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<title>Debugging with GDB: Frames In Python</title>

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<a name="Frames-In-Python"></a>
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<p>
Next: <a href="Blocks-In-Python.html#Blocks-In-Python" accesskey="n" rel="next">Blocks In Python</a>, Previous: <a href="Objfiles-In-Python.html#Objfiles-In-Python" accesskey="p" rel="previous">Objfiles In Python</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="Accessing-inferior-stack-frames-from-Python"></a>
<h4 class="subsubsection">23.2.2.25 Accessing inferior stack frames from Python</h4>

<a name="index-frames-in-python"></a>
<p>When the debugged program stops, <small>GDB</small> is able to analyze its call
stack (see <a href="Frames.html#Frames">Stack frames</a>).  The <code>gdb.Frame</code> class
represents a frame in the stack.  A <code>gdb.Frame</code> object is only valid
while its corresponding frame exists in the inferior&rsquo;s stack.  If you try
to use an invalid frame object, <small>GDB</small> will throw a <code>gdb.error</code>
exception (see <a href="Exception-Handling.html#Exception-Handling">Exception Handling</a>).
</p>
<p>Two <code>gdb.Frame</code> objects can be compared for equality with the <code>==</code>
operator, like:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) python print gdb.newest_frame() == gdb.selected_frame ()
True
</pre></div>

<p>The following frame-related functions are available in the <code>gdb</code> module:
</p>
<a name="index-gdb_002eselected_005fframe"></a>
<dl>
<dt><a name="index-gdb_002eselected_005fframe-1"></a>Function: <strong>gdb.selected_frame</strong> <em>()</em></dt>
<dd><p>Return the selected frame object.  (see <a href="Selection.html#Selection">Selecting a Frame</a>).
</p></dd></dl>

<a name="index-gdb_002enewest_005fframe"></a>
<dl>
<dt><a name="index-gdb_002enewest_005fframe-1"></a>Function: <strong>gdb.newest_frame</strong> <em>()</em></dt>
<dd><p>Return the newest frame object for the selected thread.
</p></dd></dl>

<dl>
<dt><a name="index-gdb_002eframe_005fstop_005freason_005fstring"></a>Function: <strong>gdb.frame_stop_reason_string</strong> <em>(reason)</em></dt>
<dd><p>Return a string explaining the reason why <small>GDB</small> stopped unwinding
frames, as expressed by the given <var>reason</var> code (an integer, see the
<code>unwind_stop_reason</code> method further down in this section).
</p></dd></dl>

<a name="index-gdb_002einvalidate_005fcached_005fframes"></a>
<dl>
<dt><a name="index-gdb_002einvalidate_005fcached_005fframes-1"></a>Function: <strong>gdb.invalidate_cached_frames</strong></dt>
<dd><p><small>GDB</small> internally keeps a cache of the frames that have been
unwound.  This function invalidates this cache.
</p>
<p>This function should not generally be called by ordinary Python code.
It is documented for the sake of completeness.
</p></dd></dl>

<p>A <code>gdb.Frame</code> object has the following methods:
</p>
<dl>
<dt><a name="index-Frame_002eis_005fvalid"></a>Function: <strong>Frame.is_valid</strong> <em>()</em></dt>
<dd><p>Returns true if the <code>gdb.Frame</code> object is valid, false if not.
A frame object can become invalid if the frame it refers to doesn&rsquo;t
exist anymore in the inferior.  All <code>gdb.Frame</code> methods will throw
an exception if it is invalid at the time the method is called.
</p></dd></dl>

<dl>
<dt><a name="index-Frame_002ename"></a>Function: <strong>Frame.name</strong> <em>()</em></dt>
<dd><p>Returns the function name of the frame, or <code>None</code> if it can&rsquo;t be
obtained.
</p></dd></dl>

<dl>
<dt><a name="index-Frame_002earchitecture"></a>Function: <strong>Frame.architecture</strong> <em>()</em></dt>
<dd><p>Returns the <code>gdb.Architecture</code> object corresponding to the frame&rsquo;s
architecture.  See <a href="Architectures-In-Python.html#Architectures-In-Python">Architectures In Python</a>.
</p></dd></dl>

