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Posix Semaphores are an optional part of pthreads. There is currently an implementation for Neal Walfields libpthread, which is included in the hurd sources tree. This implemention uses a mutex and a condition variable. The implmentation is in the mailing list archives at [ [http://mail.gnu.org/archive/html/bug-hurd/2002-11/msg00316.html](http://mail.gnu.org/archive/html/bug-hurd/2002-11/msg00316.html</a>)](http://mail.gnu.org/archive/html/bug-hurd/2002-11/msg00316.html).
Neal does not want to use this implementation because it adds the overhead of a condition variable. The condition variable imposes the following penalties: 1 extra spinlock/unlock 1 an extra call to a pthread cleanup function.
The first penalty has virtually no cost because we know that we will never spin trying to get this spin lock because we already have a mutex lock outside the condition variable serializing accesses to the condition variable.
The second may be more of a performance penalty, but it saves reimplmenting the code in pt-cond-signal.c pt-cond-wait.c, and pthread-timedwait.c .
-- [[Main/JamesAMorrison]] - 19 Jan 2003
Moved page to Mach web.
-- [[Main/GrantBow]] - 21 Jan 2003
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