Debugging with GDB: Events In Python

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#### 23.2.2.17 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"
        print "exit code: %d" % (event.exit_code)

    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_pre`

  Emits `gdb.InferiorCallPreEvent` which 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.

`events.inferior_call_post`

  Emits `gdb.InferiorCallPostEvent` which indicates that a function in the inferior has returned.

  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`.

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