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<title>Debugging with GDB: Unwinding Frames in Python</title>

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<a name="Unwinding-Frames-in-Python"></a>
<div class="header">
<p>
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="previous">Writing a Frame Filter</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</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="Unwinding-Frames-in-Python-1"></a>
<h4 class="subsubsection">23.2.2.12 Unwinding Frames in Python</h4>
<a name="index-unwinding-frames-in-Python"></a>

<p>In <small>GDB</small> terminology &ldquo;unwinding&rdquo; is the process of finding
the previous frame (that is, caller&rsquo;s) from the current one.  An
unwinder has three methods.  The first one checks if it can handle
given frame (&ldquo;sniff&rdquo; it).  For the frames it can sniff an unwinder
provides two additional methods: it can return frame&rsquo;s ID, and it can
fetch registers from the previous frame.  A running <small>GDB</small>
mantains a list of the unwinders and calls each unwinder&rsquo;s sniffer in
turn until it finds the one that recognizes the current frame.  There
is an API to register an unwinder.
</p>
<p>The unwinders that come with <small>GDB</small> handle standard frames.
However, mixed language applications (for example, an application
running Java Virtual Machine) sometimes use frame layouts that cannot
be handled by the <small>GDB</small> unwinders.  You can write Python code
that can handle such custom frames.
</p>
<p>You implement a frame unwinder in Python as a class with which has two
attributes, <code>name</code> and <code>enabled</code>, with obvious meanings, and
a single method <code>__call__</code>, which examines a given frame and
returns an object (an instance of <code>gdb.UnwindInfo class)</code>
describing it.  If an unwinder does not recognize a frame, it should
return <code>None</code>.  The code in <small>GDB</small> that enables writing
unwinders in Python uses this object to return frame&rsquo;s ID and previous
frame registers when <small>GDB</small> core asks for them.
</p>
<p>An unwinder should do as little work as possible.  Some otherwise
innocuous operations can cause problems (even crashes, as this code is
not not well-hardened yet).  For example, making an inferior call from
an unwinder is unadvisable, as an inferior call will reset
<small>GDB</small>&rsquo;s stack unwinding process, potentially causing re-entrant
unwinding.
</p>
<a name="Unwinder-Input"></a>
<h4 class="subheading">Unwinder Input</h4>

<p>An object passed to an unwinder (a <code>gdb.PendingFrame</code> instance)
provides a method to read frame&rsquo;s registers:
</p>
<dl>
<dt><a name="index-PendingFrame_002eread_005fregister"></a>Function: <strong>PendingFrame.read_register</strong> <em>(reg)</em></dt>
<dd><p>This method returns the contents of the register <var>reg</var> in the
frame as a <code>gdb.Value</code> object.  For a description of the
acceptable values of <var>reg</var> see
<a href="Frames-In-Python.html#gdbpy_005fframe_005fread_005fregister">Frame.read_register</a>.  If <var>reg</var>
does not name a register for the current architecture, this method
will throw an exception.
</p>
<p>Note that this method will always return a <code>gdb.Value</code> for a
valid register name.  This does not mean that the value will be valid.
For example, you may request a register that an earlier unwinder could
not unwind&mdash;the value will be unavailable.  Instead, the
<code>gdb.Value</code> returned from this method will be lazy; that is, its
underlying bits will not be fetched until it is first used.  So,
attempting to use such a value will cause an exception at the point of
use.
</p>
<p>The type of the returned <code>gdb.Value</code> depends on the register and
the architecture.  It is common for registers to have a scalar type,
like <code>long long</code>; but many other types are possible, such as
pointer, pointer-to-function, floating point or vector types.
</p></dd></dl>

<p>It also provides a factory method to create a <code>gdb.UnwindInfo</code>
instance to be returned to <small>GDB</small>:
</p>
<dl>
<dt><a name="index-PendingFrame_002ecreate_005funwind_005finfo"></a>Function: <strong>PendingFrame.create_unwind_info</strong> <em>(frame_id)</em></dt>
<dd><p>Returns a new <code>gdb.UnwindInfo</code> instance identified by given
<var>frame_id</var>.  The argument is used to build <small>GDB</small>&rsquo;s frame ID
using one of functions provided by <small>GDB</small>.  <var>frame_id</var>&rsquo;s attributes
determine which function will be used, as follows:
</p>
<dl compact="compact">
<dt><code>sp, pc</code></dt>
<dd><p>The frame is identified by the given stack address and PC.  The stack
address must be chosen so that it is constant throughout the lifetime
of the frame, so a typical choice is the value of the stack pointer at
the start of the function&mdash;in the DWARF standard, this would be the
&ldquo;Call Frame Address&rdquo;.
</p>
<p>This is the most common case by far.  The other cases are documented
for completeness but are only useful in specialized situations.
</p>
</dd>
<dt><code>sp, pc, special</code></dt>
<dd><p>The frame is identified by the stack address, the PC, and a
&ldquo;special&rdquo; address.  The special address is used on architectures
that can have frames that do not change the stack, but which are still
distinct, for example the IA-64, which has a second stack for
registers.  Both <var>sp</var> and <var>special</var> must be constant
throughout the lifetime of the frame.
</p>
</dd>
<dt><code>sp</code></dt>
<dd><p>The frame is identified by the stack address only.  Any other stack
frame with a matching <var>sp</var> will be considered to match this frame.
Inside gdb, this is called a &ldquo;wild frame&rdquo;.  You will never need
this.
</p></dd>
</dl>

