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  114. <li><a class="reference internal" href="#use-with-gdb-commands">Use with GDB commands</a></li>

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  206.   <section id="debugging-c-api-extensions-and-cpython-internals-with-gdb">

  207. <span id="gdb"></span><h1>Debugging C API extensions and CPython Internals with GDB<a class="headerlink" href="#debugging-c-api-extensions-and-cpython-internals-with-gdb" title="Link to this heading">¶</a></h1>

  208. <p>This document explains how the Python GDB extension, <code class="docutils literal notranslate"><span class="pre">python-gdb.py</span></code>, can

  209. be used with the GDB debugger to debug CPython extensions and the

  210. CPython interpreter itself.</p>

  211. <p>When debugging low-level problems such as crashes or deadlocks, a low-level

  212. debugger, such as GDB, is useful to diagnose and correct the issue.

  213. By default, GDB (or any of its front-ends) doesn’t support high-level

  214. information specific to the CPython interpreter.</p>

  215. <p>The <code class="docutils literal notranslate"><span class="pre">python-gdb.py</span></code> extension adds CPython interpreter information to GDB.

  216. The extension helps introspect the stack of currently executing Python functions.

  217. Given a Python object represented by a <span class="c-expr sig sig-inline c"><a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><span class="n">PyObject</span></a><span class="p">*</span></span> pointer,

  218. the extension surfaces the type and value of the object.</p>

  219. <p>Developers who are working on CPython extensions or tinkering with parts

  220. of CPython that are written in C can use this document to learn how to use the

  221. <code class="docutils literal notranslate"><span class="pre">python-gdb.py</span></code> extension with GDB.</p>

  222. <div class="admonition note">

  223. <p class="admonition-title">Note</p>

  224. <p>This document assumes that you are familiar with the basics of GDB and the

  225. CPython C API. It consolidates guidance from the

  226. <a class="reference external" href="https://devguide.python.org">devguide</a>  and the

  227. <a class="reference external" href="https://wiki.python.org/moin/DebuggingWithGdb">Python wiki</a>.</p>

  228. </div>

  229. <section id="prerequisites">

  230. <h2>Prerequisites<a class="headerlink" href="#prerequisites" title="Link to this heading">¶</a></h2>

  231. <p>You need to have:</p>

  232. <ul class="simple">

  233. <li><p>GDB 7 or later. (For earlier versions of GDB, see <code class="docutils literal notranslate"><span class="pre">Misc/gdbinit</span></code> in the

  234. sources of Python 3.11 or earlier.)</p></li>

  235. <li><p>GDB-compatible debugging information for Python and any extension you are

  236. debugging.</p></li>

  237. <li><p>The <code class="docutils literal notranslate"><span class="pre">python-gdb.py</span></code> extension.</p></li>

  238. </ul>

  239. <p>The extension is built with Python, but might be distributed separately or

  240. not at all. Below, we include tips for a few common systems as examples.

  241. Note that even if the instructions match your system, they might be outdated.</p>

  242. <section id="setup-with-python-built-from-source">

  243. <h3>Setup with Python built from source<a class="headerlink" href="#setup-with-python-built-from-source" title="Link to this heading">¶</a></h3>

  244. <p>When you build CPython from source, debugging information should be available,

  245. and the build should add a <code class="docutils literal notranslate"><span class="pre">python-gdb.py</span></code> file to the root directory of

  246. your repository.</p>

  247. <p>To activate support, you must add the directory containing <code class="docutils literal notranslate"><span class="pre">python-gdb.py</span></code>

  248. to GDB’s “auto-load-safe-path”.

  249. If you haven’t done this, recent versions of GDB will print out a warning

  250. with instructions on how to do this.</p>

  251. <div class="admonition note">

  252. <p class="admonition-title">Note</p>

  253. <p>If you do not see instructions for your version of GDB, put this in your

  254. configuration file (<code class="docutils literal notranslate"><span class="pre">~/.gdbinit</span></code> or <code class="docutils literal notranslate"><span class="pre">~/.config/gdb/gdbinit</span></code>):</p>

  255. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>add-auto-load-safe-path /path/to/cpython

  256. </pre></div>

  257. </div>

  258. <p>You can also add multiple paths, separated by <code class="docutils literal notranslate"><span class="pre">:</span></code>.</p>

  259. </div>

  260. </section>

  261. <section id="setup-for-python-from-a-linux-distro">

  262. <h3>Setup for Python from a Linux distro<a class="headerlink" href="#setup-for-python-from-a-linux-distro" title="Link to this heading">¶</a></h3>

  263. <p>Most Linux systems provide debug information for the system Python

  264. in a package called <code class="docutils literal notranslate"><span class="pre">python-debuginfo</span></code>, <code class="docutils literal notranslate"><span class="pre">python-dbg</span></code> or similar.