<dl>
<dt><a name="index-Frame_002etype"></a>Function: <strong>Frame.type</strong> <em>()</em></dt>
<dd><p>Returns the type of the frame.  The value can be one of:
</p><dl compact="compact">
<dt><code>gdb.NORMAL_FRAME</code></dt>
<dd><p>An ordinary stack frame.
</p>
</dd>
<dt><code>gdb.DUMMY_FRAME</code></dt>
<dd><p>A fake stack frame that was created by <small>GDB</small> when performing an
inferior function call.
</p>
</dd>
<dt><code>gdb.INLINE_FRAME</code></dt>
<dd><p>A frame representing an inlined function.  The function was inlined
into a <code>gdb.NORMAL_FRAME</code> that is older than this one.
</p>
</dd>
<dt><code>gdb.TAILCALL_FRAME</code></dt>
<dd><p>A frame representing a tail call.  See <a href="Tail-Call-Frames.html#Tail-Call-Frames">Tail Call Frames</a>.
</p>
</dd>
<dt><code>gdb.SIGTRAMP_FRAME</code></dt>
<dd><p>A signal trampoline frame.  This is the frame created by the OS when
it calls into a signal handler.
</p>
</dd>
<dt><code>gdb.ARCH_FRAME</code></dt>
<dd><p>A fake stack frame representing a cross-architecture call.
</p>
</dd>
<dt><code>gdb.SENTINEL_FRAME</code></dt>
<dd><p>This is like <code>gdb.NORMAL_FRAME</code>, but it is only used for the
newest frame.
</p></dd>
</dl>
</dd></dl>

<dl>
<dt><a name="index-Frame_002eunwind_005fstop_005freason"></a>Function: <strong>Frame.unwind_stop_reason</strong> <em>()</em></dt>
<dd><p>Return an integer representing the reason why it&rsquo;s not possible to find
more frames toward the outermost frame.  Use
<code>gdb.frame_stop_reason_string</code> to convert the value returned by this
function to a string. The value can be one of:
</p>
<dl compact="compact">
<dt><code>gdb.FRAME_UNWIND_NO_REASON</code></dt>
<dd><p>No particular reason (older frames should be available).
</p>
</dd>
<dt><code>gdb.FRAME_UNWIND_NULL_ID</code></dt>
<dd><p>The previous frame&rsquo;s analyzer returns an invalid result.  This is no
longer used by <small>GDB</small>, and is kept only for backward
compatibility.
</p>
</dd>
<dt><code>gdb.FRAME_UNWIND_OUTERMOST</code></dt>
<dd><p>This frame is the outermost.
</p>
</dd>
<dt><code>gdb.FRAME_UNWIND_UNAVAILABLE</code></dt>
<dd><p>Cannot unwind further, because that would require knowing the 
values of registers or memory that have not been collected.
</p>
</dd>
<dt><code>gdb.FRAME_UNWIND_INNER_ID</code></dt>
<dd><p>This frame ID looks like it ought to belong to a NEXT frame,
but we got it for a PREV frame.  Normally, this is a sign of
unwinder failure.  It could also indicate stack corruption.
</p>
</dd>
<dt><code>gdb.FRAME_UNWIND_SAME_ID</code></dt>
<dd><p>This frame has the same ID as the previous one.  That means
that unwinding further would almost certainly give us another
frame with exactly the same ID, so break the chain.  Normally,
this is a sign of unwinder failure.  It could also indicate
stack corruption.
</p>
</dd>
<dt><code>gdb.FRAME_UNWIND_NO_SAVED_PC</code></dt>
<dd><p>The frame unwinder did not find any saved PC, but we needed
one to unwind further.
</p>
</dd>
<dt><code>gdb.FRAME_UNWIND_MEMORY_ERROR</code></dt>
<dd><p>The frame unwinder caused an error while trying to access memory.
</p>
</dd>
<dt><code>gdb.FRAME_UNWIND_FIRST_ERROR</code></dt>
<dd><p>Any stop reason greater or equal to this value indicates some kind
of error.  This special value facilitates writing code that tests
for errors in unwinding in a way that will work correctly even if
the list of the other values is modified in future <small>GDB</small>
versions.  Using it, you could write:
</p><div class="smallexample">
<pre class="smallexample">reason = gdb.selected_frame().unwind_stop_reason ()
reason_str =  gdb.frame_stop_reason_string (reason)
if reason &gt;=  gdb.FRAME_UNWIND_FIRST_ERROR:
    print &quot;An error occured: %s&quot; % reason_str
</pre></div>
</dd>
</dl>

