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Date: Tue, 5 Jan 2010 09:37:02 -0500 (EST)
From: Daniel Eischen <deischen@freebsd.org>
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To: David Xu <davidxu@freebsd.org>
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Cc: threads@freebsd.org
Subject: Re: Underline version of semaphore functions
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On Tue, 5 Jan 2010, David Xu wrote:

> I saw underline version of semaphore functions in
> libc/include/namespace.h:
>
> #define         sem_close                       _sem_close
> #define         sem_destroy                     _sem_destroy
> #define         sem_getvalue                    _sem_getvalue
> #define         sem_init                        _sem_init
> #define         sem_open                        _sem_open
> #define         sem_post                        _sem_post
> #define         sem_timedwait                   _sem_timedwait
> #define         sem_trywait                     _sem_trywait
> #define         sem_unlink                      _sem_unlink
> #define         sem_wait                        _sem_wait
>
>
> but libc never has them:

Yes, because libpthread has them and the thread library
was needed in order to provide semaphores.  These single
underscore functions are for libc internal usage so that
libthr (libpthread) may know the difference between
semaphores created/used by libc and those created/used
by an application.  The provider of semaphores (and
mutexes, condvars, etc) may need to know who created
and uses these objects (libc or application), so for
example on a fork() or perhaps thread_safe_fork()
all those objects created by libc can be automatically
reinitialized.  Or perhaps so that we know whether
to delay thread cancellation until a thread releases
these internal locks.

-- 
DE