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<title>Debugging with GDB: Unwinding Frames in Python</title>
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<a name="Unwinding-Frames-in-Python"></a>
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<div class="header">
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<p>
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Next: <a href="Xmethods-In-Python.html#Xmethods-In-Python" accesskey="n" rel="next">Xmethods In Python</a>, Previous: <a href="Writing-a-Frame-Filter.html#Writing-a-Frame-Filter" accesskey="p" rel="prev">Writing a Frame Filter</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</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="Unwinding-Frames-in-Python-1"></a>
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<h4 class="subsubsection">23.2.2.12 Unwinding Frames in Python</h4>
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<a name="index-unwinding-frames-in-Python"></a>
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<p>In <small>GDB</small> terminology “unwinding” is the process of finding
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the previous frame (that is, caller’s) from the current one. An
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unwinder has three methods. The first one checks if it can handle
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given frame (“sniff” it). For the frames it can sniff an unwinder
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provides two additional methods: it can return frame’s ID, and it can
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fetch registers from the previous frame. A running <small>GDB</small>
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mantains a list of the unwinders and calls each unwinder’s sniffer in
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turn until it finds the one that recognizes the current frame. There
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is an API to register an unwinder.
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</p>
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<p>The unwinders that come with <small>GDB</small> handle standard frames.
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However, mixed language applications (for example, an application
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running Java Virtual Machine) sometimes use frame layouts that cannot
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be handled by the <small>GDB</small> unwinders. You can write Python code
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that can handle such custom frames.
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</p>
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<p>You implement a frame unwinder in Python as a class with which has two
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attributes, <code>name</code> and <code>enabled</code>, with obvious meanings, and
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a single method <code>__call__</code>, which examines a given frame and
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returns an object (an instance of <code>gdb.UnwindInfo class)</code>
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describing it. If an unwinder does not recognize a frame, it should
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return <code>None</code>. The code in <small>GDB</small> that enables writing
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unwinders in Python uses this object to return frame’s ID and previous
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frame registers when <small>GDB</small> core asks for them.
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</p>
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<a name="Unwinder-Input"></a>
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<h4 class="subheading">Unwinder Input</h4>
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<p>An object passed to an unwinder (a <code>gdb.PendingFrame</code> instance)
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provides a method to read frame’s registers:
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</p>
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<dl>
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<dt><a name="index-PendingFrame_002eread_005fregister"></a>Function: <strong>PendingFrame.read_register</strong> <em>(reg)</em></dt>
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<dd><p>This method returns the contents of the register <var>regn</var> in the
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frame as a <code>gdb.Value</code> object. <var>reg</var> can be either a
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register number or a register name; the values are platform-specific.
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They are usually found in the corresponding
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<samp><var>platform</var>-tdep.h</samp> file in the <small>GDB</small> source tree.
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</p></dd></dl>
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<p>It also provides a factory method to create a <code>gdb.UnwindInfo</code>
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instance to be returned to <small>GDB</small>:
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</p>
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<dl>
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<dt><a name="index-PendingFrame_002ecreate_005funwind_005finfo"></a>Function: <strong>PendingFrame.create_unwind_info</strong> <em>(frame_id)</em></dt>
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<dd><p>Returns a new <code>gdb.UnwindInfo</code> instance identified by given
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<var>frame_id</var>. The argument is used to build <small>GDB</small>’s frame ID
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using one of functions provided by <small>GDB</small>. <var>frame_id</var>’s attributes
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determine which function will be used, as follows:
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</p>
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<dl compact="compact">
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<dt><code>sp, pc, special</code></dt>
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<dd><p><code>frame_id_build_special (<var>frame_id</var>.sp, <var>frame_id</var>.pc, <var>frame_id</var>.special)</code>
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</p>
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</dd>
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<dt><code>sp, pc</code></dt>
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<dd><p><code>frame_id_build (<var>frame_id</var>.sp, <var>frame_id</var>.pc)</code>
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</p>
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<p>This is the most common case.
