01-28-2002
Shared memory and C++ Objects (cont)
I asked the next question in a former thread :
Can I safely share objects that have virtual functions (i.e. have
virtual function table pointers) between two processes ?
Where will the pointers point to in each process ?
The objects are supposed to reside in shared memory
I received a partial answer from Wizard (thanx, btw), but
was not satisfied : I am using AIX, and want to know if
I can solve the pointer problem via a OS mechanism, or,
given that this is not possible, a language interface.
What is that "shared libraries"
gizmo Wizard referred to ?
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LEARN ABOUT DEBIAN
dispatch_retain
dispatch_object(3) BSD Library Functions Manual dispatch_object(3)
NAME
dispatch_object -- General manipulation of dispatch objects
SYNOPSIS
#include <dispatch/dispatch.h>
void
dispatch_retain(dispatch_object_t object);
void
dispatch_release(dispatch_object_t object);
void
dispatch_suspend(dispatch_object_t object);
void
dispatch_resume(dispatch_object_t object);
void *
dispatch_get_context(dispatch_object_t object);
void
dispatch_set_context(dispatch_object_t object, void *context);
void
dispatch_set_finalizer_f(dispatch_object_t object, dispatch_function_t finalizer);
DESCRIPTION
Dispatch objects share functions for coordinating memory management, suspension, cancellation and context pointers. While all dispatch
objects are retainable, not all objects support suspension, context pointers or finalizers (currently only queues and sources support these
additional interfaces).
MEMORY MANGEMENT
Objects returned by creation functions in the dispatch framework may be uniformly retained and released with the functions dispatch_retain()
and dispatch_release() respectively.
The dispatch framework does not guarantee that any given client has the last or only reference to a given object. Objects may be retained
internally by the system.
SUSPENSION
The invocation of blocks on dispatch queues or dispatch sources may be suspended or resumed with the functions dispatch_suspend() and
dispatch_resume() respectively. The dispatch framework always checks the suspension status before executing a block, but such changes never
affect a block during execution (non-preemptive). Therefore the suspension of an object is asynchronous, unless it is performed from the
context of the target queue for the given object. The result of suspending or resuming an object that is not a dispatch queue or a dispatch
source is undefined.
Important: suspension applies to all aspects of the dispatch object life cycle, including the finalizer function and cancellation handler.
Therefore it is important to balance calls to dispatch_suspend() and dispatch_resume() such that the dispatch object is fully resumed when
the last reference is released. The result of releasing all references to a dispatch object while in a suspended state is undefined.
CONTEXT POINTERS
Dispatch queues and sources support supplemental context pointers. The value of the context pointer may be retrieved and updated with
dispatch_get_context() and dispatch_set_context() respectively. The dispatch_set_finalizer_f() specifies an optional per-object finalizer
function that is invoked asynchronously if the context pointer is not NULL when the last reference to the object is released. This gives the
application an opportunity to free the context data associated with the object.
The result of getting or setting the context of an object that is not a dispatch queue or a dispatch source is undefined.
SEE ALSO
dispatch(3), dispatch_group_create(3), dispatch_queue_create(3), dispatch_semaphore_create(3), dispatch_source_create(3)
Darwin May 1, 2009 Darwin