  265. For example:</p>

  266. <ul>

  267. <li><p>Fedora:</p>

  268. <blockquote>

  269. <div><div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>sudo<span class="w"> </span>dnf<span class="w"> </span>install<span class="w"> </span>gdb

  270. sudo<span class="w"> </span>dnf<span class="w"> </span>debuginfo-install<span class="w"> </span>python3

  271. </pre></div>

  272. </div>

  273. </div></blockquote>

  274. </li>

  275. <li><p>Ubuntu:</p>

  276. <blockquote>

  277. <div><div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>sudo<span class="w"> </span>apt<span class="w"> </span>install<span class="w"> </span>gdb<span class="w"> </span>python3-dbg

  278. </pre></div>

  279. </div>

  280. </div></blockquote>

  281. </li>

  282. </ul>

  283. <p>On several recent Linux systems, GDB can download debugging symbols

  284. automatically using <em>debuginfod</em>.

  285. However, this will not install the <code class="docutils literal notranslate"><span class="pre">python-gdb.py</span></code> extension;

  286. you generally do need to install the debug info package separately.</p>

  287. </section>

  288. </section>

  289. <section id="using-the-debug-build-and-development-mode">

  290. <h2>Using the Debug build and Development mode<a class="headerlink" href="#using-the-debug-build-and-development-mode" title="Link to this heading">¶</a></h2>

  291. <p>For easier debugging, you might want to:</p>

  292. <ul class="simple">

  293. <li><p>Use a <a class="reference internal" href="../using/configure.html#debug-build"><span class="std std-ref">debug build</span></a> of Python. (When building from source,

  294. use <code class="docutils literal notranslate"><span class="pre">configure</span> <span class="pre">--with-pydebug</span></code>. On Linux distros, install and run a package

  295. like <code class="docutils literal notranslate"><span class="pre">python-debug</span></code> or <code class="docutils literal notranslate"><span class="pre">python-dbg</span></code>, if available.)</p></li>

  296. <li><p>Use the runtime <a class="reference internal" href="../library/devmode.html#devmode"><span class="std std-ref">development mode</span></a> (<code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">dev</span></code>).</p></li>

  297. </ul>

  298. <p>Both enable extra assertions and disable some optimizations.

  299. Sometimes this hides the bug you are trying to find, but in most cases they

  300. make the process easier.</p>

  301. </section>

  302. <section id="using-the-python-gdb-extension">

  303. <h2>Using the <code class="docutils literal notranslate"><span class="pre">python-gdb</span></code> extension<a class="headerlink" href="#using-the-python-gdb-extension" title="Link to this heading">¶</a></h2>

  304. <p>When the extension is loaded, it provides two main features:

  305. pretty printers for Python values, and additional commands.</p>

  306. <section id="pretty-printers">

  307. <h3>Pretty-printers<a class="headerlink" href="#pretty-printers" title="Link to this heading">¶</a></h3>

  308. <p>This is what a GDB backtrace looks like (truncated) when this extension is

  309. enabled:</p>

  310. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>#0  0x000000000041a6b1 in PyObject_Malloc (nbytes=Cannot access memory at address 0x7fffff7fefe8

  311. ) at Objects/obmalloc.c:748

  312. #1  0x000000000041b7c0 in _PyObject_DebugMallocApi (id=111 &#39;o&#39;, nbytes=24) at Objects/obmalloc.c:1445

  313. #2  0x000000000041b717 in _PyObject_DebugMalloc (nbytes=24) at Objects/obmalloc.c:1412

  314. #3  0x000000000044060a in _PyUnicode_New (length=11) at Objects/unicodeobject.c:346

  315. #4  0x00000000004466aa in PyUnicodeUCS2_DecodeUTF8Stateful (s=0x5c2b8d &quot;__lltrace__&quot;, size=11, errors=0x0, consumed=

  316.     0x0) at Objects/unicodeobject.c:2531

  317. #5  0x0000000000446647 in PyUnicodeUCS2_DecodeUTF8 (s=0x5c2b8d &quot;__lltrace__&quot;, size=11, errors=0x0)

  318.     at Objects/unicodeobject.c:2495

  319. #6  0x0000000000440d1b in PyUnicodeUCS2_FromStringAndSize (u=0x5c2b8d &quot;__lltrace__&quot;, size=11)

  320.     at Objects/unicodeobject.c:551

  321. #7  0x0000000000440d94 in PyUnicodeUCS2_FromString (u=0x5c2b8d &quot;__lltrace__&quot;) at Objects/unicodeobject.c:569

