Debugging with GDB: Basic Python

Debugging with GDB: Basic Python

23.2.2.1 Basic Python

At startup, GDB overrides Python’s sys.stdout and sys.stderr to print using GDB’s output-paging streams. A Python program which outputs to one of these streams may have its output interrupted by the user (see Screen Size). In this situation, a Python KeyboardInterrupt exception is thrown.

Some care must be taken when writing Python code to run in GDB. Two things worth noting in particular:

GDB install handlers for SIGCHLD and SIGINT. Python code must not override these, or even change the options using sigaction. If your program changes the handling of these signals, GDB will most likely stop working correctly. Note that it is unfortunately common for GUI toolkits to install a SIGCHLD handler.
GDB takes care to mark its internal file descriptors as close-on-exec. However, this cannot be done in a thread-safe way on all platforms. Your Python programs should be aware of this and should both create new file descriptors with the close-on-exec flag set and arrange to close unneeded file descriptors before starting a child process.

GDB introduces a new Python module, named gdb. All methods and classes added by GDB are placed in this module. GDB automatically imports the gdb module for use in all scripts evaluated by the python command.

Variable: gdb.PYTHONDIR: A string containing the python directory (see Python).

Function: gdb.execute (command [, from_tty [, to_string]])

Evaluate command, a string, as a GDB CLI command. If a GDB exception happens while command runs, it is translated as described in Exception Handling.

The from_tty flag specifies whether GDB ought to consider this command as having originated from the user invoking it interactively. It must be a boolean value. If omitted, it defaults to False.

By default, any output produced by command is sent to GDB’s standard output (and to the log output if logging is turned on). If the to_string parameter is True, then output will be collected by gdb.execute and returned as a string. The default is False, in which case the return value is None. If to_string is True, the GDB virtual terminal will be temporarily set to unlimited width and height, and its pagination will be disabled; see Screen Size.

Function: gdb.breakpoints (): Return a sequence holding all of GDB’s breakpoints. See Breakpoints In Python, for more information. In GDB version 7.11 and earlier, this function returned None if there were no breakpoints. This peculiarity was subsequently fixed, and now gdb.breakpoints returns an empty sequence in this case.

Function: gdb.rbreak (regex [, minsyms [, throttle, [, symtabs ]]]): Return a Python list holding a collection of newly set gdb.Breakpoint objects matching function names defined by the regex pattern. If the minsyms keyword is True, all system functions (those not explicitly defined in the inferior) will also be included in the match. The throttle keyword takes an integer that defines the maximum number of pattern matches for functions matched by the regex pattern. If the number of matches exceeds the integer value of throttle, a RuntimeError will be raised and no breakpoints will be created. If throttle is not defined then there is no imposed limit on the maximum number of matches and breakpoints to be created. The symtabs keyword takes a Python iterable that yields a collection of gdb.Symtab objects and will restrict the search to those functions only contained within the gdb.Symtab objects.

Function: gdb.parameter (parameter)

Return the value of a GDB parameter given by its name, a string; the parameter name string may contain spaces if the parameter has a multi-part name. For example, ‘print object’ is a valid parameter name.

If the named parameter does not exist, this function throws a gdb.error (see Exception Handling). Otherwise, the parameter’s value is converted to a Python value of the appropriate type, and returned.

Function: gdb.history (number)

Return a value from GDB’s value history (see Value History). The number argument indicates which history element to return. If number is negative, then GDB 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 number is zero, then GDB will return the most recent element. If the element specified by number doesn’t exist in the value history, a gdb.error exception will be raised.

If no exception is raised, the return value is always an instance of gdb.Value (see Values From Inferior).

Function: gdb.parse_and_eval (expression)

Parse expression, which must be a string, as an expression in the current language, evaluate it, and return the result as a gdb.Value.

This function can be useful when implementing a new command (see Commands In Python), as it provides a way to parse the command’s argument as an expression. It is also useful simply to compute values, for example, it is the only way to get the value of a convenience variable (see Convenience Vars) as a gdb.Value.

Function: gdb.find_pc_line (pc): Return the gdb.Symtab_and_line object corresponding to the pc value. See Symbol Tables In Python. If an invalid value of pc is passed as an argument, then the symtab and line attributes of the returned gdb.Symtab_and_line object will be None and 0 respectively.

Function: gdb.post_event (event)

Put event, a callable object taking no arguments, into GDB’s internal event queue. This callable will be invoked at some later point, during GDB’s event processing. Events posted using post_event 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 GDB.

GDB is not thread-safe. If your Python program uses multiple threads, you must be careful to only call GDB-specific functions in the GDB thread. post_event ensures this. For example:

(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

Function: gdb.write (string [, stream])

Print a string to GDB’s paginated output stream. The optional stream determines the stream to print to. The default stream is GDB’s standard output stream. Possible stream values are:

gdb.STDOUT: GDB’s standard output stream.
gdb.STDERR: GDB’s standard error stream.
gdb.STDLOG: GDB’s log stream (see Logging Output).

Writing to sys.stdout or sys.stderr will automatically call this function and will automatically direct the output to the relevant stream.

Function: gdb.flush ()

Flush the buffer of a GDB paginated stream so that the contents are displayed immediately. GDB will flush the contents of a stream automatically when it encounters a newline in the buffer. The optional stream determines the stream to flush. The default stream is GDB’s standard output stream. Possible stream values are:

gdb.STDOUT: GDB’s standard output stream.
gdb.STDERR: GDB’s standard error stream.
gdb.STDLOG: GDB’s log stream (see Logging Output).

Flushing sys.stdout or sys.stderr will automatically call this function for the relevant stream.

Function: gdb.target_charset (): Return the name of the current target character set (see Character Sets). This differs from gdb.parameter('target-charset') in that ‘auto’ is never returned.

Function: gdb.target_wide_charset (): Return the name of the current target wide character set (see Character Sets). This differs from gdb.parameter('target-wide-charset') in that ‘auto’ is never returned.

Function: gdb.solib_name (address): Return the name of the shared library holding the given address as a string, or None.

Function: gdb.decode_line [expression]: Return locations of the line specified by expression, 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 expression (or None if the expression has been fully parsed). The second element contains either None or another tuple that contains all the locations that match the expression represented as gdb.Symtab_and_line objects (see Symbol Tables In Python). If expression is provided, it is decoded the way that GDB’s inbuilt break or edit commands do (see Specify Location).

Function: gdb.prompt_hook (current_prompt)

If prompt_hook is callable, GDB will call the method assigned to this operation before a prompt is displayed by GDB.

The parameter current_prompt contains the current GDB prompt. This method must return a Python string, or None. If a string is returned, the GDB prompt will be set to that string. If None is returned, GDB will continue to use the current prompt.

Some prompts cannot be substituted in GDB. Secondary prompts such as those used by readline for command input, and annotation related prompts are prohibited from being changed.