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<title>Debugging with GDB: Frame Decorator API</title>
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<a name="Frame-Decorator-API"></a>
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<div class="header">
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<p>
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Next: <a href="Writing-a-Frame-Filter.html#Writing-a-Frame-Filter" accesskey="n" rel="next">Writing a Frame Filter</a>, Previous: <a href="Frame-Filter-API.html#Frame-Filter-API" accesskey="p" rel="previous">Frame Filter API</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="Decorating-Frames"></a>
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<h4 class="subsubsection">23.2.2.10 Decorating Frames</h4>
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<a name="index-frame-decorator-api"></a>
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<p>Frame decorators are sister objects to frame filters (see <a href="Frame-Filter-API.html#Frame-Filter-API">Frame Filter API</a>). Frame decorators are applied by a frame filter and can
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only be used in conjunction with frame filters.
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</p>
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<p>The purpose of a frame decorator is to customize the printed content
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of each <code>gdb.Frame</code> in commands where frame filters are executed.
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This concept is called decorating a frame. Frame decorators decorate
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a <code>gdb.Frame</code> with Python code contained within each API call.
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This separates the actual data contained in a <code>gdb.Frame</code> from
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the decorated data produced by a frame decorator. This abstraction is
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necessary to maintain integrity of the data contained in each
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<code>gdb.Frame</code>.
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</p>
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<p>Frame decorators have a mandatory interface, defined below.
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</p>
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<p><small>GDB</small> already contains a frame decorator called
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<code>FrameDecorator</code>. This contains substantial amounts of
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boilerplate code to decorate the content of a <code>gdb.Frame</code>. It is
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recommended that other frame decorators inherit and extend this
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object, and only to override the methods needed.
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</p>
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<a name="index-gdb_002eFrameDecorator"></a>
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<p><code>FrameDecorator</code> is defined in the Python module
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<code>gdb.FrameDecorator</code>, so your code can import it like:
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</p><div class="smallexample">
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<pre class="smallexample">from gdb.FrameDecorator import FrameDecorator
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</pre></div>
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<dl>
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<dt><a name="index-FrameDecorator_002eelided"></a>Function: <strong>FrameDecorator.elided</strong> <em>(self)</em></dt>
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<dd>
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<p>The <code>elided</code> method groups frames together in a hierarchical
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system. An example would be an interpreter, where multiple low-level
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frames make up a single call in the interpreted language. In this
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example, the frame filter would elide the low-level frames and present
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a single high-level frame, representing the call in the interpreted
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language, to the user.
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</p>
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<p>The <code>elided</code> function must return an iterable and this iterable
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must contain the frames that are being elided wrapped in a suitable
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frame decorator. If no frames are being elided this function may
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return an empty iterable, or <code>None</code>. Elided frames are indented
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from normal frames in a <code>CLI</code> backtrace, or in the case of
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<code>GDB/MI</code>, are placed in the <code>children</code> field of the eliding
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frame.
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</p>
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<p>It is the frame filter’s task to also filter out the elided frames from
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the source iterator. This will avoid printing the frame twice.
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</p></dd></dl>
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<dl>
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<dt><a name="index-FrameDecorator_002efunction"></a>Function: <strong>FrameDecorator.function</strong> <em>(self)</em></dt>
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<dd>
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<p>This method returns the name of the function in the frame that is to
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be printed.
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</p>
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<p>This method must return a Python string describing the function, or
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<code>None</code>.
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</p>
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<p>If this function returns <code>None</code>, <small>GDB</small> will not print any
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data for this field.
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</p></dd></dl>
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<dl>
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<dt><a name="index-FrameDecorator_002eaddress"></a>Function: <strong>FrameDecorator.address</strong> <em>(self)</em></dt>
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<dd>
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<p>This method returns the address of the frame that is to be printed.
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</p>
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<p>This method must return a Python numeric integer type of sufficient
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size to describe the address of the frame, or <code>None</code>.
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</p>
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<p>If this function returns a <code>None</code>, <small>GDB</small> will not print
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any data for this field.
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</p></dd></dl>
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<dl>
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<dt><a name="index-FrameDecorator_002efilename"></a>Function: <strong>FrameDecorator.filename</strong> <em>(self)</em></dt>
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<dd>
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<p>This method returns the filename and path associated with this frame.
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</p>
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<p>This method must return a Python string containing the filename and
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the path to the object file backing the frame, or <code>None</code>.