  322. #8  0x0000000000584abd in PyDict_GetItemString (v=

  323.     {&#39;Yuck&#39;: &lt;type at remote 0xad4730&gt;, &#39;__builtins__&#39;: &lt;module at remote 0x7ffff7fd5ee8&gt;, &#39;__file__&#39;: &#39;Lib/test/crashers/nasty_eq_vs_dict.py&#39;, &#39;__package__&#39;: None, &#39;y&#39;: &lt;Yuck(i=0) at remote 0xaacd80&gt;, &#39;dict&#39;: {0: 0, 1: 1, 2: 2, 3: 3}, &#39;__cached__&#39;: None, &#39;__name__&#39;: &#39;__main__&#39;, &#39;z&#39;: &lt;Yuck(i=0) at remote 0xaace60&gt;, &#39;__doc__&#39;: None}, key=

  324.     0x5c2b8d &quot;__lltrace__&quot;) at Objects/dictobject.c:2171

  325. </pre></div>

  326. </div>

  327. <p>Notice how the dictionary argument to <code class="docutils literal notranslate"><span class="pre">PyDict_GetItemString</span></code> is displayed

  328. as its <code class="docutils literal notranslate"><span class="pre">repr()</span></code>, rather than an opaque <code class="docutils literal notranslate"><span class="pre">PyObject</span> <span class="pre">*</span></code> pointer.</p>

  329. <p>The extension works by supplying a custom printing routine for values of type

  330. <code class="docutils literal notranslate"><span class="pre">PyObject</span> <span class="pre">*</span></code>.  If you need to access lower-level details of an object, then

  331. cast the value to a pointer of the appropriate type.  For example:</p>

  332. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) p globals

  333. $1 = {&#39;__builtins__&#39;: &lt;module at remote 0x7ffff7fb1868&gt;, &#39;__name__&#39;:

  334. &#39;__main__&#39;, &#39;ctypes&#39;: &lt;module at remote 0x7ffff7f14360&gt;, &#39;__doc__&#39;: None,

  335. &#39;__package__&#39;: None}

  336. 

  337. (gdb) p *(PyDictObject*)globals

  338. $2 = {ob_refcnt = 3, ob_type = 0x3dbdf85820, ma_fill = 5, ma_used = 5,

  339. ma_mask = 7, ma_table = 0x63d0f8, ma_lookup = 0x3dbdc7ea70

  340. &lt;lookdict_string&gt;, ma_smalltable = {{me_hash = 7065186196740147912,

  341. me_key = &#39;__builtins__&#39;, me_value = &lt;module at remote 0x7ffff7fb1868&gt;},

  342. {me_hash = -368181376027291943, me_key = &#39;__name__&#39;,

  343. me_value =&#39;__main__&#39;}, {me_hash = 0, me_key = 0x0, me_value = 0x0},

  344. {me_hash = 0, me_key = 0x0, me_value = 0x0},

  345. {me_hash = -9177857982131165996, me_key = &#39;ctypes&#39;,

  346. me_value = &lt;module at remote 0x7ffff7f14360&gt;},

  347. {me_hash = -8518757509529533123, me_key = &#39;__doc__&#39;, me_value = None},

  348. {me_hash = 0, me_key = 0x0, me_value = 0x0}, {

  349.   me_hash = 6614918939584953775, me_key = &#39;__package__&#39;, me_value = None}}}

  350. </pre></div>

  351. </div>

  352. <p>Note that the pretty-printers do not actually call <code class="docutils literal notranslate"><span class="pre">repr()</span></code>.

  353. For basic types, they try to match its result closely.</p>

  354. <p>An area that can be confusing is that the custom printer for some types look a

  355. lot like GDB’s built-in printer for standard types.  For example, the

  356. pretty-printer for a Python <code class="docutils literal notranslate"><span class="pre">int</span></code> (<span class="c-expr sig sig-inline c"><a class="reference internal" href="../c-api/long.html#c.PyLongObject" title="PyLongObject"><span class="n">PyLongObject</span></a><span class="p">*</span></span>)

  357. gives a representation that is not distinguishable from one of a

  358. regular machine-level integer:</p>

  359. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) p some_machine_integer

  360. $3 = 42

  361. 