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</p>
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</dd>
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<dt><code>sp</code></dt>
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<dd><p><code>frame_id_build_wild (<var>frame_id</var>.sp)</code>
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</p></dd>
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</dl>
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<p>The attribute values should be <code>gdb.Value</code>
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</p>
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</dd></dl>
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<a name="Unwinder-Output_003a-UnwindInfo"></a>
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<h4 class="subheading">Unwinder Output: UnwindInfo</h4>
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<p>Use <code>PendingFrame.create_unwind_info</code> method described above to
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create a <code>gdb.UnwindInfo</code> instance. Use the following method to
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specify caller registers that have been saved in this frame:
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</p>
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<dl>
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<dt><a name="index-gdb_002eUnwindInfo_002eadd_005fsaved_005fregister"></a>Function: <strong>gdb.UnwindInfo.add_saved_register</strong> <em>(reg, value)</em></dt>
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<dd><p><var>reg</var> identifies the register. It can be a number or a name, just
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as for the <code>PendingFrame.read_register</code> method above.
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<var>value</var> is a register value (a <code>gdb.Value</code> object).
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</p></dd></dl>
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<a name="Unwinder-Skeleton-Code"></a>
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<h4 class="subheading">Unwinder Skeleton Code</h4>
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<p><small>GDB</small> comes with the module containing the base <code>Unwinder</code>
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class. Derive your unwinder class from it and structure the code as
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follows:
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</p>
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<div class="smallexample">
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<pre class="smallexample">from gdb.unwinders import Unwinder
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class FrameId(object):
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def __init__(self, sp, pc):
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self.sp = sp
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self.pc = pc
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class MyUnwinder(Unwinder):
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def __init__(....):
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supe(MyUnwinder, self).__init___(<expects unwinder name argument>)
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def __call__(pending_frame):
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if not <we recognize frame>:
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return None
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# Create UnwindInfo. Usually the frame is identified by the stack
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# pointer and the program counter.
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sp = pending_frame.read_register(<SP number>)
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pc = pending_frame.read_register(<PC number>)
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unwind_info = pending_frame.create_unwind_info(FrameId(sp, pc))
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# Find the values of the registers in the caller's frame and
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# save them in the result:
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unwind_info.add_saved_register(<register>, <value>)
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....
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# Return the result:
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return unwind_info
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</pre></div>
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<a name="Registering-a-Unwinder"></a>
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<h4 class="subheading">Registering a Unwinder</h4>
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<p>An object file, a program space, and the <small>GDB</small> proper can have
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unwinders registered with it.
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</p>
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<p>The <code>gdb.unwinders</code> module provides the function to register a
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unwinder:
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</p>
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<dl>
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<dt><a name="index-gdb_002eunwinder_002eregister_005funwinder"></a>Function: <strong>gdb.unwinder.register_unwinder</strong> <em>(locus, unwinder, replace=False)</em></dt>
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<dd><p><var>locus</var> is specifies an object file or a program space to which
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<var>unwinder</var> is added. Passing <code>None</code> or <code>gdb</code> adds
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<var>unwinder</var> to the <small>GDB</small>’s global unwinder list. The newly
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added <var>unwinder</var> will be called before any other unwinder from the
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same locus. Two unwinders in the same locus cannot have the same
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name. An attempt to add a unwinder with already existing name raises
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an exception unless <var>replace</var> is <code>True</code>, in which case the
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old unwinder is deleted.
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</p></dd></dl>
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<a name="Unwinder-Precedence"></a>
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<h4 class="subheading">Unwinder Precedence</h4>
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<p><small>GDB</small> first calls the unwinders from all the object files in no
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particular order, then the unwinders from the current program space,
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and finally the unwinders from <small>GDB</small>.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Xmethods-In-Python.html#Xmethods-In-Python" accesskey="n" rel="next">Xmethods In Python</a>, Previous: <a href="Writing-a-Frame-Filter.html#Writing-a-Frame-Filter" accesskey="p" rel="prev">Writing a Frame Filter</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</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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