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<h5 class="subsubsection">23.2.2.12 Unwinding Frames in Python</h5>

<p><a name="index-unwinding-frames-in-Python-2034"></a>
In <span class="sc">gdb</span> terminology &ldquo;unwinding&rdquo; is the process of finding
the previous frame (that is, caller'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's ID, and it can
fetch registers from the previous frame.  A running <span class="sc">gdb</span>
mantains a list of the unwinders and calls each unwinder's sniffer in
turn until it finds the one that recognizes the current frame.  There
is an API to register an unwinder.

   <p>The unwinders that come with <span class="sc">gdb</span> 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 <span class="sc">gdb</span> unwinders.  You can write Python code
that can handle such custom frames.

   <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 <span class="sc">gdb</span> that enables writing
unwinders in Python uses this object to return frame's ID and previous
frame registers when <span class="sc">gdb</span> core asks for them.

   <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
<span class="sc">gdb</span>'s stack unwinding process, potentially causing re-entrant
unwinding.

<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's registers:

<div class="defun">
&mdash; Function: <b>PendingFrame.read_register</b> (<var>reg</var>)<var><a name="index-PendingFrame_002eread_005fregister-2035"></a></var><br>
<blockquote><p>This method returns the contents of the register <var>reg</var> in the
frame as a <code>gdb.Value</code> object.  <var>reg</var> can be either a
register number or a register name; the values are platform-specific. 
They are usually found in the corresponding
<samp><var>platform</var><span class="file">-tdep.h</span></samp> file in the <span class="sc">gdb</span> source tree.  If
<var>reg</var> does not name a register for the current architecture, this
method will throw an exception.

        <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>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></blockquote></div>

   <p>It also provides a factory method to create a <code>gdb.UnwindInfo</code>
instance to be returned to <span class="sc">gdb</span>:

<div class="defun">
&mdash; Function: <b>PendingFrame.create_unwind_info</b> (<var>frame_id</var>)<var><a name="index-PendingFrame_002ecreate_005funwind_005finfo-2036"></a></var><br>
<blockquote><p>Returns a new <code>gdb.UnwindInfo</code> instance identified by given
<var>frame_id</var>.  The argument is used to build <span class="sc">gdb</span>'s frame ID
using one of functions provided by <span class="sc">gdb</span>.  <var>frame_id</var>'s attributes
determine which function will be used, as follows:

          <dl>
<dt><code>sp, pc</code><dd>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>This is the most common case by far.  The other cases are documented
for completeness but are only useful in specialized situations.

          <br><dt><code>sp, pc, special</code><dd>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.

          <br><dt><code>sp</code><dd>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. 
</dl>

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

        </blockquote></div>

<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:

<div class="defun">
&mdash; Function: <b>gdb.UnwindInfo.add_saved_register</b> (<var>reg, value</var>)<var><a name="index-gdb_002eUnwindInfo_002eadd_005fsaved_005fregister-2037"></a></var><br>
<blockquote><p><var>reg</var> identifies the register.  It can be a number or a name, just
as for the <code>PendingFrame.read_register</code> method above. 
<var>value</var> is a register value (a <code>gdb.Value</code> object). 
</p></blockquote></div>

<h4 class="subheading">Unwinder Skeleton Code</h4>

<p><span class="sc">gdb</span> comes with the module containing the base <code>Unwinder</code>
class.  Derive your unwinder class from it and structure the code as
follows:

<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__(....):
             supe(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>
   <h4 class="subheading">Registering a Unwinder</h4>

<p>An object file, a program space, and the <span class="sc">gdb</span> proper can have
unwinders registered with it.

   <p>The <code>gdb.unwinders</code> module provides the function to register a
unwinder:

<div class="defun">
&mdash; Function: <b>gdb.unwinder.register_unwinder</b> (<var>locus, unwinder, replace=False</var>)<var><a name="index-gdb_002eunwinder_002eregister_005funwinder-2038"></a></var><br>
<blockquote><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 <span class="sc">gdb</span>'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></blockquote></div>

<h4 class="subheading">Unwinder Precedence</h4>

<p><span class="sc">gdb</span> 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 <span class="sc">gdb</span>.

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