Events In Python (Debugging with GDB)

23.3.2.17 Events In Python

GDB provides a general event facility so that Python code can be notified of various state changes, particularly changes that occur in the inferior.

An event is just an object that describes some state change. The type of the object and its attributes will vary depending on the details of the change. All the existing events are described below.

In order to be notified of an event, you must register an event handler with an event registry. An event registry is an object in the gdb.events module which dispatches particular events. A registry provides methods to register and unregister event handlers:

Function: EventRegistry.connect (object); Add the given callable object to the registry. This object will be called when an event corresponding to this registry occurs.

Function: EventRegistry.disconnect (object); Remove the given object from the registry. Once removed, the object will no longer receive notifications of events.

Here is an example:

def exit_handler (event):
    print ("event type: exit")
    if hasattr (event, 'exit_code'):
        print ("exit code: %d" % (event.exit_code))
    else:
        print ("exit code not available")

gdb.events.exited.connect (exit_handler)

In the above example we connect our handler exit_handler to the registry events.exited. Once connected, exit_handler gets called when the inferior exits. The argument event in this example is of type gdb.ExitedEvent. As you can see in the example the ExitedEvent object has an attribute which indicates the exit code of the inferior.

The following is a listing of the event registries that are available and details of the events they emit:

events.cont

Emits gdb.ThreadEvent.

Some events can be thread specific when GDB is running in non-stop mode. When represented in Python, these events all extend gdb.ThreadEvent. Note, this event is not emitted directly; instead, events which are emitted by this or other modules might extend this event. Examples of these events are gdb.BreakpointEvent and gdb.ContinueEvent.

Variable: ThreadEvent.inferior_thread: In non-stop mode this attribute will be set to the specific thread which was involved in the emitted event. Otherwise, it will be set to None.

Emits gdb.ContinueEvent which extends gdb.ThreadEvent.

This event indicates that the inferior has been continued after a stop. For inherited attribute refer to gdb.ThreadEvent above.

events.exited

Emits events.ExitedEvent which indicates that the inferior has exited. events.ExitedEvent has two attributes:

Variable: ExitedEvent.exit_code: An integer representing the exit code, if available, which the inferior has returned. (The exit code could be unavailable if, for example, GDB detaches from the inferior.) If the exit code is unavailable, the attribute does not exist.

Variable: ExitedEvent.inferior: A reference to the inferior which triggered the exited event.

events.stop

Emits gdb.StopEvent which extends gdb.ThreadEvent.

Indicates that the inferior has stopped. All events emitted by this registry extend StopEvent. As a child of gdb.ThreadEvent, gdb.StopEvent will indicate the stopped thread when GDB is running in non-stop mode. Refer to gdb.ThreadEvent above for more details.

Emits gdb.SignalEvent which extends gdb.StopEvent.

This event indicates that the inferior or one of its threads has received as signal. gdb.SignalEvent has the following attributes:

Variable: SignalEvent.stop_signal: A string representing the signal received by the inferior. A list of possible signal values can be obtained by running the command info signals in the GDB command prompt.

Also emits gdb.BreakpointEvent which extends gdb.StopEvent.

gdb.BreakpointEvent event indicates that one or more breakpoints have been hit, and has the following attributes:

Variable: BreakpointEvent.breakpoints: A sequence containing references to all the breakpoints (type gdb.Breakpoint) that were hit. See Breakpoints In Python, for details of the gdb.Breakpoint object.

Variable: BreakpointEvent.breakpoint: A reference to the first breakpoint that was hit. This function is maintained for backward compatibility and is now deprecated in favor of the gdb.BreakpointEvent.breakpoints attribute.

events.new_objfile

Emits gdb.NewObjFileEvent which indicates that a new object file has been loaded by GDB. gdb.NewObjFileEvent has one attribute:

Variable: NewObjFileEvent.new_objfile: A reference to the object file (gdb.Objfile) which has been loaded. See Objfiles In Python, for details of the gdb.Objfile object.

events.clear_objfiles

Emits gdb.ClearObjFilesEvent which indicates that the list of object files for a program space has been reset. gdb.ClearObjFilesEvent has one attribute:

Variable: ClearObjFilesEvent.progspace: A reference to the program space (gdb.Progspace) whose objfile list has been cleared. See Progspaces In Python.

events.inferior_call

Emits events just before and after a function in the inferior is called by GDB. Before an inferior call, this emits an event of type gdb.InferiorCallPreEvent, and after an inferior call, this emits an event of type gdb.InferiorCallPostEvent.

gdb.InferiorCallPreEvent

Indicates that a function in the inferior is about to be called.

Variable: InferiorCallPreEvent.ptid: The thread in which the call will be run.

Variable: InferiorCallPreEvent.address: The location of the function to be called.

gdb.InferiorCallPostEvent

Indicates that a function in the inferior has just been called.

Variable: InferiorCallPostEvent.ptid: The thread in which the call was run.

Variable: InferiorCallPostEvent.address: The location of the function that was called.

events.memory_changed

Emits gdb.MemoryChangedEvent which indicates that the memory of the inferior has been modified by the GDB user, for instance via a command like set *addr = value /@w . The event has the following attributes:

Variable: MemoryChangedEvent.address; The start address of the changed region.

Variable: MemoryChangedEvent.length; Length in bytes of the changed region.

events.register_changed

Emits gdb.RegisterChangedEvent which indicates that a register in the inferior has been modified by the GDB user.

Variable: RegisterChangedEvent.frame; A gdb.Frame object representing the frame in which the register was modified.

Variable: RegisterChangedEvent.regnum; Denotes which register was modified.

events.breakpoint_created

This is emitted when a new breakpoint has been created. The argument that is passed is the new gdb.Breakpoint object.

events.breakpoint_modified

This is emitted when a breakpoint has been modified in some way. The argument that is passed is the new gdb.Breakpoint object.

events.breakpoint_deleted

This is emitted when a breakpoint has been deleted. The argument that is passed is the gdb.Breakpoint object. When this event is emitted, the gdb.Breakpoint object will already be in its invalid state; that is, the is_valid method will return False.

events.before_prompt

This event carries no payload. It is emitted each time GDB presents a prompt to the user.

events.new_inferior

This is emitted when a new inferior is created. Note that the inferior is not necessarily running; in fact, it may not even have an associated executable.

The event is of type gdb.NewInferiorEvent. This has a single attribute:

Variable: NewInferiorEvent.inferior; The new inferior, a gdb.Inferior object.

events.inferior_deleted

This is emitted when an inferior has been deleted. Note that this is not the same as process exit; it is notified when the inferior itself is removed, say via remove-inferiors.

The event is of type gdb.InferiorDeletedEvent. This has a single attribute:

Variable: NewInferiorEvent.inferior; The inferior that is being removed, a gdb.Inferior object.

events.new_thread

This is emitted when GDB notices a new thread. The event is of type gdb.NewThreadEvent, which extends gdb.ThreadEvent. This has a single attribute:

Variable: NewThreadEvent.inferior_thread; The new thread.

events.gdb_exiting

This is emitted when GDB exits. This event is not emitted if GDB exits as a result of an internal error, or after an unexpected signal. The event is of type gdb.GdbExitingEvent, which has a single attribute:

Variable: GdbExitingEvent.exit_code; An integer, the value of the exit code GDB will return.