[haiku-3rdparty-dev] Re: Problem sending/receiving app defined msgs
- From: Ingo Weinhold <ingo_weinhold@xxxxxx>
- To: haiku-3rdparty-dev@xxxxxxxxxxxxx
- Date: Sun, 05 Dec 2010 15:28:50 +0100
On 2010-12-05 at 14:15:49 [+0100], Axel Dörfler <axeld@xxxxxxxxxxxxxxxx>
wrote:
> Using the window as pointer is indeed not thread-safe - creating a
> BMessenger out of it is, though.
Formulated as generally as that the statement isn't quite accurate. In both
cases the how is relevant. Only dereferencing the window pointer is
universally unsafe (unless, e.g. by locking, it has been ensured first that
the window exists and won't go away, of course).
There are several methods that can be invoked, which don't dereference the
pointer uncheckedly and are thus safe to be called using the pointer. Most
relevantly for this discussion PostMessage(), but also Lock() (which
returns false in case the window is already toast) and all methods that
first call Lock() and check its return value.
It is important to note that BMessenger(window) is an *unsafe* call, since
the window is passed in as a BHandler* which has to be dereferenced to get
the respective looper. BMessenger(NULL, window) is safe, but also has a
different semantics (targeting the preferred handler which is not
necessarily the window). Unlike window->PostMessage(..., window) using
BMessenger(window, window) is unsafe in the current Haiku implementation
(though that could be changed).
While using a potentially invalid window pointer with PostMessage() or the
NULL-BHandler* BMessenger() constructor is safe in that it won't crash, it
still has an issue: If the window was deleted and in the meantime a new
BWindow (or BLooper or any derived class) has been created at the same
address in memory, the message will be sent to the new object. The only
really safe method is to create a BMessenger as long as the window object
is still valid and use that for sending messages. After the destruction of
the window sending messages via the messenger will simply fail.
CU, Ingo
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