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<a name="Debugging-information-in-a-special-section"></a>
<h3 class="section">18.3 Debugging information in a special section</h3>
<a name="index-separate-debug-sections"></a>
<a name="index-_002egnu_005fdebugdata-section"></a>

<p>Some systems ship pre-built executables and libraries that have a
special &lsquo;<samp>.gnu_debugdata</samp>&rsquo; section.  This feature is called
<em>MiniDebugInfo</em>.  This section holds an LZMA-compressed object and
is used to supply extra symbols for backtraces.
</p>
<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,
<small>GDB</small> does not currently put restrictions on what sort of
debugging information might be included in the section.
</p>
<p><small>GDB</small> 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 <small>GDB</small> was configured with LZMA support.
</p>
<p>This section can be easily created using <code>objcopy</code> and other
standard utilities:
</p>
<div class="smallexample">
<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 &quot;D&quot; symbols, for the benefit
# of platforms like PowerPC64 that use function descriptors.)
nm <var>binary</var> --format=posix --defined-only \
  | awk '{ if ($2 == &quot;T&quot; || $2 == &quot;t&quot; || $2 == &quot;D&quot;) 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></div>

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