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</p>
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<p>If this function returns a <code>None</code>, <small>GDB</small> will not print
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any data for this field.
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</p></dd></dl>
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<dl>
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<dt><a name="index-FrameDecorator_002eline"></a>Function: <strong>FrameDecorator.line</strong> <em>(self):</em></dt>
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<dd>
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<p>This method returns the line number associated with the current
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position within the function addressed by this frame.
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</p>
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<p>This method must return a Python integer type, or <code>None</code>.
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</p>
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<p>If this function returns a <code>None</code>, <small>GDB</small> will not print
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any data for this field.
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</p></dd></dl>
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<dl>
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<dt><a name="index-FrameDecorator_002eframe_005fargs"></a>Function: <strong>FrameDecorator.frame_args</strong> <em>(self)</em></dt>
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<dd><a name="frame_005fargs"></a>
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<p>This method must return an iterable, or <code>None</code>. Returning an
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empty iterable, or <code>None</code> means frame arguments will not be
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printed for this frame. This iterable must contain objects that
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implement two methods, described here.
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</p>
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<p>This object must implement a <code>argument</code> method which takes a
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single <code>self</code> parameter and must return a <code>gdb.Symbol</code>
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(see <a href="Symbols-In-Python.html#Symbols-In-Python">Symbols In Python</a>), or a Python string. The object must also
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implement a <code>value</code> method which takes a single <code>self</code>
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parameter and must return a <code>gdb.Value</code> (see <a href="Values-From-Inferior.html#Values-From-Inferior">Values From Inferior</a>), a Python value, or <code>None</code>. If the <code>value</code>
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method returns <code>None</code>, and the <code>argument</code> method returns a
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<code>gdb.Symbol</code>, <small>GDB</small> will look-up and print the value of
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the <code>gdb.Symbol</code> automatically.
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</p>
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<p>A brief example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">class SymValueWrapper():
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def __init__(self, symbol, value):
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self.sym = symbol
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self.val = value
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def value(self):
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return self.val
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def symbol(self):
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return self.sym
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class SomeFrameDecorator()
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...
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...
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def frame_args(self):
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args = []
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try:
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block = self.inferior_frame.block()
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except:
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return None
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# Iterate over all symbols in a block. Only add
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# symbols that are arguments.
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for sym in block:
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if not sym.is_argument:
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continue
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args.append(SymValueWrapper(sym,None))
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# Add example synthetic argument.
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args.append(SymValueWrapper(``foo'', 42))
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return args
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</pre></div>
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</dd></dl>
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<dl>
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<dt><a name="index-FrameDecorator_002eframe_005flocals"></a>Function: <strong>FrameDecorator.frame_locals</strong> <em>(self)</em></dt>
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<dd>
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<p>This method must return an iterable or <code>None</code>. Returning an
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empty iterable, or <code>None</code> means frame local arguments will not be
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printed for this frame.
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</p>
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<p>The object interface, the description of the various strategies for
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reading frame locals, and the example are largely similar to those
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described in the <code>frame_args</code> function, (see <a href="#frame_005fargs">The
|
frame filter frame_args function</a>). Below is a modified example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">class SomeFrameDecorator()
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...
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...
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def frame_locals(self):
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vars = []
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try:
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block = self.inferior_frame.block()
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except:
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return None
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# Iterate over all symbols in a block. Add all
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# symbols, except arguments.
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for sym in block:
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if sym.is_argument:
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continue
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vars.append(SymValueWrapper(sym,None))
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# Add an example of a synthetic local variable.
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vars.append(SymValueWrapper(``bar'', 99))
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return vars
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</pre></div>
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</dd></dl>
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<dl>
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<dt><a name="index-FrameDecorator_002einferior_005fframe"></a>Function: <strong>FrameDecorator.inferior_frame</strong> <em>(self):</em></dt>
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<dd>
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<p>This method must return the underlying <code>gdb.Frame</code> that this
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frame decorator is decorating. <small>GDB</small> requires the underlying
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frame for internal frame information to determine how to print certain
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values when printing a frame.
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</p></dd></dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Writing-a-Frame-Filter.html#Writing-a-Frame-Filter" accesskey="n" rel="next">Writing a Frame Filter</a>, Previous: <a href="Frame-Filter-API.html#Frame-Filter-API" accesskey="p" rel="previous">Frame Filter API</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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