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| -rw-r--r-- | BUGS | 35 |
1 files changed, 34 insertions, 1 deletions
@@ -102,4 +102,37 @@ Known bugs the pthreads library.
-
+3. sem_getvalue sometimes returns lower value than expected
+
+ The following comments from the code explain the problem:
+
+ /*
+ * 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.
+ */
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