because the
|
---|
72 | <command>./configure</command> runs of both GCC and Glibc perform
|
---|
73 | various feature tests on the assembler and linker to determine which
|
---|
74 | software features to enable or disable. This is more important than
|
---|
75 | one might first realize. An incorrectly configured GCC or Glibc can
|
---|
76 | result in a subtly broken toolchain, where the impact of such breakage
|
---|
77 | might not show up until near the end of the build of an entire
|
---|
78 | distribution. A test suite failure will usually highlight this error
|
---|
79 | before too much additional work is performed.</para>
|
---|
80 |
|
---|
81 | <beginpage/>
|
---|
82 |
|
---|
83 | <para>Binutils installs its assembler and linker in two locations,
|
---|
84 | <filename class="directory">/tools/bin</filename> and <filename
|
---|
85 | class="directory">/tools/$TARGET_TRIPLET/bin</filename>. The tools in
|
---|
86 | one location are hard linked to the other. An important facet of the
|
---|
87 | linker is its library search order. Detailed information can be
|
---|
88 | obtained from <command>ld</command> by passing it the
|
---|
89 | <parameter>--verbose</parameter> flag. For example, an <userinput>ld
|
---|
90 | --verbose | grep SEARCH</userinput> will illustrate the current search
|
---|
91 | paths and their order. It shows which files are linked by
|
---|
92 | <command>ld</command> by compiling a dummy program and passing the
|
---|
93 | <parameter>--verbose</parameter> switch to the linker. For example,
|
---|
94 | <userinput>gcc dummy.c -Wl,--verbose 2>&1 | grep
|
---|
95 | succeeded</userinput> will show all the files successfully opened
|
---|
96 | during the linking.</para>
|
---|
97 |
|
---|
98 | <para>The next package installed is GCC. An example of what can be
|
---|
99 | seen during its run of <command>./configure</command> is:</para>
|
---|
100 |
|
---|
101 | <screen><computeroutput>checking what assembler to use...
|
---|
102 | /tools/i686-pc-linux-gnu/bin/as
|
---|
103 | checking what linker to use... /tools/i686-pc-linux-gnu/bin/ld</computeroutput></screen>
|
---|
104 |
|
---|
105 | <para>This is important for the reasons mentioned above. It also
|
---|
106 | demonstrates that GCC's configure script does not search the PATH
|
---|
107 | directories to find which tools to use. However, during the actual
|
---|
108 | operation of <command>gcc</command> itself, the same
|
---|
109 | search paths are not necessarily used. To find out which standard
|
---|
110 | linker <command>gcc</command> will use, run: <userinput>gcc
|
---|
111 | -print-prog-name=ld</userinput>.</para>
|
---|
112 |
|
---|
113 | <para>Detailed information can be obtained from <command>gcc</command>
|
---|
114 | by passing it the <parameter>-v</parameter> command line option while
|
---|
115 | compiling a dummy program. For example, <userinput>gcc -v
|
---|
116 | dummy.c</userinput> will show detailed information about the
|
---|
117 | preprocessor, compilation, and assembly stages, including
|
---|
118 | <command>gcc</command>'s included search paths and their order.</para>
|
---|
119 |
|
---|
120 | <para>The next package installed is Glibc. The most important
|
---|
121 | considerations for building Glibc are the compiler, binary tools, and
|
---|
122 | kernel headers. The compiler is generally not an issue since Glibc
|
---|
123 | will always use the <command>gcc</command> found in a
|
---|
124 | <envar>PATH</envar> directory.
|
---|
125 | The binary tools and kernel headers can be a bit more complicated.
