lfs-book/chapter06/adjusting.xml

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<sect1 id="ch-system-adjusting">
  <?dbhtml filename="adjusting.html"?>

  <title>Adjusting the Toolchain</title>

  <para>Now that the final C libraries have been installed, it is time to adjust
  the toolchain so that it will link any
  newly compiled program against these new libraries.</para>

  <para>First, backup the <filename class="directory">/tools</filename> linker,
  and replace it with the adjusted linker we made in chapter 5. We'll also create
  a link to its counterpart in
  <filename class="directory">/tools/$(uname -m)-pc-linux-gnu/bin</filename>:</para>

<screen><userinput>mv -v /tools/bin/{ld,ld-old}
mv -v /tools/$(uname -m)-pc-linux-gnu/bin/{ld,ld-old}
mv -v /tools/bin/{ld-new,ld}
ln -sv /tools/bin/ld /tools/$(uname -m)-pc-linux-gnu/bin/ld</userinput></screen>

  <para>the next command amends the GCC specs file to achieve three goals:
  first point GCC to the new dynamic linker. Simply deleting all instances of
  <quote>/tools</quote> should leave us with the correct path to the dynamic
  linker. Second, let GCC know where to find the Glibc start files. Third,
  add the /usr/include directory at the end of the default search path, so
  that header files added in chapter 6 are found.
  A <command>sed</command> command accomplishes this:</para>

<screen><userinput>gcc -dumpspecs | sed -e 's@/tools@@g'                   \
    -e '/\*startfile_prefix_spec:/{n;s@.*@/usr/lib/ @}' \
    -e '/\*cpp:/{n;s@$@ -idirafter /usr/include@}' &gt;      \
    `dirname $(gcc --print-libgcc-file-name)`/specs</userinput></screen>

  <para>It is a good idea to visually inspect the specs file to verify the
  intended change was actually made.</para>

  <para>It is imperative at this point to ensure that the basic
  functions (compiling and linking) of the adjusted toolchain are working
  as expected. To do this, perform the following sanity checks:</para>

<screen os="a"><userinput>echo 'int main(){}' &gt; dummy.c
cc dummy.c -v -Wl,--verbose &amp;&gt; dummy.log
readelf -l a.out | grep ': /lib'</userinput></screen>

  <para os="b">There should be no errors,
  and the output of the last command will be (allowing for
  platform-specific differences in dynamic linker name):</para>

<screen os="c"><computeroutput>[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]</computeroutput></screen>

  <para>Note that on 64-bit systems <filename class="directory">/lib</filename> is
  the location of our dynamic linker, but is accessed via a symbolic link
  in /lib64.</para>

  <note><para>On 32-bit systems the interpreter should be
  /lib/ld-linux.so.2.</para></note>

  <para os="d">Now make sure that we're setup to use the correct start files:</para>

<screen os="e"><userinput>grep -o '/usr/lib.*/crt[1in].*succeeded' dummy.log</userinput></screen>

  <para os="f">The output of the last command should be:</para>

<screen><computeroutput>/usr/lib/../lib/crt1.o succeeded
/usr/lib/../lib/crti.o succeeded
/usr/lib/../lib/crtn.o succeeded</computeroutput></screen>

  <para os="g">Verify that the compiler is searching for the correct header
  files:</para>

<screen><userinput>grep -B4 '^ /usr/include' dummy.log</userinput></screen>

  <para os="h">This command should return the following output:</para>

<screen><computeroutput>#include &lt;...&gt; search starts here:
 /tools/lib/gcc/x86_64-pc-linux-gnu/&gcc-version;/include
 /tools/include
 /tools/lib/gcc/x86_64-pc-linux-gnu/&gcc-version;/include-fixed
 /usr/include</computeroutput></screen>

  <note><para>On a 32 bit system, x86_64 is replaced with i686.</para></note>

  <para os="i">Next, verify that the new linker is being used with the correct search paths:</para>

<screen os="j"><userinput>grep 'SEARCH.*/usr/lib' dummy.log |sed 's|; |\n|g'</userinput></screen>

  <para os="k">References to paths that have components with '-linux-gnu' should
  be ignored, but otherwise the output of the last command should be:</para>

<screen><computeroutput>SEARCH_DIR("/usr/lib")
SEARCH_DIR("/lib")</computeroutput></screen>

  <para os="l">Next make sure that we're using the correct libc:</para>

<screen os="m"><userinput>grep "/lib.*/libc.so.6 " dummy.log</userinput></screen>

  <para os="n">The output of the last command should be:</para>

<screen os="o"><computeroutput>attempt to open /lib/libc.so.6 succeeded</computeroutput></screen>

  <para os="p">Lastly, make sure GCC is using the correct dynamic linker:</para>

<screen os="q"><userinput>grep found dummy.log</userinput></screen>

  <para os="r">The output of the last command should be (allowing for
  platform-specific differences in dynamic linker name):</para>

<screen os="s"><computeroutput>found ld-linux-x86-64.so.2 at /lib/ld-linux-x86-64.so.2</computeroutput></screen>

  <para os="t">If the output does not appear as shown above or is not received
  at all, then something is seriously wrong. Investigate and retrace the
  steps to find out where the problem is and correct it. The most likely
  reason is that something went wrong with the specs file adjustment. Any
  issues will need to be resolved before continuing with the process.</para>

  <para os="u">Once everything is working correctly, clean up the test files:</para>

<screen os="v"><userinput>rm -v dummy.c a.out dummy.log</userinput></screen>

</sect1>