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<title>Debugging with GDB: Basic Python</title>
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<a name="Basic-Python"></a>
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<div class="header">
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<p>
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Next: <a href="Exception-Handling.html#Exception-Handling" accesskey="n" rel="next">Exception Handling</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="Basic-Python-1"></a>
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<h4 class="subsubsection">23.2.2.1 Basic Python</h4>
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<a name="index-python-stdout"></a>
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<a name="index-python-pagination"></a>
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<p>At startup, <small>GDB</small> overrides Python’s <code>sys.stdout</code> and
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<code>sys.stderr</code> to print using <small>GDB</small>’s output-paging streams.
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A Python program which outputs to one of these streams may have its
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output interrupted by the user (see <a href="Screen-Size.html#Screen-Size">Screen Size</a>). In this
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situation, a Python <code>KeyboardInterrupt</code> exception is thrown.
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</p>
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<p>Some care must be taken when writing Python code to run in
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<small>GDB</small>. Two things worth noting in particular:
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</p>
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<ul>
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<li> <small>GDB</small> install handlers for <code>SIGCHLD</code> and <code>SIGINT</code>.
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Python code must not override these, or even change the options using
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<code>sigaction</code>. If your program changes the handling of these
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signals, <small>GDB</small> will most likely stop working correctly. Note
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that it is unfortunately common for GUI toolkits to install a
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<code>SIGCHLD</code> handler.
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</li><li> <small>GDB</small> takes care to mark its internal file descriptors as
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close-on-exec. However, this cannot be done in a thread-safe way on
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all platforms. Your Python programs should be aware of this and
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should both create new file descriptors with the close-on-exec flag
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set and arrange to close unneeded file descriptors before starting a
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child process.
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</li></ul>
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<a name="index-python-functions"></a>
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<a name="index-python-module"></a>
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<a name="index-gdb-module"></a>
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<p><small>GDB</small> introduces a new Python module, named <code>gdb</code>. All
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methods and classes added by <small>GDB</small> are placed in this module.
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<small>GDB</small> automatically <code>import</code>s the <code>gdb</code> module for
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use in all scripts evaluated by the <code>python</code> command.
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</p>
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<p>Some types of the <code>gdb</code> module come with a textual representation
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(accessible through the <code>repr</code> or <code>str</code> functions). These are
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offered for debugging purposes only, expect them to change over time.
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</p>
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<a name="index-gdb_002ePYTHONDIR"></a>
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<dl>
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<dt><a name="index-gdb_002ePYTHONDIR-1"></a>Variable: <strong>gdb.PYTHONDIR</strong></dt>
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<dd><p>A string containing the python directory (see <a href="Python.html#Python">Python</a>).
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</p></dd></dl>
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<a name="index-gdb_002eexecute"></a>
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<dl>
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<dt><a name="index-gdb_002eexecute-1"></a>Function: <strong>gdb.execute</strong> <em>(command <span class="roman">[</span>, from_tty <span class="roman">[</span>, to_string<span class="roman">]]</span>)</em></dt>
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<dd><p>Evaluate <var>command</var>, a string, as a <small>GDB</small> CLI command.
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If a GDB exception happens while <var>command</var> runs, it is
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translated as described in <a href="Exception-Handling.html#Exception-Handling">Exception Handling</a>.
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</p>
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<p>The <var>from_tty</var> flag specifies whether <small>GDB</small> ought to consider this
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command as having originated from the user invoking it interactively.
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It must be a boolean value. If omitted, it defaults to <code>False</code>.
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</p>
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<p>By default, any output produced by <var>command</var> is sent to
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<small>GDB</small>’s standard output (and to the log output if logging is
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turned on). If the <var>to_string</var> parameter is
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<code>True</code>, then output will be collected by <code>gdb.execute</code> and
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returned as a string. The default is <code>False</code>, in which case the
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return value is <code>None</code>. If <var>to_string</var> is <code>True</code>, the
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<small>GDB</small> virtual terminal will be temporarily set to unlimited width
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and height, and its pagination will be disabled; see <a href="Screen-Size.html#Screen-Size">Screen Size</a>.
