Subj : Re: Boss Told Me You Can't Use Semaphore, But I Have To Do Something like It!
To   : comp.programming.threads
From : Charles Bryant
Date : Tue Jul 19 2005 11:19 pm

In article <1121807491.741451.96600@g47g2000cwa.googlegroups.com>,
Sean Kelly <sean@f4.ca> wrote:
>Instead of a 1, why not write the app's PID into this memory segment.
>So each app waits for 0, writes its PID, then reads to see if its PID
>is set.  If so, it continues.  Not as nice as a semaphore,
...

Not as nice especially because it won't work. Suppose there are two
presecces, A and B. They might run as follows:
	A sees 0
	A is suspended
				B runs
				B sees 0
				B writes B
				B sees B
				B assumes it has the lock
				B is suspended
	A runs
	A writes A
	A sees A
	A assumes it has the lock

	Oops!

It is extremely difficult to invent correct concurrent algorithms -
which is why it's nearly always best to use things like semaphores
which have been designed by experts and extensively tested.

.