  362. (gdb) p some_python_integer

  363. $4 = 42

  364. </pre></div>

  365. </div>

  366. <p>The internal structure can be revealed with a cast to <span class="c-expr sig sig-inline c"><a class="reference internal" href="../c-api/long.html#c.PyLongObject" title="PyLongObject"><span class="n">PyLongObject</span></a><span class="p">*</span></span>:</p>

  367. <blockquote>

  368. <div><p>(gdb) p <em>(PyLongObject</em>)some_python_integer

  369. $5 = {ob_base = {ob_base = {ob_refcnt = 8, ob_type = 0x3dad39f5e0}, ob_size = 1},

  370. ob_digit = {42}}</p>

  371. </div></blockquote>

  372. <p>A similar confusion can arise with the <code class="docutils literal notranslate"><span class="pre">str</span></code> type, where the output looks a

  373. lot like gdb’s built-in printer for <code class="docutils literal notranslate"><span class="pre">char</span> <span class="pre">*</span></code>:</p>

  374. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) p ptr_to_python_str

  375. $6 = &#39;__builtins__&#39;

  376. </pre></div>

  377. </div>

  378. <p>The pretty-printer for <code class="docutils literal notranslate"><span class="pre">str</span></code> instances defaults to using single-quotes (as

  379. does Python’s <code class="docutils literal notranslate"><span class="pre">repr</span></code> for strings) whereas the standard printer for <code class="docutils literal notranslate"><span class="pre">char</span> <span class="pre">*</span></code>

  380. values uses double-quotes and contains a hexadecimal address:</p>

  381. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) p ptr_to_char_star

  382. $7 = 0x6d72c0 &quot;hello world&quot;

  383. </pre></div>

  384. </div>

  385. <p>Again, the implementation details can be revealed with a cast to

  386. <span class="c-expr sig sig-inline c"><a class="reference internal" href="../c-api/unicode.html#c.PyUnicodeObject" title="PyUnicodeObject"><span class="n">PyUnicodeObject</span></a><span class="p">*</span></span>:</p>

  387. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) p *(PyUnicodeObject*)$6

  388. $8 = {ob_base = {ob_refcnt = 33, ob_type = 0x3dad3a95a0}, length = 12,

  389. str = 0x7ffff2128500, hash = 7065186196740147912, state = 1, defenc = 0x0}

  390. </pre></div>

  391. </div>

  392. </section>

  393. <section id="py-list">

  394. <h3><code class="docutils literal notranslate"><span class="pre">py-list</span></code><a class="headerlink" href="#py-list" title="Link to this heading">¶</a></h3>

  395. <blockquote>

  396. <div><p>The extension adds a <code class="docutils literal notranslate"><span class="pre">py-list</span></code> command, which

  397. lists the Python source code (if any) for the current frame in the selected

  398. thread.  The current line is marked with a “&gt;”:</p>

  399. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) py-list

  400.  901        if options.profile:

  401.  902            options.profile = False

  402.  903            profile_me()

  403.  904            return

  404.  905

  405. &gt;906        u = UI()

  406.  907        if not u.quit:

  407.  908            try:

  408.  909                gtk.main()

  409.  910            except KeyboardInterrupt:

  410.  911                # properly quit on a keyboard interrupt...

  411. </pre></div>

  412. </div>

  413. <p>Use <code class="docutils literal notranslate"><span class="pre">py-list</span> <span class="pre">START</span></code> to list at a different line number within the Python

  414. source, and <code class="docutils literal notranslate"><span class="pre">py-list</span> <span class="pre">START,END</span></code> to list a specific range of lines within

  415. the Python source.</p>

  416. </div></blockquote>

  417. </section>

  418. <section id="py-up-and-py-down">

  419. <h3><code class="docutils literal notranslate"><span class="pre">py-up</span></code> and <code class="docutils literal notranslate"><span class="pre">py-down</span></code><a class="headerlink" href="#py-up-and-py-down" title="Link to this heading">¶</a></h3>

  420. <blockquote>

  421. <div><p>The <code class="docutils literal notranslate"><span class="pre">py-up</span></code> and <code class="docutils literal notranslate"><span class="pre">py-down</span></code> commands are analogous to GDB’s regular <code class="docutils literal notranslate"><span class="pre">up</span></code>

  422. and <code class="docutils literal notranslate"><span class="pre">down</span></code> commands, but try to move at the level of CPython frames, rather

  423. than C frames.</p>

  424. <p>GDB is not always able to read the relevant frame information, depending on

  425. the optimization level with which CPython was compiled. Internally, the

  426. commands look for C frames that are executing the default frame evaluation

  427. function (that is, the core bytecode interpreter loop within CPython) and

  428. look up the value of the related <code class="docutils literal notranslate"><span class="pre">PyFrameObject</span> <span class="pre">*</span></code>.</p>

  429. <p>They emit the frame number (at the C level) within the thread.</p>

  430. <p>For example:</p>

  431. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) py-up

  432. #37 Frame 0x9420b04, for file /usr/lib/python2.6/site-packages/

  433. gnome_sudoku/main.py, line 906, in start_game ()

  434.     u = UI()

  435. (gdb) py-up

  436. #40 Frame 0x948e82c, for file /usr/lib/python2.6/site-packages/

  437. gnome_sudoku/gnome_sudoku.py, line 22, in start_game(main=&lt;module at remote 0xb771b7f4&gt;)

  438.     main.start_game()

  439. (gdb) py-up

  440. Unable to find an older python frame

  441. </pre></div>

  442. </div>

  443. <p>so we’re at the top of the Python stack.</p>

  444. <p>The frame numbers correspond to those displayed by GDB’s standard

  445. <code class="docutils literal notranslate"><span class="pre">backtrace</span></code> command.