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</p></dd></dl>
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<a name="index-gdb_002ebreakpoints"></a>
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<dl>
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<dt><a name="index-gdb_002ebreakpoints-1"></a>Function: <strong>gdb.breakpoints</strong> <em>()</em></dt>
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<dd><p>Return a sequence holding all of <small>GDB</small>’s breakpoints.
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See <a href="Breakpoints-In-Python.html#Breakpoints-In-Python">Breakpoints In Python</a>, for more information. In <small>GDB</small>
|
version 7.11 and earlier, this function returned <code>None</code> if there
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were no breakpoints. This peculiarity was subsequently fixed, and now
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<code>gdb.breakpoints</code> returns an empty sequence in this case.
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</p></dd></dl>
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<dl>
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<dt><a name="index-gdb_002erbreak"></a>Function: <strong>gdb.rbreak</strong> <em>(regex <span class="roman">[</span>, minsyms <span class="roman">[</span>, throttle, <span class="roman">[</span>, symtabs <span class="roman">]]]</span>)</em></dt>
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<dd><p>Return a Python list holding a collection of newly set
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<code>gdb.Breakpoint</code> objects matching function names defined by the
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<var>regex</var> pattern. If the <var>minsyms</var> keyword is <code>True</code>, all
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system functions (those not explicitly defined in the inferior) will
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also be included in the match. The <var>throttle</var> keyword takes an
|
integer that defines the maximum number of pattern matches for
|
functions matched by the <var>regex</var> pattern. If the number of
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matches exceeds the integer value of <var>throttle</var>, a
|
<code>RuntimeError</code> will be raised and no breakpoints will be created.
|
If <var>throttle</var> is not defined then there is no imposed limit on the
|
maximum number of matches and breakpoints to be created. The
|
<var>symtabs</var> keyword takes a Python iterable that yields a collection
|
of <code>gdb.Symtab</code> objects and will restrict the search to those
|
functions only contained within the <code>gdb.Symtab</code> objects.
|
</p></dd></dl>
|
|
<a name="index-gdb_002eparameter"></a>
|
<dl>
|
<dt><a name="index-gdb_002eparameter-1"></a>Function: <strong>gdb.parameter</strong> <em>(parameter)</em></dt>
|
<dd><p>Return the value of a <small>GDB</small> <var>parameter</var> given by its name,
|
a string; the parameter name string may contain spaces if the parameter has a
|
multi-part name. For example, ‘<samp>print object</samp>’ is a valid
|
parameter name.
|
</p>
|
<p>If the named parameter does not exist, this function throws a
|
<code>gdb.error</code> (see <a href="Exception-Handling.html#Exception-Handling">Exception Handling</a>). Otherwise, the
|
parameter’s value is converted to a Python value of the appropriate
|
type, and returned.
|
</p></dd></dl>
|
|
<a name="index-gdb_002ehistory"></a>
|
<dl>
|
<dt><a name="index-gdb_002ehistory-1"></a>Function: <strong>gdb.history</strong> <em>(number)</em></dt>
|
<dd><p>Return a value from <small>GDB</small>’s value history (see <a href="Value-History.html#Value-History">Value History</a>). The <var>number</var> argument indicates which history element to return.
|
If <var>number</var> is negative, then <small>GDB</small> will take its absolute value
|
and count backward from the last element (i.e., the most recent element) to
|
find the value to return. If <var>number</var> is zero, then <small>GDB</small> will
|
return the most recent element. If the element specified by <var>number</var>
|
doesn’t exist in the value history, a <code>gdb.error</code> exception will be
|
raised.
|
</p>
|
<p>If no exception is raised, the return value is always an instance of
|
<code>gdb.Value</code> (see <a href="Values-From-Inferior.html#Values-From-Inferior">Values From Inferior</a>).