|
---|
126 | Therefore, take no risks and use the available configure switches to
|
---|
127 | enforce the correct selections. After the run of
|
---|
128 | <command>./configure</command>, check the contents of the
|
---|
129 | <filename>config.make</filename> file in the <filename
|
---|
130 | class="directory">glibc-build</filename> directory for all important
|
---|
131 | details. Note the use of <parameter>CC="gcc -B/tools/bin/"</parameter>
|
---|
132 | to control which binary tools are used and the use of the
|
---|
133 | <parameter>-nostdinc</parameter> and <parameter>-isystem</parameter>
|
---|
134 | flags to control the compiler's include search path. These items
|
---|
135 | highlight an important aspect of the Glibc package—it is very
|
---|
136 | self-sufficient in terms of its build machinery and generally does not
|
---|
137 | rely on toolchain defaults.</para>
|
---|
138 |
|
---|
139 | <para>After the Glibc installation, make some adjustments to ensure
|
---|
140 | that searching and linking take place only within the <filename
|
---|
141 | class="directory">/tools</filename> prefix. Install an adjusted
|
---|
142 | <command>ld</command>, which has a hard-wired search path limited to
|
---|
143 | <filename class="directory">/tools/lib</filename>. Then amend
|
---|
144 | <command>gcc</command>'s specs file to point to the new dynamic linker
|
---|
145 | in <filename class="directory">/tools/lib</filename>. This last step
|
---|
146 | is vital to the whole process. As mentioned above, a hard-wired path
|
---|
147 | to a dynamic linker is embedded into every Executable and Link Format
|
---|
148 | (ELF)-shared executable. This can be inspected by running:
|
---|
149 | <userinput>readelf -l <name of binary> | grep
|
---|
150 | interpreter</userinput>. Amending gcc's specs file
|
---|
151 | ensures that every program compiled from here through the end of this
|
---|
152 | chapter will use the new dynamic linker in <filename
|
---|
153 | class="directory">/tools/lib</filename>.</para>
|
---|
154 |
|
---|
155 | <para>The need to use the new dynamic linker is also the reason why
|
---|
156 | the Specs patch is applied for the second pass of GCC. Failure to do
|
---|
157 | so will result in the GCC programs themselves having the name of the
|
---|
158 | dynamic linker from the host system's <filename
|
---|
159 | class="directory">/lib</filename> directory embedded into them, which
|
---|
160 | would defeat the goal of getting away from the host.</para>
|
---|
161 |
|
---|
162 | <para>During the second pass of Binutils, we are able to utilize the
|
---|
163 | <parameter>--with-lib-path</parameter> configure switch to control
|
---|
164 | <command>ld</command>'s library search path. From this point onwards,
|
---|
165 | the core toolchain is self-contained and self-hosted. The remainder of
|
---|
166 | the <xref linkend="chapter-temporary-tools"/> packages all build
|
---|
167 | against the new Glibc in <filename
|
---|
168 | class="directory">/tools</filename>.</para>
|
---|
169 |
|
---|
170 | <beginpage/>
|
---|
171 |
|
---|
172 | <para>Upon entering the chroot environment in <xref
|
---|
173 | linkend="chapter-building-system"/>, the first major package to be
|
---|
174 | installed is Glibc, due to its self-sufficient nature mentioned above.
|
---|
175 | Once this Glibc is installed into <filename
|
---|
176 | class="directory">/usr</filename>, perform a quick changeover of the
|
---|
177 | toolchain defaults, then proceed in building the rest of the target
|
---|
178 | LFS system.</para>
|
---|
179 |
|
---|
180 | <!-- Removed as part of the fix for bug 1061 - we no longer build pass1
|
---|
181 | packages statically, therefore this explanation isn't required -->
|
---|
182 |
|
---|
183 | <!--<sect2>
|
---|
184 | <title>Notes on Static Linking</title>
|
---|
185 |
|
---|
186 | <para>Besides their specific task, most programs have to perform many
|
---|
187 | common and sometimes trivial operations. These include allocating
|
---|
188 | memory, searching directories, reading and writing files, string
|
---|
189 | handling, pattern matching, arithmetic, and other tasks. Instead of
|
---|
190 | obliging each program to reinvent the wheel, the GNU system provides
|
---|
191 | all these basic functions in ready-made libraries. The major library
|
---|
192 | on any Linux system is Glibc.</para>
|
---|
193 |
|
---|
194 | <para>There are two primary ways of linking the functions from a
|
---|
195 | library to a program that uses them—statically or dynamically. When
|
---|
196 | a program is linked statically, the code of the used functions is
|
---|
197 | included in the executable, resulting in a rather bulky program. When
|
---|
198 | a program is dynamically linked, it includes a reference to the
|
---|
199 | dynamic linker, the name of the library, and the name of the function,
|
---|
200 | resulting in a much smaller executable. A third option is to use the
|
---|
201 | programming interface of the dynamic linker (see <filename>dlopen(3)</filename>
|
---|
202 | for more information).</para>
|
---|
203 |
|
---|
204 | <para>Dynamic linking is the default on Linux and has three major
|
---|
205 | advantages over static linking. First, only one copy of the executable
|
---|
206 | library code is needed on the hard disk, instead of having multiple
|
---|
207 | copies of the same code included in several programs, thus saving
|
---|
208 | disk space. Second, when several programs use the same library
|
---|
209 | function at the same time, only one copy of the function's code is
|
---|
210 | required in core, thus saving memory space. Third, when a library
|
---|
211 | function gets a bug fixed or is otherwise improved, only the one
|
---|
212 | library needs to be recompiled instead of recompiling all programs
|
---|
213 | that make use of the improved function.</para>
|
---|
214 |
|
---|
215 | <para>If dynamic linking has several advantages, why then do we
|
---|
216 | statically link the first two packages in this chapter? The reasons
|
---|
217 | are threefold—historical, educational, and technical. The
|
---|
218 | historical reason is that earlier versions of LFS statically linked
|
---|
219 | every program in this chapter. Educationally, knowing the difference
|
---|
220 | between static and dynamic linking is useful. The technical benefit is
|
---|
221 | a gained element of independence from the host, meaning that those
|
---|
222 | programs can be used independently of the host system. However, it is
|
---|
223 | worth noting that an overall successful LFS build can still be
|
---|
224 | achieved when the first two packages are built dynamically.</para>
|
---|
225 |
|
---|
226 | </sect2>-->
|
---|
227 |
|
---|
228 | </sect1>
|
---|
229 |
|
---|