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<title>Debugging with GDB: Index Files</title>
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<a name="Index-Files"></a>
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<div class="header">
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<p>
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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="previous">MiniDebugInfo</a>, Up: <a href="GDB-Files.html#GDB-Files" accesskey="u" rel="up">GDB Files</a> [<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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</div>
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<hr>
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<a name="Index-Files-Speed-Up-GDB"></a>
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<h3 class="section">18.5 Index Files Speed Up <small>GDB</small></h3>
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<a name="index-index-files"></a>
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<a name="index-_002egdb_005findex-section"></a>
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<p>When <small>GDB</small> finds a symbol file, it scans the symbols in the
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file in order to construct an internal symbol table. This lets most
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<small>GDB</small> operations work quickly—at the cost of a delay early
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on. For large programs, this delay can be quite lengthy, so
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<small>GDB</small> provides a way to build an index, which speeds up
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startup.
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</p>
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<p>For convenience, <small>GDB</small> comes with a program,
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<code>gdb-add-index</code>, which can be used to add the index to a
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symbol file. It takes the symbol file as its only argument:
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</p>
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<div class="smallexample">
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<pre class="smallexample">$ gdb-add-index symfile
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</pre></div>
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<p>See <a href="gdb_002dadd_002dindex-man.html#gdb_002dadd_002dindex">gdb-add-index</a>.
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</p>
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<p>It is also possible to do the work manually. Here is what
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<code>gdb-add-index</code> does behind the curtains.
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</p>
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<p>The index is stored as a section in the symbol file. <small>GDB</small> can
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write the index to a file, then you can put it into the symbol file
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using <code>objcopy</code>.
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</p>
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<p>To create an index file, use the <code>save gdb-index</code> command:
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</p>
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<dl compact="compact">
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<dt><code>save gdb-index [-dwarf-5] <var>directory</var></code></dt>
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<dd><a name="index-save-gdb_002dindex"></a>
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<p>Create index files for all symbol files currently known by
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<small>GDB</small>. For each known <var>symbol-file</var>, this command by
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default creates it produces a single file
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<samp><var>symbol-file</var>.gdb-index</samp>. If you invoke this command with
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the <samp>-dwarf-5</samp> option, it produces 2 files:
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<samp><var>symbol-file</var>.debug_names</samp> and
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<samp><var>symbol-file</var>.debug_str</samp>. The files are created in the
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given <var>directory</var>.
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</p></dd>
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</dl>
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<p>Once you have created an index file you can merge it into your symbol
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file, here named <samp>symfile</samp>, using <code>objcopy</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">$ objcopy --add-section .gdb_index=symfile.gdb-index \
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--set-section-flags .gdb_index=readonly symfile symfile
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</pre></div>
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<p>Or for <code>-dwarf-5</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">$ objcopy --dump-section .debug_str=symfile.debug_str.new symfile
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$ cat symfile.debug_str >>symfile.debug_str.new
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$ objcopy --add-section .debug_names=symfile.gdb-index \
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--set-section-flags .debug_names=readonly \
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--update-section .debug_str=symfile.debug_str.new symfile symfile
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</pre></div>
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<p><small>GDB</small> will normally ignore older versions of <samp>.gdb_index</samp>
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sections that have been deprecated. Usually they are deprecated because
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they are missing a new feature or have performance issues.
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To tell <small>GDB</small> to use a deprecated index section anyway
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specify <code>set use-deprecated-index-sections on</code>.
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The default is <code>off</code>.
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This can speed up startup, but may result in some functionality being lost.
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See <a href="Index-Section-Format.html#Index-Section-Format">Index Section Format</a>.
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</p>
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<p><em>Warning:</em> Setting <code>use-deprecated-index-sections</code> to <code>on</code>
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must be done before gdb reads the file. The following will not work:
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</p>
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<div class="smallexample">
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<pre class="smallexample">$ gdb -ex "set use-deprecated-index-sections on" <program>
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</pre></div>
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<p>Instead you must do, for example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">$ gdb -iex "set use-deprecated-index-sections on" <program>
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</pre></div>
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<p>Indices only work when using DWARF debugging information, not stabs.
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</p>
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<a name="Automatic-Symbol-Index-Cache"></a>
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<h4 class="subsection">18.5.1 Automatic Symbol Index Cache</h4>
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<a name="index-automatic-symbol-index-cache"></a>
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<p>It is possible for <small>GDB</small> to automatically save a copy of this index in a
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cache on disk and retrieve it from there when loading the same binary in the
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future. This feature can be turned on with <kbd>set index-cache on</kbd>. The
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following commands can be used to tweak the behavior of the index cache.
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</p>
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<dl compact="compact">
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<dd>
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<a name="index-set-index_002dcache"></a>
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</dd>
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<dt><code>set index-cache on</code></dt>
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<dt><code>set index-cache off</code></dt>
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<dd><p>Enable or disable the use of the symbol index cache.
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</p>
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</dd>
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<dt><code>set index-cache directory <var>directory</var></code></dt>
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<dd><a name="index-show-index_002dcache"></a>
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</dd>
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<dt><code>show index-cache directory</code></dt>
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<dd><p>Set/show the directory where index files will be saved.
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</p>
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<p>The default value for this directory depends on the host platform. On
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most systems, the index is cached in the <samp>gdb</samp> subdirectory of
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the directory pointed to by the <code>XDG_CACHE_HOME</code> environment
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variable, if it is defined, else in the <samp>.cache/gdb</samp> subdirectory
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of your home directory. However, on some systems, the default may
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differ according to local convention.
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</p>
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<p>There is no limit on the disk space used by index cache. It is perfectly safe
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to delete the content of that directory to free up disk space.
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</p>
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</dd>
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<dt><code>show index-cache stats</code></dt>
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<dd><p>Print the number of cache hits and misses since the launch of <small>GDB</small>.
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</p>
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</dd>
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</dl>
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<div class="header">
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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="previous">MiniDebugInfo</a>, Up: <a href="GDB-Files.html#GDB-Files" accesskey="u" rel="up">GDB Files</a> [<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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