I inserted comments below. You need to make sure that the critical
section cannot be entered by another thread at the same time.
Otherwise a race condition can occur that result in two camera objects
being created.
Typically this is done by having a mutex associated with the singleton
that you lock at the start of the critical section, and unlock at the
end. Don't forget to unlock before the return inside the critical
section or you will deadlock the third time you call this function.
However, the mutex is not necessary if there is already a mutex around
the call to createInstance() and there is no other code that accesses
the singleton.
sp<CameraHardwareInterface> CameraHal::createInstance()
{
LOG_FUNCTION_NAME
// BEGIN CRITICAL SECTION
if (singleton != 0) {
sp<CameraHardwareInterface> hardware = singleton.promote();
if (hardware != 0) {
return hardware;
}
}
sp<CameraHardwareInterface> hardware(new CameraHal());
singleton = hardware;
// END CRITICAL SECTION
return hardware;
}
On May 19, 6:42 pm, xie <yili....@gmail.com> wrote:
> thanks very much ,there must have a mutex to protect the singleton~~~
>
> ye, it actually has 2 references : the singleton and the fuction return
> value ,but the singeleton is a wp pointer . It just add a weak
> reference , not a strong reference.Do you mean that the caller to
> createInstance() will hold a strong reference when the return value goes
> out of scope ?
>
> 在 2009-05-19二的 08:00 -0700，Dave Sparks写道：
>
> > Yes, it actually has 2 references upon exit: The singleton and the
> > function return value. Presumably the caller to createInstance() also
> > holds onto a reference when the return value goes out of scope.
>
> > I hope there is a mutex protecting the singleton somewhere because
> > this code has a race condition otherwise.
>
> > On May 19, 5:15 am, Freepine <freep...@gmail.com> wrote:
> > > Its reference count gets increased by copy constructor & operator = while
> > > returned from createInstance().
>
> > > On Tue, May 19, 2009 at 4:15 PM, xie <yili....@gmail.com> wrote:
>
> > > > hi Dianne~~
>
> > > > thanks for you answer~~ i have implmented the camera hal with v4l2,
> > > > although i used the below source, i still have some confusion. I think
> > > > the below source is a singleton mode, if the object have been freed we
> > > > will creat a new object . when i creat a new object with this code :
>
> > > > sp<CameraHardwareInterface> hardware(new CameraHal());
>
> > > > i think the "hardware" is a temp variable, it will be deleted when the
> > > > function return. And the sp is a smartpointer ,it will free the object.
> > > > But i am wrong in fact , the object have not been deleted. Can you tell
> > > > my where i am wrong ~~
>
> > > > thanks very much
>
> > > > sp<CameraHardwareInterface> CameraHal::createInstance()
> > > > {
> > > > LOG_FUNCTION_NAME
>
> > > > if (singleton != 0) {
> > > > sp<CameraHardwareInterface> hardware = singleton.promote();
> > > > if (hardware != 0) {
> > > > return hardware;
> > > > }
> > > > }
>
> > > > sp<CameraHardwareInterface> hardware(new CameraHal());
>
> > > > singleton = hardware;
> > > > return hardware;
> > > > }
>
> > > > 在 2009-05-19二的 00:43 -0700，Dianne Hackborn写道：
> > > > > sp == strong pointer, wp == weak pointer.
>
> > > > > The object will remain around while there are strong pointers; it is
> > > > > destroyed once the last one is released. All you can do with a weak
> > > > > pointer is comparison and attempting to promote to a strong pointer;
> > > > > the latter will fail if there are no other strong pointers on the
> > > > > object.
>
> > > > > On Mon, May 18, 2009 at 11:56 PM, xie <yili....@gmail.com> wrote:
>
> > > > > Dear all :
>
> > > > > when i read the android source, i find that "<sp>" is not only
> > > > > a smart
> > > > > pointer,it manage a strong ref count and i also find that the
> > > > > <wp>
> > > > > manage a weak ref count .
>
> > > > > who can tell me how the two kinds of pointer works together,
> > > > > when will
> > > > > the object be freed?
>
> > > > > thanks a lot
>
> > > > > --
> > > > > Dianne Hackborn
> > > > > Android framework engineer
> > > > > hack...@android.com
>
> > > > > Note: please don't send private questions to me, as I don't have time
> > > > > to provide private support, and so won't reply to such e-mails. All
> > > > > such questions should be posted on public forums, where I and others
> > > > > can see and answer them.
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