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| <h3 class="section">18.5 Index Files Speed Up <span class="sc">gdb</span></h3> |
| |
| <p><a name="index-index-files-1292"></a><a name="index-g_t_0040samp_007b_002egdb_005findex_007d-section-1293"></a> |
| When <span class="sc">gdb</span> finds a symbol file, it scans the symbols in the |
| file in order to construct an internal symbol table. This lets most |
| <span class="sc">gdb</span> operations work quickly—at the cost of a delay early |
| on. For large programs, this delay can be quite lengthy, so |
| <span class="sc">gdb</span> provides a way to build an index, which speeds up |
| startup. |
| |
| <p>For convenience, <span class="sc">gdb</span> comes with a program, |
| <samp><span class="command">gdb-add-index</span></samp>, which can be used to add the index to a |
| symbol file. It takes the symbol file as its only argument: |
| |
| <pre class="smallexample"> $ gdb-add-index symfile |
| </pre> |
| <p>See <a href="gdb_002dadd_002dindex.html#gdb_002dadd_002dindex">gdb-add-index</a>. |
| |
| <p>It is also possible to do the work manually. Here is what |
| <samp><span class="command">gdb-add-index</span></samp> does behind the curtains. |
| |
| <p>The index is stored as a section in the symbol file. <span class="sc">gdb</span> can |
| write the index to a file, then you can put it into the symbol file |
| using <samp><span class="command">objcopy</span></samp>. |
| |
| <p>To create an index file, use the <code>save gdb-index</code> command: |
| |
| <dl> |
| <dt><code>save gdb-index [-dwarf-5] </code><var>directory</var><dd><a name="index-save-gdb_002dindex-1294"></a>Create index files for all symbol files currently known by |
| <span class="sc">gdb</span>. For each known <var>symbol-file</var>, this command by |
| default creates it produces a single file |
| <samp><var>symbol-file</var><span class="file">.gdb-index</span></samp>. If you invoke this command with |
| the <samp><span class="option">-dwarf-5</span></samp> option, it produces 2 files: |
| <samp><var>symbol-file</var><span class="file">.debug_names</span></samp> and |
| <samp><var>symbol-file</var><span class="file">.debug_str</span></samp>. The files are created in the |
| given <var>directory</var>. |
| </dl> |
| |
| <p>Once you have created an index file you can merge it into your symbol |
| file, here named <samp><span class="file">symfile</span></samp>, using <samp><span class="command">objcopy</span></samp>: |
| |
| <pre class="smallexample"> $ objcopy --add-section .gdb_index=symfile.gdb-index \ |
| --set-section-flags .gdb_index=readonly symfile symfile |
| </pre> |
| <p>Or for <code>-dwarf-5</code>: |
| |
| <pre class="smallexample"> $ objcopy --dump-section .debug_str=symfile.debug_str.new symfile |
| $ cat symfile.debug_str >>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> |
| <p><span class="sc">gdb</span> will normally ignore older versions of <samp><span class="file">.gdb_index</span></samp> |
| sections that have been deprecated. Usually they are deprecated because |
| they are missing a new feature or have performance issues. |
| To tell <span class="sc">gdb</span> 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><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: |
| |
| <pre class="smallexample"> $ gdb -ex "set use-deprecated-index-sections on" <program> |
| </pre> |
| <p>Instead you must do, for example, |
| |
| <pre class="smallexample"> $ gdb -iex "set use-deprecated-index-sections on" <program> |
| </pre> |
| <p>There are currently some limitation on indices. They only work when |
| for DWARF debugging information, not stabs. And, they do not |
| currently work for programs using Ada. |
| |
| <h4 class="subsection">18.5.1 Automatic symbol index cache</h4> |
| |
| <p>It is possible for <span class="sc">gdb</span> 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. |
| |
| <dl> |
| <dt><code>set index-cache on</code><dt><code>set index-cache off</code><dd>Enable or disable the use of the symbol index cache. |
| |
| <br><dt><code>set index-cache directory </code><var>directory</var><dt><code>show index-cache directory</code><dd>Set/show the directory where index files will be saved. |
| |
| <p>The default value for this directory depends on the host platform. On |
| most systems, the index is cached in the <samp><span class="file">gdb</span></samp> subdirectory of |
| the directory pointed to by the <samp><span class="env">XDG_CACHE_HOME</span></samp> environment |
| variable, if it is defined, else in the <samp><span class="file">.cache/gdb</span></samp> subdirectory |
| of your home directory. However, on some systems, the default may |
| differ according to local convention. |
| |
| <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. |
| |
| <br><dt><code>show index-cache stats</code><dd>Print the number of cache hits and misses since the launch of <span class="sc">gdb</span>. |
| |
| </dl> |
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