<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
<html>
|
<!-- Copyright (C) 1988-2021 Free Software Foundation, Inc.
|
|
Permission is granted to copy, distribute and/or modify this document
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
any later version published by the Free Software Foundation; with the
|
Invariant Sections being "Free Software" and "Free Software Needs
|
Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
|
and with the Back-Cover Texts as in (a) below.
|
|
(a) The FSF's Back-Cover Text is: "You are free to copy and modify
|
this GNU Manual. Buying copies from GNU Press supports the FSF in
|
developing GNU and promoting software freedom." -->
|
<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ -->
|
<head>
|
<title>Debugging with GDB: Unwinding Frames in Python</title>
|
|
<meta name="description" content="Debugging with GDB: Unwinding Frames in Python">
|
<meta name="keywords" content="Debugging with GDB: Unwinding Frames in Python">
|
<meta name="resource-type" content="document">
|
<meta name="distribution" content="global">
|
<meta name="Generator" content="makeinfo">
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
<link href="index.html#Top" rel="start" title="Top">
|
<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
<link href="Python-API.html#Python-API" rel="up" title="Python API">
|
<link href="Xmethods-In-Python.html#Xmethods-In-Python" rel="next" title="Xmethods In Python">
|
<link href="Writing-a-Frame-Filter.html#Writing-a-Frame-Filter" rel="previous" title="Writing a Frame Filter">
|
<style type="text/css">
|
<!--
|
a.summary-letter {text-decoration: none}
|
blockquote.smallquotation {font-size: smaller}
|
div.display {margin-left: 3.2em}
|
div.example {margin-left: 3.2em}
|
div.indentedblock {margin-left: 3.2em}
|
div.lisp {margin-left: 3.2em}
|
div.smalldisplay {margin-left: 3.2em}
|
div.smallexample {margin-left: 3.2em}
|
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
|
div.smalllisp {margin-left: 3.2em}
|
kbd {font-style:oblique}
|
pre.display {font-family: inherit}
|
pre.format {font-family: inherit}
|
pre.menu-comment {font-family: serif}
|
pre.menu-preformatted {font-family: serif}
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
pre.smallexample {font-size: smaller}
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
pre.smalllisp {font-size: smaller}
|
span.nocodebreak {white-space:nowrap}
|
span.nolinebreak {white-space:nowrap}
|
span.roman {font-family:serif; font-weight:normal}
|
span.sansserif {font-family:sans-serif; font-weight:normal}
|
ul.no-bullet {list-style: none}
|
-->
|
</style>
|
|
|
</head>
|
|
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
|
<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> [<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 “unwinding” 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 (“sniff” 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 <small>GDB</small>
|
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>
|
<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’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>’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’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—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>’s frame ID
|
using one of functions provided by <small>GDB</small>. <var>frame_id</var>’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—in the DWARF standard, this would be the
|
“Call Frame Address”.
|
</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
|
“special” 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 “wild frame”. 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___(<expects unwinder name argument>)
|
|
def __call__(pending_frame):
|
if not <we recognize frame>:
|
return None
|
# Create UnwindInfo. Usually the frame is identified by the stack
|
# pointer and the program counter.
|
sp = pending_frame.read_register(<SP number>)
|
pc = pending_frame.read_register(<PC number>)
|
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(<register>, <value>)
|
....
|
|
# 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>’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> [<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>
|
|
|
|
</body>
|
</html>
|