<p>Each attribute value should be an instance of <code>gdb.Value</code>.
</p>
</dd></dl>

<dl>
<dt><a name="index-PendingFrame_002earchitecture"></a>Function: <strong>PendingFrame.architecture</strong> <em>()</em></dt>
<dd><p>Return the <code>gdb.Architecture</code> (see <a href="Architectures-In-Python.html#Architectures-In-Python">Architectures In Python</a>)
for this <code>gdb.PendingFrame</code>.  This represents the architecture of
the particular frame being unwound.
</p></dd></dl>

<a name="Unwinder-Output_003a-UnwindInfo"></a>
<h4 class="subheading">Unwinder Output: UnwindInfo</h4>

<p>Use <code>PendingFrame.create_unwind_info</code> method described above to
create a <code>gdb.UnwindInfo</code> instance.  Use the following method to
specify caller registers that have been saved in this frame:
</p>
<dl>
<dt><a name="index-gdb_002eUnwindInfo_002eadd_005fsaved_005fregister"></a>Function: <strong>gdb.UnwindInfo.add_saved_register</strong> <em>(reg, value)</em></dt>
<dd><p><var>reg</var> identifies the register, for a description of the acceptable
values see <a href="Frames-In-Python.html#gdbpy_005fframe_005fread_005fregister">Frame.read_register</a>.
<var>value</var> is a register value (a <code>gdb.Value</code> object).
</p></dd></dl>

<a name="Unwinder-Skeleton-Code"></a>
<h4 class="subheading">Unwinder Skeleton Code</h4>

<p><small>GDB</small> comes with the module containing the base <code>Unwinder</code>
class.  Derive your unwinder class from it and structure the code as
follows:
</p>
<div class="smallexample">
<pre class="smallexample">from gdb.unwinders import Unwinder

class FrameId(object):
    def __init__(self, sp, pc):
        self.sp = sp
        self.pc = pc


class MyUnwinder(Unwinder):
    def __init__(....):
        super(MyUnwinder, self).__init___(&lt;expects unwinder name argument&gt;)

    def __call__(pending_frame):
        if not &lt;we recognize frame&gt;:
            return None
        # Create UnwindInfo.  Usually the frame is identified by the stack 
        # pointer and the program counter.
        sp = pending_frame.read_register(&lt;SP number&gt;)
        pc = pending_frame.read_register(&lt;PC number&gt;)
        unwind_info = pending_frame.create_unwind_info(FrameId(sp, pc))

        # Find the values of the registers in the caller's frame and 
        # save them in the result:
        unwind_info.add_saved_register(&lt;register&gt;, &lt;value&gt;)
        ....

        # Return the result:
        return unwind_info

</pre></div>

<a name="Registering-a-Unwinder"></a>
<h4 class="subheading">Registering a Unwinder</h4>

<p>An object file, a program space, and the <small>GDB</small> proper can have
unwinders registered with it.
</p>
<p>The <code>gdb.unwinders</code> module provides the function to register a
unwinder:
</p>
<dl>
<dt><a name="index-gdb_002eunwinder_002eregister_005funwinder"></a>Function: <strong>gdb.unwinder.register_unwinder</strong> <em>(locus, unwinder, replace=False)</em></dt>
<dd><p><var>locus</var> is specifies an object file or a program space to which
<var>unwinder</var> is added.  Passing <code>None</code> or <code>gdb</code> adds
<var>unwinder</var> to the <small>GDB</small>&rsquo;s global unwinder list.  The newly
added <var>unwinder</var> will be called before any other unwinder from the
same locus.  Two unwinders in the same locus cannot have the same
name.  An attempt to add a unwinder with already existing name raises
an exception unless <var>replace</var> is <code>True</code>, in which case the
old unwinder is deleted.
</p></dd></dl>

<a name="Unwinder-Precedence"></a>
<h4 class="subheading">Unwinder Precedence</h4>

<p><small>GDB</small> first calls the unwinders from all the object files in no
particular order, then the unwinders from the current program space,
and finally the unwinders from <small>GDB</small>.
</p>
<hr>
<div class="header">
<p>
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="previous">Writing a Frame Filter</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</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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