  446. The command skips C frames which are not executing Python code.</p>

  447. <p>Going back down:</p>

  448. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) py-down

  449. #37 Frame 0x9420b04, for file /usr/lib/python2.6/site-packages/gnome_sudoku/main.py, line 906, in start_game ()

  450.     u = UI()

  451. (gdb) py-down

  452. #34 (unable to read python frame information)

  453. (gdb) py-down

  454. #23 (unable to read python frame information)

  455. (gdb) py-down

  456. #19 (unable to read python frame information)

  457. (gdb) py-down

  458. #14 Frame 0x99262ac, for file /usr/lib/python2.6/site-packages/gnome_sudoku/game_selector.py, line 201, in run_swallowed_dialog (self=&lt;NewOrSavedGameSelector(new_game_model=&lt;gtk.ListStore at remote 0x98fab44&gt;, puzzle=None, saved_games=[{&#39;gsd.auto_fills&#39;: 0, &#39;tracking&#39;: {}, &#39;trackers&#39;: {}, &#39;notes&#39;: [], &#39;saved_at&#39;: 1270084485, &#39;game&#39;: &#39;7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 0 0 0 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5\n7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 1 8 3 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5&#39;, &#39;gsd.impossible_hints&#39;: 0, &#39;timer.__absolute_start_time__&#39;: &lt;float at remote 0x984b474&gt;, &#39;gsd.hints&#39;: 0, &#39;timer.active_time&#39;: &lt;float at remote 0x984b494&gt;, &#39;timer.total_time&#39;: &lt;float at remote 0x984b464&gt;}], dialog=&lt;gtk.Dialog at remote 0x98faaa4&gt;, saved_game_model=&lt;gtk.ListStore at remote 0x98fad24&gt;, sudoku_maker=&lt;SudokuMaker(terminated=False, played=[], batch_siz...(truncated)

  459.             swallower.run_dialog(self.dialog)

  460. (gdb) py-down

  461. #11 Frame 0x9aead74, for file /usr/lib/python2.6/site-packages/gnome_sudoku/dialog_swallower.py, line 48, in run_dialog (self=&lt;SwappableArea(running=&lt;gtk.Dialog at remote 0x98faaa4&gt;, main_page=0) at remote 0x98fa6e4&gt;, d=&lt;gtk.Dialog at remote 0x98faaa4&gt;)

  462.             gtk.main()

  463. (gdb) py-down

  464. #8 (unable to read python frame information)

  465. (gdb) py-down

  466. Unable to find a newer python frame

  467. </pre></div>

  468. </div>

  469. <p>and we’re at the bottom of the Python stack.</p>

  470. <p>Note that in Python 3.12 and newer, the same C stack frame can be used for

  471. multiple Python stack frames. This means that <code class="docutils literal notranslate"><span class="pre">py-up</span></code> and <code class="docutils literal notranslate"><span class="pre">py-down</span></code>

  472. may move multiple Python frames at once. For example:</p>

  473. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) py-up

  474. #6 Frame 0x7ffff7fb62b0, for file /tmp/rec.py, line 5, in recursive_function (n=0)

  475.    time.sleep(5)

  476. #6 Frame 0x7ffff7fb6240, for file /tmp/rec.py, line 7, in recursive_function (n=1)

  477.    recursive_function(n-1)

  478. #6 Frame 0x7ffff7fb61d0, for file /tmp/rec.py, line 7, in recursive_function (n=2)

  479.    recursive_function(n-1)

  480. #6 Frame 0x7ffff7fb6160, for file /tmp/rec.py, line 7, in recursive_function (n=3)

  481.    recursive_function(n-1)

  482. #6 Frame 0x7ffff7fb60f0, for file /tmp/rec.py, line 7, in recursive_function (n=4)

  483.    recursive_function(n-1)

  484. #6 Frame 0x7ffff7fb6080, for file /tmp/rec.py, line 7, in recursive_function (n=5)

  485.    recursive_function(n-1)

