## Appendix A gdb Currently available observers

### A.1 Implementation rationale

An observer is an entity which is interested in being notified when GDB reaches certain states, or certain events occur in GDB. The entity being observed is called the subject. To receive notifications, the observer attaches a callback to the subject. One subject can have several observers.

`observer.c` implements an internal generic low-level event notification mechanism. This generic event notification mechanism is then re-used to implement the exported high-level notification management routines for all possible notifications.

The current implementation of the generic observer provides support for contextual data. This contextual data is given to the subject when attaching the callback. In return, the subject will provide this contextual data back to the observer as a parameter of the callback.

Note that the current support for the contextual data is only partial, as it lacks a mechanism that would deallocate this data when the callback is detached. This is not a problem so far, as this contextual data is only used internally to hold a function pointer. Later on, if a certain observer needs to provide support for user-level contextual data, then the generic notification mechanism will need to be enhanced to allow the observer to provide a routine to deallocate the data when attaching the callback.

The observer implementation is also currently not reentrant. In particular, it is therefore not possible to call the attach or detach routines during a notification.

### A.2 Debugging

Observer notifications can be traced using the command ``set debug observer 1`' (see Optional messages about internal happenings).

### A.3 `normal_stop` Notifications

gdb notifies all `normal_stop` observers when the inferior execution has just stopped, the associated messages and annotations have been printed, and the control is about to be returned to the user.

Note that the `normal_stop` notification is not emitted when the execution stops due to a breakpoint, and this breakpoint has a condition that is not met. If the breakpoint has any associated commands list, the commands are executed after the notification is emitted.

The following interfaces are available to manage observers:

— Function: extern struct observer *observer_attach_event (observer_event_ftype *f)

> Using the function f, create an observer that is notified when ever event occurs, return the observer.

— Function: extern void observer_detach_event (struct observer *observer);

> Remove observer from the list of observers to be notified when event occurs.

— Function: extern void observer_notify_event (void);

> Send a notification to all event observers.

The following observable events are defined:

— Function: void normal_stop (struct bpstats *bs)

> The inferior has stopped for real.

— Function: void target_changed (struct target_ops *target)

> The target's register contents have changed.

— Function: void executable_changed (void *unused_args)

> The executable being debugged by GDB has changed: The user decided to debug a different program, or the program he was debugging has been modified since being loaded by the debugger (by being recompiled, for instance).

— Function: void inferior_created (struct target_ops *objfile, int from_tty)

> gdb has just connected to an inferior. For ``run`', gdb calls this observer while the inferior is still stopped at the entry-point instruction. For ``attach`' and ``core`', gdb calls this observer immediately after connecting to the inferior, and before any information on the inferior has been printed.

— Function: void solib_loaded (struct so_list *solib)

> The shared library specified by solib has been loaded. Note that when gdb calls this observer, the library's symbols probably haven't been loaded yet.

— Function: void solib_unloaded (struct so_list *solib)

> The shared library specified by solib has been unloaded.

— Function: void arch_changed (struct gdbarch *old, struct gdbarch *new)

> The current architecture has just been changed. old is the prior architecture, and new is the architecture now current.

— Function: void new_objfile (struct objfile *objfile)

> The symbol file specified by objfile has been loaded. Called with objfile equal to `NULL` to indicate previously loaded symbol table data has now been invalidated.