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-rw-r--r--sem_getvalue.c116
1 files changed, 73 insertions, 43 deletions
diff --git a/sem_getvalue.c b/sem_getvalue.c
index 297bc6c..9d37c38 100644
--- a/sem_getvalue.c
+++ b/sem_getvalue.c
@@ -11,32 +11,32 @@
*
* -------------------------------------------------------------
*
- * Pthreads-win32 - POSIX Threads Library for Win32
- * Copyright(C) 1998 John E. Bossom
- * Copyright(C) 1999,2002 Pthreads-win32 contributors
+ * 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
+ * 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
+ * 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 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.
+ * 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
+ * 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"
@@ -55,20 +55,20 @@ sem_getvalue(sem_t * sem, int * sval)
*
* Return value
*
- * 0 sval has been set.
- * -1 failed, error in errno
+ * 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,
+ * EINVAL 'sem' is not a valid semaphore,
+ * ENOSYS this function is not supported,
*
*
* PARAMETERS
*
- * sem pointer to an instance of sem_t
+ * sem pointer to an instance of sem_t
*
- * sval pointer to int.
+ * sval pointer to int.
*
* DESCRIPTION
* This function stores the current count value of the semaphore
@@ -86,27 +86,57 @@ sem_getvalue(sem_t * sem, int * sval)
{
#ifdef NEED_SEM
-
+
result = ENOSYS;
-
+
#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( (*sem)->sem, 0 ) == WAIT_TIMEOUT )
- {
- /* Failed - must be zero. */
- value = 0;
- }
+ {
+ /* Failed - must be zero */
+ value = 0;
+ }
else
- {
- /* Decremented sema - release it and note the value. */
- (void) ReleaseSemaphore( (*sem)->sem, 1L, &value );
- value++;
- }
-
+ {
+ /* Decremented semaphore - release it and note the value */
+ (void) ReleaseSemaphore( (*sem)->sem, 1L, &value);
+ value++;
+ }
+
#endif
}
-
-
+
+
if ( result != 0 )
{
errno = result;
@@ -115,5 +145,5 @@ sem_getvalue(sem_t * sem, int * sval)
*sval = value;
return 0;
-
+
} /* sem_getvalue */