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<a name="Index-Files"></a>
<div class="header">
<p>
Next: <a href="Symbol-Errors.html#Symbol-Errors" accesskey="n" rel="next">Symbol Errors</a>, Previous: <a href="MiniDebugInfo.html#MiniDebugInfo" accesskey="p" rel="prev">MiniDebugInfo</a>, Up: <a href="GDB-Files.html#GDB-Files" accesskey="u" rel="up">GDB Files</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Index-Files-Speed-Up-GDB"></a>
<h3 class="section">18.5 Index Files Speed Up <small>GDB</small></h3>
<a name="index-index-files"></a>
<a name="index-_002egdb_005findex-section"></a>

<p>When <small>GDB</small> finds a symbol file, it scans the symbols in the
file in order to construct an internal symbol table.  This lets most
<small>GDB</small> operations work quickly&mdash;at the cost of a delay early
on.  For large programs, this delay can be quite lengthy, so
<small>GDB</small> provides a way to build an index, which speeds up
startup.
</p>
<p>For convenience, <small>GDB</small> comes with a program,
<code>gdb-add-index</code>, which can be used to add the index to a
symbol file.  It takes the symbol file as its only argument:
</p>
<div class="smallexample">
<pre class="smallexample">$ gdb-add-index symfile
</pre></div>

<p>See <a href="gdb_002dadd_002dindex-man.html#gdb_002dadd_002dindex">gdb-add-index</a>.
</p>
<p>It is also possible to do the work manually.  Here is what
<code>gdb-add-index</code> does behind the curtains.
</p>
<p>The index is stored as a section in the symbol file.  <small>GDB</small> can
write the index to a file, then you can put it into the symbol file
using <code>objcopy</code>.
</p>
<p>To create an index file, use the <code>save gdb-index</code> command:
</p>
<dl compact="compact">
<dt><code>save gdb-index [-dwarf-5] <var>directory</var></code></dt>
<dd><a name="index-save-gdb_002dindex"></a>
<p>Create index files for all symbol files currently known by
<small>GDB</small>.  For each known <var>symbol-file</var>, this command by
default creates it produces a single file
<samp><var>symbol-file</var>.gdb-index</samp>.  If you invoke this command with
the <samp>-dwarf-5</samp> option, it produces 2 files:
<samp><var>symbol-file</var>.debug_names</samp> and
<samp><var>symbol-file</var>.debug_str</samp>.  The files are created in the
given <var>directory</var>.
</p></dd>
</dl>

<p>Once you have created an index file you can merge it into your symbol
file, here named <samp>symfile</samp>, using <code>objcopy</code>:
</p>
<div class="smallexample">
<pre class="smallexample">$ objcopy --add-section .gdb_index=symfile.gdb-index \
    --set-section-flags .gdb_index=readonly symfile symfile
</pre></div>

<p>Or for <code>-dwarf-5</code>:
</p>
<div class="smallexample">
<pre class="smallexample">$ objcopy --dump-section .debug_str=symfile.debug_str.new symfile
$ cat symfile.debug_str &gt;&gt;symfile.debug_str.new
$ objcopy --add-section .debug_names=symfile.gdb-index \
    --set-section-flags .debug_names=readonly \
    --update-section .debug_str=symfile.debug_str.new symfile symfile
</pre></div>

<p><small>GDB</small> will normally ignore older versions of <samp>.gdb_index</samp>
sections that have been deprecated.  Usually they are deprecated because
they are missing a new feature or have performance issues.
To tell <small>GDB</small> to use a deprecated index section anyway
specify <code>set use-deprecated-index-sections on</code>.
The default is <code>off</code>.
This can speed up startup, but may result in some functionality being lost.
See <a href="Index-Section-Format.html#Index-Section-Format">Index Section Format</a>.
</p>
<p><em>Warning:</em> Setting <code>use-deprecated-index-sections</code> to <code>on</code>
must be done before gdb reads the file.  The following will not work:
</p>
<div class="smallexample">
<pre class="smallexample">$ gdb -ex &quot;set use-deprecated-index-sections on&quot; &lt;program&gt;
</pre></div>

<p>Instead you must do, for example,
</p>
<div class="smallexample">
<pre class="smallexample">$ gdb -iex &quot;set use-deprecated-index-sections on&quot; &lt;program&gt;
</pre></div>

<p>Indices only work when using DWARF debugging information, not stabs.
</p>
<a name="Automatic-symbol-index-cache"></a>
<h4 class="subsection">18.5.1 Automatic symbol index cache</h4>

<a name="index-automatic-symbol-index-cache"></a>
<p>It is possible for <small>GDB</small> to automatically save a copy of this index in a
cache on disk and retrieve it from there when loading the same binary in the
future.  This feature can be turned on with <kbd>set index-cache on</kbd>.  The
following commands can be used to tweak the behavior of the index cache.
</p>
<dl compact="compact">
<dd>
<a name="index-set-index_002dcache"></a>
</dd>
<dt><code>set index-cache on</code></dt>
<dt><code>set index-cache off</code></dt>
<dd><p>Enable or disable the use of the symbol index cache.
</p>
</dd>
<dt><code>set index-cache directory <var>directory</var></code></dt>
<dd><a name="index-show-index_002dcache"></a>
</dd>
<dt><code>show index-cache directory</code></dt>
<dd><p>Set/show the directory where index files will be saved.
</p>
<p>The default value for this directory depends on the host platform.  On
most systems, the index is cached in the <samp>gdb</samp> subdirectory of
the directory pointed to by the <code>XDG_CACHE_HOME</code> environment
variable, if it is defined, else in the <samp>.cache/gdb</samp> subdirectory
of your home directory.  However, on some systems, the default may
differ according to local convention.
</p>
<p>There is no limit on the disk space used by index cache.  It is perfectly safe
to delete the content of that directory to free up disk space.
</p>
</dd>
<dt><code>show index-cache stats</code></dt>
<dd><p>Print the number of cache hits and misses since the launch of <small>GDB</small>.
</p>
</dd>
</dl>

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Next: <a href="Symbol-Errors.html#Symbol-Errors" accesskey="n" rel="next">Symbol Errors</a>, Previous: <a href="MiniDebugInfo.html#MiniDebugInfo" accesskey="p" rel="prev">MiniDebugInfo</a>, Up: <a href="GDB-Files.html#GDB-Files" accesskey="u" rel="up">GDB Files</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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