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>Next, amend the GCC specs file so that it points to the new
  dynamic linker. Simply deleting all instances of <quote>/tools</quote> should
  leave us with the correct path to the dynamic linker. Also adjust the specs file
  so that GCC knows where to find the correct headers and Glibc start files.
  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@$@ -isystem /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 -B1 '^ /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:
 /usr/include</computeroutput></screen>

  <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>