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<h3 class="section">18.4 Debugging information in a special section</h3>

<p><a name="index-separate-debug-sections-1290"></a><a name="index-g_t_0040samp_007b_002egnu_005fdebugdata_007d-section-1291"></a>
Some systems ship pre-built executables and libraries that have a
special &lsquo;<samp><span class="samp">.gnu_debugdata</span></samp>&rsquo; section.  This feature is called
<dfn>MiniDebugInfo</dfn>.  This section holds an LZMA-compressed object and
is used to supply extra symbols for backtraces.

   <p>The intent of this section is to provide extra minimal debugging
information for use in simple backtraces.  It is not intended to be a
replacement for full separate debugging information (see <a href="Separate-Debug-Files.html#Separate-Debug-Files">Separate Debug Files</a>).  The example below shows the intended use; however,
<span class="sc">gdb</span> does not currently put restrictions on what sort of
debugging information might be included in the section.

   <p><span class="sc">gdb</span> has support for this extension.  If the section exists,
then it is used provided that no other source of debugging information
can be found, and that <span class="sc">gdb</span> was configured with LZMA support.

   <p>This section can be easily created using <samp><span class="command">objcopy</span></samp> and other
standard utilities:

<pre class="smallexample">     # Extract the dynamic symbols from the main binary, there is no need
     # to also have these in the normal symbol table.
     nm -D <var>binary</var> --format=posix --defined-only \
       | awk '{ print $1 }' | sort &gt; dynsyms
     
     # Extract all the text (i.e. function) symbols from the debuginfo.
     # (Note that we actually also accept "D" symbols, for the benefit
     # of platforms like PowerPC64 that use function descriptors.)
     nm <var>binary</var> --format=posix --defined-only \
       | awk '{ if ($2 == "T" || $2 == "t" || $2 == "D") print $1 }' \
       | sort &gt; funcsyms
     
     # Keep all the function symbols not already in the dynamic symbol
     # table.
     comm -13 dynsyms funcsyms &gt; keep_symbols
     
     # Separate full debug info into debug binary.
     objcopy --only-keep-debug <var>binary</var> debug
     
     # Copy the full debuginfo, keeping only a minimal set of symbols and
     # removing some unnecessary sections.
     objcopy -S --remove-section .gdb_index --remove-section .comment \
       --keep-symbols=keep_symbols debug mini_debuginfo
     
     # Drop the full debug info from the original binary.
     strip --strip-all -R .comment <var>binary</var>
     
     # Inject the compressed data into the .gnu_debugdata section of the
     # original binary.
     xz mini_debuginfo
     objcopy --add-section .gnu_debugdata=mini_debuginfo.xz <var>binary</var>
</pre>
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