23.3.2.18 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.

Some events can be thread specific when GDB is running in non-stop mode. When represented in Python, these events all extend gdb.ThreadEvent. This event is a base class and is never 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. gdb.ThreadEvent holds the following attributes:

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.

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

events.cont

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 gdb.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.

gdb.StopEvent has the following additional attributes:

Variable: StopEvent.details

A dictionary holding any details relevant to the stop. The exact keys and values depend on the type of stop, but are identical to the corresponding MI output (see GDB/MI Async Records).

A dictionary was used for this (rather than adding attributes directly to the event object) so that the MI keys could be used unchanged.

When a StopEvent results from a finish command, it will also hold the return value from the function, if that is available. This will be an entry named ‘return-value’ in the details dictionary. The value of this entry will be a gdb.Value object.

Emits gdb.SignalEvent, which extends gdb.StopEvent.

This event indicates that the inferior or one of its threads has received a 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 attribute 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.free_objfile

Emits gdb.FreeObjFileEvent which indicates that an object file is about to be removed from GDB. One reason this can happen is when the inferior calls dlclose. gdb.FreeObjFileEvent has one attribute:

Variable: FreeObjFileEvent.objfile

A reference to the object file (gdb.Objfile) which will be unloaded. 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. 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: InferiorDeletedEvent.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.thread_exited

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

Variable: ThreadExitedEvent.inferior_thread

The exiting 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.

events.connection_removed

This is emitted when GDB removes a connection (see Connections In Python). The event is of type gdb.ConnectionEvent. This has a single read-only attribute:

Variable: ConnectionEvent.connection

The gdb.TargetConnection that is being removed.

events.executable_changed

Emits gdb.ExecutableChangedEvent which indicates that the gdb.Progspace.executable_filename has changed.

This event is emitted when either the value of gdb.Progspace.executable_filename has changed to name a different file, or the executable file named by gdb.Progspace.executable_filename has changed on disk, and GDB has therefore reloaded it.

Variable: ExecutableChangedEvent.progspace

The gdb.Progspace in which the current executable has changed. The file name of the updated executable will be visible in gdb.Progspace.executable_filename (see Progspaces In Python).

Variable: ExecutableChangedEvent.reload

This attribute will be True if the value of gdb.Progspace.executable_filename didn’t change, but the file it names changed on disk instead, and GDB reloaded it.

When this attribute is False, the value in gdb.Progspace.executable_filename was changed to name a different file.

Remember that GDB tracks the executable file and the symbol file separately, these are visible as gdb.Progspace.executable_filename and gdb.Progspace.filename respectively. When using the file command, GDB updates both of these fields, but the executable file is updated first, so when this event is emitted, the executable filename will have changed, but the symbol filename might still hold its previous value.

events.new_progspace

This is emitted when GDB adds a new program space (see Program Spaces In Python). The event is of type gdb.NewProgspaceEvent, and has a single read-only attribute:

Variable: NewProgspaceEvent.progspace

The gdb.Progspace that was added to GDB.

No NewProgspaceEvent is emitted for the very first program space, which is assigned to the first inferior. This first program space is created within GDB before any Python scripts are sourced.

events.free_progspace

This is emitted when GDB removes a program space (see Program Spaces In Python), for example as a result of the remove-inferiors command (see remove-inferiors). The event is of type gdb.FreeProgspaceEvent, and has a single read-only attribute:

Variable: FreeProgspaceEvent.progspace

The gdb.Progspace that is about to be removed from GDB.