|
</p></dd></dl>
|
|
<a name="index-gdb_002econvenience_005fvariable"></a>
|
<dl>
|
<dt><a name="index-gdb_002econvenience_005fvariable-1"></a>Function: <strong>gdb.convenience_variable</strong> <em>(name)</em></dt>
|
<dd><p>Return the value of the convenience variable (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience Vars</a>) named <var>name</var>. <var>name</var> must be a string. The name
|
should not include the ‘<samp>$</samp>’ that is used to mark a convenience
|
variable in an expression. If the convenience variable does not
|
exist, then <code>None</code> is returned.
|
</p></dd></dl>
|
|
<a name="index-gdb_002eset_005fconvenience_005fvariable"></a>
|
<dl>
|
<dt><a name="index-gdb_002eset_005fconvenience_005fvariable-1"></a>Function: <strong>gdb.set_convenience_variable</strong> <em>(name, value)</em></dt>
|
<dd><p>Set the value of the convenience variable (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience Vars</a>)
|
named <var>name</var>. <var>name</var> must be a string. The name should not
|
include the ‘<samp>$</samp>’ that is used to mark a convenience variable in an
|
expression. If <var>value</var> is <code>None</code>, then the convenience
|
variable is removed. Otherwise, if <var>value</var> is not a
|
<code>gdb.Value</code> (see <a href="Values-From-Inferior.html#Values-From-Inferior">Values From Inferior</a>), it is is converted
|
using the <code>gdb.Value</code> constructor.
|
</p></dd></dl>
|
|
<a name="index-gdb_002eparse_005fand_005feval"></a>
|
<dl>
|
<dt><a name="index-gdb_002eparse_005fand_005feval-1"></a>Function: <strong>gdb.parse_and_eval</strong> <em>(expression)</em></dt>
|
<dd><p>Parse <var>expression</var>, which must be a string, as an expression in
|
the current language, evaluate it, and return the result as a
|
<code>gdb.Value</code>.
|
</p>
|
<p>This function can be useful when implementing a new command
|
(see <a href="Commands-In-Python.html#Commands-In-Python">Commands In Python</a>), as it provides a way to parse the
|
command’s argument as an expression. It is also useful simply to
|
compute values.
|
</p></dd></dl>
|
|
<a name="index-gdb_002efind_005fpc_005fline"></a>
|
<dl>
|
<dt><a name="index-gdb_002efind_005fpc_005fline-1"></a>Function: <strong>gdb.find_pc_line</strong> <em>(pc)</em></dt>
|
<dd><p>Return the <code>gdb.Symtab_and_line</code> object corresponding to the
|
<var>pc</var> value. See <a href="Symbol-Tables-In-Python.html#Symbol-Tables-In-Python">Symbol Tables In Python</a>. If an invalid
|
value of <var>pc</var> is passed as an argument, then the <code>symtab</code> and
|
<code>line</code> attributes of the returned <code>gdb.Symtab_and_line</code> object
|
will be <code>None</code> and 0 respectively. This is identical to
|
<code>gdb.current_progspace().find_pc_line(pc)</code> and is included for
|
historical compatibility.
|
</p></dd></dl>
|
|
<a name="index-gdb_002epost_005fevent"></a>
|
<dl>
|
<dt><a name="index-gdb_002epost_005fevent-1"></a>Function: <strong>gdb.post_event</strong> <em>(event)</em></dt>
|
<dd><p>Put <var>event</var>, a callable object taking no arguments, into
|
<small>GDB</small>’s internal event queue. This callable will be invoked at
|
some later point, during <small>GDB</small>’s event processing. Events
|
posted using <code>post_event</code> will be run in the order in which they
|
were posted; however, there is no way to know when they will be
|
processed relative to other events inside <small>GDB</small>.