  486. #6 Frame 0x7ffff7fb6020, for file /tmp/rec.py, line 9, in &lt;module&gt; ()

  487.    recursive_function(5)

  488. (gdb) py-up

  489. Unable to find an older python frame

  490. </pre></div>

  491. </div>

  492. </div></blockquote>

  493. </section>

  494. <section id="py-bt">

  495. <h3><code class="docutils literal notranslate"><span class="pre">py-bt</span></code><a class="headerlink" href="#py-bt" title="Link to this heading">¶</a></h3>

  496. <blockquote>

  497. <div><p>The <code class="docutils literal notranslate"><span class="pre">py-bt</span></code> command attempts to display a Python-level backtrace of the

  498. current thread.</p>

  499. <p>For example:</p>

  500. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) py-bt

  501. #8 (unable to read python frame information)

  502. #11 Frame 0x9aead74, for file /usr/lib/python2.6/site-packages/gnome_sudoku/dialog_swallower.py, line 48, in run_dialog (self=&lt;SwappableArea(running=&lt;gtk.Dialog at remote 0x98faaa4&gt;, main_page=0) at remote 0x98fa6e4&gt;, d=&lt;gtk.Dialog at remote 0x98faaa4&gt;)

  503.             gtk.main()

  504. #14 Frame 0x99262ac, for file /usr/lib/python2.6/site-packages/gnome_sudoku/game_selector.py, line 201, in run_swallowed_dialog (self=&lt;NewOrSavedGameSelector(new_game_model=&lt;gtk.ListStore at remote 0x98fab44&gt;, puzzle=None, saved_games=[{&#39;gsd.auto_fills&#39;: 0, &#39;tracking&#39;: {}, &#39;trackers&#39;: {}, &#39;notes&#39;: [], &#39;saved_at&#39;: 1270084485, &#39;game&#39;: &#39;7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 0 0 0 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5\n7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 1 8 3 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5&#39;, &#39;gsd.impossible_hints&#39;: 0, &#39;timer.__absolute_start_time__&#39;: &lt;float at remote 0x984b474&gt;, &#39;gsd.hints&#39;: 0, &#39;timer.active_time&#39;: &lt;float at remote 0x984b494&gt;, &#39;timer.total_time&#39;: &lt;float at remote 0x984b464&gt;}], dialog=&lt;gtk.Dialog at remote 0x98faaa4&gt;, saved_game_model=&lt;gtk.ListStore at remote 0x98fad24&gt;, sudoku_maker=&lt;SudokuMaker(terminated=False, played=[], batch_siz...(truncated)

  505.             swallower.run_dialog(self.dialog)

  506. #19 (unable to read python frame information)

  507. #23 (unable to read python frame information)

  508. #34 (unable to read python frame information)

  509. #37 Frame 0x9420b04, for file /usr/lib/python2.6/site-packages/gnome_sudoku/main.py, line 906, in start_game ()

  510.     u = UI()

  511. #40 Frame 0x948e82c, for file /usr/lib/python2.6/site-packages/gnome_sudoku/gnome_sudoku.py, line 22, in start_game (main=&lt;module at remote 0xb771b7f4&gt;)

  512.     main.start_game()

  513. </pre></div>

  514. </div>

  515. <p>The frame numbers correspond to those displayed by GDB’s standard

  516. <code class="docutils literal notranslate"><span class="pre">backtrace</span></code> command.</p>

  517. </div></blockquote>

  518. </section>

  519. <section id="py-print">

  520. <h3><code class="docutils literal notranslate"><span class="pre">py-print</span></code><a class="headerlink" href="#py-print" title="Link to this heading">¶</a></h3>

  521. <blockquote>

  522. <div><p>The <code class="docutils literal notranslate"><span class="pre">py-print</span></code> command looks up a Python name and tries to print it.

  523. It looks in locals within the current thread, then globals, then finally

  524. builtins:</p>

  525. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) py-print self

  526. local &#39;self&#39; = &lt;SwappableArea(running=&lt;gtk.Dialog at remote 0x98faaa4&gt;,

  527. main_page=0) at remote 0x98fa6e4&gt;

  528. (gdb) py-print __name__

  529. global &#39;__name__&#39; = &#39;gnome_sudoku.dialog_swallower&#39;

  530. (gdb) py-print len

  531. builtin &#39;len&#39; = &lt;built-in function len&gt;

  532. (gdb) py-print scarlet_pimpernel

  533. &#39;scarlet_pimpernel&#39; not found

  534. </pre></div>

  535. </div>

  536. <p>If the current C frame corresponds to multiple Python frames, <code class="docutils literal notranslate"><span class="pre">py-print</span></code>