</dd></dl>

<dl>
<dt><a name="index-Frame_002epc"></a>Function: <strong>Frame.pc</strong> <em>()</em></dt>
<dd><p>Returns the frame&rsquo;s resume address.
</p></dd></dl>

<dl>
<dt><a name="index-Frame_002eblock"></a>Function: <strong>Frame.block</strong> <em>()</em></dt>
<dd><p>Return the frame&rsquo;s code block.  See <a href="Blocks-In-Python.html#Blocks-In-Python">Blocks In Python</a>.  If the frame
does not have a block &ndash; for example, if there is no debugging
information for the code in question &ndash; then this will throw an
exception.
</p></dd></dl>

<dl>
<dt><a name="index-Frame_002efunction"></a>Function: <strong>Frame.function</strong> <em>()</em></dt>
<dd><p>Return the symbol for the function corresponding to this frame.
See <a href="Symbols-In-Python.html#Symbols-In-Python">Symbols In Python</a>.
</p></dd></dl>

<dl>
<dt><a name="index-Frame_002eolder"></a>Function: <strong>Frame.older</strong> <em>()</em></dt>
<dd><p>Return the frame that called this frame.
</p></dd></dl>

<dl>
<dt><a name="index-Frame_002enewer"></a>Function: <strong>Frame.newer</strong> <em>()</em></dt>
<dd><p>Return the frame called by this frame.
</p></dd></dl>

<dl>
<dt><a name="index-Frame_002efind_005fsal"></a>Function: <strong>Frame.find_sal</strong> <em>()</em></dt>
<dd><p>Return the frame&rsquo;s symtab and line object.
See <a href="Symbol-Tables-In-Python.html#Symbol-Tables-In-Python">Symbol Tables In Python</a>.
</p></dd></dl>

<a name="gdbpy_005fframe_005fread_005fregister"></a><dl>
<dt><a name="index-Frame_002eread_005fregister"></a>Function: <strong>Frame.read_register</strong> <em>(register)</em></dt>
<dd><p>Return the value of <var>register</var> in this frame.  Returns a
<code>Gdb.Value</code> object.  Throws an exception if <var>register</var> does
not exist.  The <var>register</var> argument must be one of the following:
</p><ol>
<li> A string that is the name of a valid register (e.g., <code>'sp'</code> or
<code>'rax'</code>).
</li><li> A <code>gdb.RegisterDescriptor</code> object (see <a href="Registers-In-Python.html#Registers-In-Python">Registers In Python</a>).
</li><li> A <small>GDB</small> internal, platform specific number.  Using these
numbers is supported for historic reasons, but is not recommended as
future changes to <small>GDB</small> could change the mapping between
numbers and the registers they represent, breaking any Python code
that uses the platform-specific numbers.  The numbers are usually
found in the corresponding <samp><var>platform</var>-tdep.h</samp> file in the
<small>GDB</small> source tree.
</li></ol>
<p>Using a string to access registers will be slightly slower than the
other two methods as <small>GDB</small> must look up the mapping between
name and internal register number.  If performance is critical
consider looking up and caching a <code>gdb.RegisterDescriptor</code>
object.
</p></dd></dl>

<dl>
<dt><a name="index-Frame_002eread_005fvar"></a>Function: <strong>Frame.read_var</strong> <em>(variable <span class="roman">[</span>, block<span class="roman">]</span>)</em></dt>
<dd><p>Return the value of <var>variable</var> in this frame.  If the optional
argument <var>block</var> is provided, search for the variable from that
block; otherwise start at the frame&rsquo;s current block (which is
determined by the frame&rsquo;s current program counter).  The <var>variable</var>
argument must be a string or a <code>gdb.Symbol</code> object; <var>block</var> must be a
<code>gdb.Block</code> object.
</p></dd></dl>

<dl>
<dt><a name="index-Frame_002eselect"></a>Function: <strong>Frame.select</strong> <em>()</em></dt>
<dd><p>Set this frame to be the selected frame.  See <a href="Stack.html#Stack">Examining the
Stack</a>.
</p></dd></dl>

<hr>
<div class="header">
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Next: <a href="Blocks-In-Python.html#Blocks-In-Python" accesskey="n" rel="next">Blocks In Python</a>, Previous: <a href="Objfiles-In-Python.html#Objfiles-In-Python" accesskey="p" rel="previous">Objfiles In Python</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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