|
</p>
|
<p><small>GDB</small> is not thread-safe. If your Python program uses multiple
|
threads, you must be careful to only call <small>GDB</small>-specific
|
functions in the <small>GDB</small> thread. <code>post_event</code> ensures
|
this. For example:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">(gdb) python
|
>import threading
|
>
|
>class Writer():
|
> def __init__(self, message):
|
> self.message = message;
|
> def __call__(self):
|
> gdb.write(self.message)
|
>
|
>class MyThread1 (threading.Thread):
|
> def run (self):
|
> gdb.post_event(Writer("Hello "))
|
>
|
>class MyThread2 (threading.Thread):
|
> def run (self):
|
> gdb.post_event(Writer("World\n"))
|
>
|
>MyThread1().start()
|
>MyThread2().start()
|
>end
|
(gdb) Hello World
|
</pre></div>
|
</dd></dl>
|
|
<a name="index-gdb_002ewrite"></a>
|
<dl>
|
<dt><a name="index-gdb_002ewrite-1"></a>Function: <strong>gdb.write</strong> <em>(string <span class="roman">[</span>, stream])</em></dt>
|
<dd><p>Print a string to <small>GDB</small>’s paginated output stream. The
|
optional <var>stream</var> determines the stream to print to. The default
|
stream is <small>GDB</small>’s standard output stream. Possible stream
|
values are:
|
</p>
|
<dl compact="compact">
|
<dd><a name="index-STDOUT"></a>
|
<a name="index-gdb_002eSTDOUT"></a>
|
</dd>
|
<dt><code>gdb.STDOUT</code></dt>
|
<dd><p><small>GDB</small>’s standard output stream.
|
</p>
|
<a name="index-STDERR"></a>
|
<a name="index-gdb_002eSTDERR"></a>
|
</dd>
|
<dt><code>gdb.STDERR</code></dt>
|
<dd><p><small>GDB</small>’s standard error stream.
|
</p>
|
<a name="index-STDLOG"></a>
|
<a name="index-gdb_002eSTDLOG"></a>
|
</dd>
|
<dt><code>gdb.STDLOG</code></dt>
|
<dd><p><small>GDB</small>’s log stream (see <a href="Logging-Output.html#Logging-Output">Logging Output</a>).
|
</p></dd>
|
</dl>
|
|
<p>Writing to <code>sys.stdout</code> or <code>sys.stderr</code> will automatically
|
call this function and will automatically direct the output to the
|
relevant stream.
|
</p></dd></dl>
|
|
<a name="index-gdb_002eflush"></a>
|
<dl>
|
<dt><a name="index-gdb_002eflush-1"></a>Function: <strong>gdb.flush</strong> <em>()</em></dt>
|
<dd><p>Flush the buffer of a <small>GDB</small> paginated stream so that the
|
contents are displayed immediately. <small>GDB</small> will flush the
|
contents of a stream automatically when it encounters a newline in the
|
buffer. The optional <var>stream</var> determines the stream to flush. The
|
default stream is <small>GDB</small>’s standard output stream. Possible
|
stream values are:
|
</p>
|
<dl compact="compact">
|
<dd><a name="index-STDOUT-1"></a>
|
<a name="index-gdb_002eSTDOUT-1"></a>
|
</dd>
|
<dt><code>gdb.STDOUT</code></dt>
|
<dd><p><small>GDB</small>’s standard output stream.
|
</p>
|
<a name="index-STDERR-1"></a>
|
<a name="index-gdb_002eSTDERR-1"></a>
|
</dd>
|
<dt><code>gdb.STDERR</code></dt>
|
<dd><p><small>GDB</small>’s standard error stream.
|
</p>
|
<a name="index-STDLOG-1"></a>
|
<a name="index-gdb_002eSTDLOG-1"></a>
|
</dd>
|
<dt><code>gdb.STDLOG</code></dt>
|
<dd><p><small>GDB</small>’s log stream (see <a href="Logging-Output.html#Logging-Output">Logging Output</a>).