  537. only considers the first one.</p>

  538. </div></blockquote>

  539. </section>

  540. <section id="py-locals">

  541. <h3><code class="docutils literal notranslate"><span class="pre">py-locals</span></code><a class="headerlink" href="#py-locals" title="Link to this heading">¶</a></h3>

  542. <blockquote>

  543. <div><p>The <code class="docutils literal notranslate"><span class="pre">py-locals</span></code> command looks up all Python locals within the current

  544. Python frame in the selected thread, and prints their representations:</p>

  545. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) py-locals

  546. self = &lt;SwappableArea(running=&lt;gtk.Dialog at remote 0x98faaa4&gt;,

  547. main_page=0) at remote 0x98fa6e4&gt;

  548. d = &lt;gtk.Dialog at remote 0x98faaa4&gt;

  549. </pre></div>

  550. </div>

  551. <p>If the current C frame corresponds to multiple Python frames, locals from

  552. all of them will be shown:</p>

  553. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) py-locals

  554. Locals for recursive_function

  555. n = 0

  556. Locals for recursive_function

  557. n = 1

  558. Locals for recursive_function

  559. n = 2

  560. Locals for recursive_function

  561. n = 3

  562. Locals for recursive_function

  563. n = 4

  564. Locals for recursive_function

  565. n = 5

  566. Locals for &lt;module&gt;

  567. </pre></div>

  568. </div>

  569. </div></blockquote>

  570. </section>

  571. </section>

  572. <section id="use-with-gdb-commands">

  573. <h2>Use with GDB commands<a class="headerlink" href="#use-with-gdb-commands" title="Link to this heading">¶</a></h2>

  574. <p>The extension commands complement GDB’s built-in commands.

  575. For example, you can use a frame numbers shown by <code class="docutils literal notranslate"><span class="pre">py-bt</span></code> with the <code class="docutils literal notranslate"><span class="pre">frame</span></code>

  576. command to go a specific frame within the selected thread, like this:</p>

  577. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) py-bt

  578. (output snipped)

  579. #68 Frame 0xaa4560, for file Lib/test/regrtest.py, line 1548, in &lt;module&gt; ()

  580.         main()

  581. (gdb) frame 68

  582. #68 0x00000000004cd1e6 in PyEval_EvalFrameEx (f=Frame 0xaa4560, for file Lib/test/regrtest.py, line 1548, in &lt;module&gt; (), throwflag=0) at Python/ceval.c:2665

  583. 2665                            x = call_function(&amp;sp, oparg);

  584. (gdb) py-list

  585. 1543        # Run the tests in a context manager that temporary changes the CWD to a

  586. 1544        # temporary and writable directory. If it&#39;s not possible to create or

  587. 1545        # change the CWD, the original CWD will be used. The original CWD is

  588. 1546        # available from test_support.SAVEDCWD.

  589. 1547        with test_support.temp_cwd(TESTCWD, quiet=True):

  590. &gt;1548            main()

  591. </pre></div>

  592. </div>

  593. <p>The <code class="docutils literal notranslate"><span class="pre">info</span> <span class="pre">threads</span></code> command will give you a list of the threads within the

  594. process, and you can use the <code class="docutils literal notranslate"><span class="pre">thread</span></code> command to select a different one:</p>

  595. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) info threads

  596.   105 Thread 0x7fffefa18710 (LWP 10260)  sem_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/sem_wait.S:86

  597.   104 Thread 0x7fffdf5fe710 (LWP 10259)  sem_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/sem_wait.S:86

  598. * 1 Thread 0x7ffff7fe2700 (LWP 10145)  0x00000038e46d73e3 in select () at ../sysdeps/unix/syscall-template.S:82

  599. </pre></div>

  600. </div>

  601. <p>You can use <code class="docutils literal notranslate"><span class="pre">thread</span> <span class="pre">apply</span> <span class="pre">all</span> <span class="pre">COMMAND</span></code> or (<code class="docutils literal notranslate"><span class="pre">t</span> <span class="pre">a</span> <span class="pre">a</span> <span class="pre">COMMAND</span></code> for short) to run

  602. a command on all threads.  With <code class="docutils literal notranslate"><span class="pre">py-bt</span></code>, this lets you see what every

  603. thread is doing at the Python level:</p>

  604. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>(gdb) t a a py-bt

  605. 

