Re: Sharing a semaphore between users

From:

"Alexander Grigoriev" <alegr@earthlink.net>

Newsgroups:

microsoft.public.vc.mfc

Date:

Wed, 25 Jun 2008 07:55:41 -0700

Message-ID:

<urK5JOt1IHA.2292@TK2MSFTNGP03.phx.gbl>

"Joseph M. Newcomer" <newcomer@flounder.com> wrote in message
news:d3e3645od2dfcbs5dcjh5aohclr7ghabn5@4ax.com...

Some questions, see below...

SecAtt.nLength=sizeof(SECURITY_DESCRIPTOR);
if ((g_hSem=OpenSemaphore(SEMAPHORE_ALL_ACCESS,TRUE,"Global\
\MyApp"))==NULL)
{
g_hSem = CreateSemaphore(&SecAtt,3,3,"Global\\MyApp");
}

****
In the above sequence, suppose I have two sequences in thread A and B that
can be
represented as o (open) and c (create), where ~ means the operation fails

if the sequences are

Ao~Bo~AcBc
Ao~Bo~BcAc

Why the OpenSemaphore call at all? Why not just CreateSemaphore?
CreateSemaphore says if
the named semaphore already exists, it applies SEMAPHORE_ALL_ACCESS, which
is all that is
happening here.

So what has been gained here? Note the window between open and create
allows the other
thread to create it before the first thread gets to create it.

There is a subtle problem with Create<object> (addressed in Vista, IIRC),
that if the object already exists, the OS will try to open it with
ALL_ACCESS, which may not be granted, though it doesn't apply to this
example. This is why it makes sense to try Open<object> first.