|
</p>
|
</dd>
|
</dl>
|
|
<p>Flushing <code>sys.stdout</code> or <code>sys.stderr</code> will automatically
|
call this function for the relevant stream.
|
</p></dd></dl>
|
|
<a name="index-gdb_002etarget_005fcharset"></a>
|
<dl>
|
<dt><a name="index-gdb_002etarget_005fcharset-1"></a>Function: <strong>gdb.target_charset</strong> <em>()</em></dt>
|
<dd><p>Return the name of the current target character set (see <a href="Character-Sets.html#Character-Sets">Character Sets</a>). This differs from <code>gdb.parameter('target-charset')</code> in
|
that ‘<samp>auto</samp>’ is never returned.
|
</p></dd></dl>
|
|
<a name="index-gdb_002etarget_005fwide_005fcharset"></a>
|
<dl>
|
<dt><a name="index-gdb_002etarget_005fwide_005fcharset-1"></a>Function: <strong>gdb.target_wide_charset</strong> <em>()</em></dt>
|
<dd><p>Return the name of the current target wide character set
|
(see <a href="Character-Sets.html#Character-Sets">Character Sets</a>). This differs from
|
<code>gdb.parameter('target-wide-charset')</code> in that ‘<samp>auto</samp>’ is
|
never returned.
|
</p></dd></dl>
|
|
<a name="index-gdb_002esolib_005fname"></a>
|
<dl>
|
<dt><a name="index-gdb_002esolib_005fname-1"></a>Function: <strong>gdb.solib_name</strong> <em>(address)</em></dt>
|
<dd><p>Return the name of the shared library holding the given <var>address</var>
|
as a string, or <code>None</code>. This is identical to
|
<code>gdb.current_progspace().solib_name(address)</code> and is included for
|
historical compatibility.
|
</p></dd></dl>
|
|
<a name="index-gdb_002edecode_005fline"></a>
|
<dl>
|
<dt><a name="index-gdb_002edecode_005fline-1"></a>Function: <strong>gdb.decode_line</strong> <em>(<span class="roman">[</span>expression<span class="roman">]</span>)</em></dt>
|
<dd><p>Return locations of the line specified by <var>expression</var>, or of the
|
current line if no argument was given. This function returns a Python
|
tuple containing two elements. The first element contains a string
|
holding any unparsed section of <var>expression</var> (or <code>None</code> if
|
the expression has been fully parsed). The second element contains
|
either <code>None</code> or another tuple that contains all the locations
|
that match the expression represented as <code>gdb.Symtab_and_line</code>
|
objects (see <a href="Symbol-Tables-In-Python.html#Symbol-Tables-In-Python">Symbol Tables In Python</a>). If <var>expression</var> is
|
provided, it is decoded the way that <small>GDB</small>’s inbuilt
|
<code>break</code> or <code>edit</code> commands do (see <a href="Specify-Location.html#Specify-Location">Specify Location</a>).
|
</p></dd></dl>
|
|
<dl>
|
<dt><a name="index-gdb_002eprompt_005fhook"></a>Function: <strong>gdb.prompt_hook</strong> <em>(current_prompt)</em></dt>
|
<dd><a name="prompt_005fhook"></a>
|
<p>If <var>prompt_hook</var> is callable, <small>GDB</small> will call the method
|
assigned to this operation before a prompt is displayed by
|
<small>GDB</small>.
|
</p>
|
<p>The parameter <code>current_prompt</code> contains the current <small>GDB</small>
|
prompt. This method must return a Python string, or <code>None</code>. If
|
a string is returned, the <small>GDB</small> prompt will be set to that
|
string. If <code>None</code> is returned, <small>GDB</small> will continue to use
|
the current prompt.
|
</p>
|
<p>Some prompts cannot be substituted in <small>GDB</small>. Secondary prompts
|
such as those used by readline for command input, and annotation
|
related prompts are prohibited from being changed.
|
</p></dd></dl>
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