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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:
Add the given callable object to the registry. This object will be called when an event corresponding to this registry occurs.
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:
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.contEmits 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.exitedEmits events.ExitedEvent, which indicates that the inferior has
exited. events.ExitedEvent has two attributes:
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.
A reference to the inferior which triggered the exited event.
events.stopEmits 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:
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:
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:
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.
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_objfileEmits gdb.NewObjFileEvent which indicates that a new object file has
been loaded by GDB. gdb.NewObjFileEvent has one attribute:
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_objfileEmits 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:
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_objfilesEmits gdb.ClearObjFilesEvent which indicates that the list of object
files for a program space has been reset.
gdb.ClearObjFilesEvent has one attribute:
A reference to the program space (gdb.Progspace) whose objfile list has
been cleared. See Progspaces In Python.
events.inferior_callEmits 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.InferiorCallPreEventIndicates that a function in the inferior is about to be called.
The thread in which the call will be run.
The location of the function to be called.
gdb.InferiorCallPostEventIndicates that a function in the inferior has just been called.
The thread in which the call was run.
The location of the function that was called.
events.memory_changedEmits 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:
The start address of the changed region.
Length in bytes of the changed region.
events.register_changedEmits gdb.RegisterChangedEvent which indicates that a register in the
inferior has been modified by the GDB user.
A gdb.Frame object representing the frame in which the register was modified.
Denotes which register was modified.
events.breakpoint_createdThis is emitted when a new breakpoint has been created. The argument
that is passed is the new gdb.Breakpoint object.
events.breakpoint_modifiedThis is emitted when a breakpoint has been modified in some way. The
argument that is passed is the new gdb.Breakpoint object.
events.breakpoint_deletedThis 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_promptThis event carries no payload. It is emitted each time GDB presents a prompt to the user.
events.new_inferiorThis 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:
The new inferior, a gdb.Inferior object.
events.inferior_deletedThis 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:
The inferior that is being removed, a gdb.Inferior object.
events.new_threadThis is emitted when GDB notices a new thread. The event is of
type gdb.NewThreadEvent, which extends gdb.ThreadEvent.
This has a single attribute:
The new thread.
events.thread_exitedThis 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:
The exiting thread.
events.gdb_exitingThis 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:
An integer, the value of the exit code GDB will return.
events.connection_removedThis 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:
The gdb.TargetConnection that is being removed.
events.executable_changedEmits 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.
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).
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_progspaceThis 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:
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_progspaceThis 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:
The gdb.Progspace that is about to be removed from
GDB.
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