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/*
* -------------------------------------------------------------
*
* Module: sem_getvalue.c
*
* Purpose:
* Semaphores aren't actually part of PThreads.
* They are defined by the POSIX Standard:
*
* POSIX 1003.1-2001
*
* -------------------------------------------------------------
*
* Pthreads-win32 - POSIX Threads Library for Win32
* Copyright(C) 1998 John E. Bossom
* Copyright(C) 1999,2002 Pthreads-win32 contributors
*
* Contact Email: rpj@ise.canberra.edu.au
*
* The current list of contributors is contained
* in the file CONTRIBUTORS included with the source
* code distribution. The list can also be seen at the
* following World Wide Web location:
* http://sources.redhat.com/pthreads-win32/contributors.html
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library in the file COPYING.LIB;
* if not, write to the Free Software Foundation, Inc.,
* 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
*/
#include "pthread.h"
#include "semaphore.h"
#include "implement.h"
int
sem_getvalue(sem_t * sem, int * sval)
/*
* ------------------------------------------------------
* DOCPUBLIC
* This function stores the current count value of the
* semaphore.
* RESULTS
*
* Return value
*
* 0 sval has been set.
* -1 failed, error in errno
*
* in global errno
*
* EINVAL 'sem' is not a valid semaphore,
* ENOSYS this function is not supported,
*
*
* PARAMETERS
*
* sem pointer to an instance of sem_t
*
* sval pointer to int.
*
* DESCRIPTION
* This function stores the current count value of the semaphore
* pointed to by sem in the int pointed to by sval.
*/
{
int result = 0;
long value;
if ( sem == NULL || *sem == NULL || sval == NULL )
{
result = EINVAL;
}
else
{
register sem_t s = *sem;
#ifdef NEED_SEM
EnterCriticalSection(&s->sem_lock_cs);
value = s->value;
LeaveCriticalSection(&s->sem_lock_cs);
#else
/*
* There appears to be NO atomic means of determining the
* value of the semaphore. Using only ReleaseSemaphore()
* with either a zero or oversized count parameter has been
* suggested but this trick doesn't produce consistent results
* across Windows versions (the technique uses values that are
* knowingly illegal but hopes to extract the current value
* anyway - the zero parameter appears to work for Win9x but
* neither work reliably for WinNT).
*
* The intrusive method below will give false results
* at times but it at least errs on the side of
* caution. Competing threads might occasionally believe
* the semaphore has a count of one less than it actually
* would have and possibly block momentarily unecessarily,
* but they will never see a higher semaphore value than
* there should be.
*
*
* Multiple threads calling sem_getvalue() at the same time
* may not all return the same value (assuming no calls to
* other semaphore routines). They will always return the
* correct value or a lesser value. This problem could be fixed
* with a global or a per-semaphore critical section here.
*
* An equally approximate but IMO slightly riskier approach
* would be to keep a separate per-semaphore counter and
* decrement/increment it inside of sem_wait() and sem_post()
* etc using the Interlocked* functions.
*/
if ( WaitForSingleObject( s->sem, 0 ) == WAIT_TIMEOUT )
{
/* Failed - must be zero */
value = 0;
}
else
{
/* Decremented semaphore - release it and note the value */
(void) ReleaseSemaphore( s->sem, 1L, &value);
value++;
}
#endif
}
if ( result != 0 )
{
errno = result;
return -1;
}
*sval = value;
return 0;
} /* sem_getvalue */
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