*** empty log message ***

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 */