Subj : Re: when should use mutex/semaphore
To   : comp.programming.threads
From : David Butenhof
Date : Wed Jan 12 2005 05:35 pm

sachin_mzn@yahoo.com wrote:
> 
> Sometimes I am confused about usage of mutex and semaphore.
> When we should use one. Is mutex is best suitable for threads and
> semaphore for process.

The best (though perhaps not the most easily applied) advice is to use 
the synchronization primitive that best fits your need. ;-)

> What is diffrence between binary semaphore and mutex?

A mutex is a binary semaphore with some additional consistency state. To 
put the distinction in simple terms, a binary semaphore is "locked" or 
"unlocked", while a mutex is "owned" or "released". When a binary 
semaphore is "locked", you can unlock from any code path in any thread; 
there's no consistency checks. When a mutex is "owned", only the thread 
that owns it can release it.

In many implementations, a mutex really is just a binary semaphore, and 
the code doesn't detect or enforce ownership violations. But tracking 
ownership provides the ability to, for example, return EDEADLK when a 
thread attempts to lock a mutex it already owns instead of blocking 
indefinitely. The consistency checks also protect against code errors 
such as skipping a lock operation on some path, which can lead to a 
thread accidentally unlocking a mutex that's already owned by another 
thread.

> When should we use counting semaphore?

When you have a counted resource, such that you're writing 
synchronization code that reads essentially "lock, increment, unlock" 
and "lock, decrease, unlock", you may be better off with a counting 
semaphore that does all this in one operation; especially if you're 
waiting at a boundary condition. For example, if you have a fixed queue 
of 10 entries that one thread produces for another thread to consume, a 
counting semaphore with an initial value of 10 might be just what you want.

> Is there any good book/tutorial on
> multithreading/semaphore/mutex/condition variable available online?

Lots of tutorials online, some of them are OK but limited in scope 
(often setting the stage for a particular class project), while others 
are out of date, poorly structured, or outright incorrect. You can 
search and evaluate; I'm sure Google will be happy to report any number 
of them you want.

There are also several good printed books that you can buy in a store or 
online.

-- 
Dave Butenhof, David.Butenhof@hp.com
HP Utility Pricing software, POSIX thread consultant
Manageability Solutions Lab (MSL), Hewlett-Packard Company
110 Spit Brook Road, ZK2/3-Q18, Nashua, NH 03062

.