Subj : Two semaphore questions
To   : comp.programming.threads
From : Ian Pilcher
Date : Sat Mar 05 2005 10:02 am

Two newbie questions about (realtime) semaphores:

1.  I initially wanted to use a semaphore as the reference counter for
     a object shared by multiple threads, which seems to be the intended
     use.  The last thread to dereference the object must free it, how-
     ever, and I believe that this requires a method of atomically
     decrementing the semaphore and retrieving its value, something which
     doesn't seem to exist.

     Have I missed something?  (I've fallen back to using a mutex-
     protected int.)

2.  Sort of the reverse of #1.  I'm currently using a mutex-protected
     int as the global exit flag in my application.  From an API point of
     view, a semaphore looks simpler -- sem_post to set the flag and
     sem_getvalue to check it.

     Other than the need to initialize the semaphore, is there a reason
     not to use the semaphore?

Thanks!

-- 
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Ian Pilcher                                        i.pilcher@comcast.net
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