  606. Thread 105 (Thread 0x7fffefa18710 (LWP 10260)):

  607. #5 Frame 0x7fffd00019d0, for file /home/david/coding/python-svn/Lib/threading.py, line 155, in _acquire_restore (self=&lt;_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=&lt;thread.lock at remote 0x858770&gt;, _RLock__count=1) at remote 0xd7ff40&gt;, count_owner=(1, 140737213728528), count=1, owner=140737213728528)

  608.         self.__block.acquire()

  609. #8 Frame 0x7fffac001640, for file /home/david/coding/python-svn/Lib/threading.py, line 269, in wait (self=&lt;_Condition(_Condition__lock=&lt;_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=&lt;thread.lock at remote 0x858770&gt;, _RLock__count=1) at remote 0xd7ff40&gt;, acquire=&lt;instancemethod at remote 0xd80260&gt;, _is_owned=&lt;instancemethod at remote 0xd80160&gt;, _release_save=&lt;instancemethod at remote 0xd803e0&gt;, release=&lt;instancemethod at remote 0xd802e0&gt;, _acquire_restore=&lt;instancemethod at remote 0xd7ee60&gt;, _Verbose__verbose=False, _Condition__waiters=[]) at remote 0xd7fd10&gt;, timeout=None, waiter=&lt;thread.lock at remote 0x858a90&gt;, saved_state=(1, 140737213728528))

  610.             self._acquire_restore(saved_state)

  611. #12 Frame 0x7fffb8001a10, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 348, in f ()

  612.             cond.wait()

  613. #16 Frame 0x7fffb8001c40, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 37, in task (tid=140737213728528)

  614.                 f()

  615. 

  616. Thread 104 (Thread 0x7fffdf5fe710 (LWP 10259)):

  617. #5 Frame 0x7fffe4001580, for file /home/david/coding/python-svn/Lib/threading.py, line 155, in _acquire_restore (self=&lt;_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=&lt;thread.lock at remote 0x858770&gt;, _RLock__count=1) at remote 0xd7ff40&gt;, count_owner=(1, 140736940992272), count=1, owner=140736940992272)

  618.         self.__block.acquire()

  619. #8 Frame 0x7fffc8002090, for file /home/david/coding/python-svn/Lib/threading.py, line 269, in wait (self=&lt;_Condition(_Condition__lock=&lt;_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=&lt;thread.lock at remote 0x858770&gt;, _RLock__count=1) at remote 0xd7ff40&gt;, acquire=&lt;instancemethod at remote 0xd80260&gt;, _is_owned=&lt;instancemethod at remote 0xd80160&gt;, _release_save=&lt;instancemethod at remote 0xd803e0&gt;, release=&lt;instancemethod at remote 0xd802e0&gt;, _acquire_restore=&lt;instancemethod at remote 0xd7ee60&gt;, _Verbose__verbose=False, _Condition__waiters=[]) at remote 0xd7fd10&gt;, timeout=None, waiter=&lt;thread.lock at remote 0x858860&gt;, saved_state=(1, 140736940992272))

  620.             self._acquire_restore(saved_state)

  621. #12 Frame 0x7fffac001c90, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 348, in f ()

  622.             cond.wait()

  623. #16 Frame 0x7fffac0011c0, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 37, in task (tid=140736940992272)

  624.                 f()

  625. 

  626. Thread 1 (Thread 0x7ffff7fe2700 (LWP 10145)):

  627. #5 Frame 0xcb5380, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 16, in _wait ()

  628.     time.sleep(0.01)

  629. #8 Frame 0x7fffd00024a0, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 378, in _check_notify (self=&lt;ConditionTests(_testMethodName=&#39;test_notify&#39;, _resultForDoCleanups=&lt;TestResult(_original_stdout=&lt;cStringIO.StringO at remote 0xc191e0&gt;, skipped=[], _mirrorOutput=False, testsRun=39, buffer=False, _original_stderr=&lt;file at remote 0x7ffff7fc6340&gt;, _stdout_buffer=&lt;cStringIO.StringO at remote 0xc9c7f8&gt;, _stderr_buffer=&lt;cStringIO.StringO at remote 0xc9c790&gt;, _moduleSetUpFailed=False, expectedFailures=[], errors=[], _previousTestClass=&lt;type at remote 0x928310&gt;, unexpectedSuccesses=[], failures=[], shouldStop=False, failfast=False) at remote 0xc185a0&gt;, _threads=(0,), _cleanups=[], _type_equality_funcs={&lt;type at remote 0x7eba00&gt;: &lt;instancemethod at remote 0xd750e0&gt;, &lt;type at remote 0x7e7820&gt;: &lt;instancemethod at remote 0xd75160&gt;, &lt;type at remote 0x7e30e0&gt;: &lt;instancemethod at remote 0xd75060&gt;, &lt;type at remote 0x7e7d20&gt;: &lt;instancemethod at remote 0xd751e0&gt;, &lt;type at remote 0x7f19e0...(truncated)

  630.         _wait()

  631. </pre></div>

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