diff --git a/Misc/NEWS b/Misc/NEWS
index ccbf416e3a4..726777ec0a8 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -200,6 +200,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #9586: Redefine SEM_FAILED on MacOSX to keep compiler happy.
+
 - Issue #16787: Increase asyncore and asynchat default output buffers size, to
   decrease CPU usage and increase throughput.
 
diff --git a/Modules/_multiprocessing/semaphore.c b/Modules/_multiprocessing/semaphore.c
index ccd5f017eca..eb3fa0ceb58 100644
--- a/Modules/_multiprocessing/semaphore.c
+++ b/Modules/_multiprocessing/semaphore.c
@@ -186,6 +186,13 @@ semlock_release(SemLockObject *self, PyObject *args)
 #define SEM_GETVALUE(sem, pval) sem_getvalue(sem, pval)
 #define SEM_UNLINK(name) sem_unlink(name)
 
+/* OS X 10.4 defines SEM_FAILED as -1 instead of (sem_t *)-1;  this gives
+   compiler warnings, and (potentially) undefined behaviour. */
+#ifdef __APPLE__
+#  undef SEM_FAILED
+#  define SEM_FAILED ((sem_t *)-1)
+#endif
+
 #ifndef HAVE_SEM_UNLINK
 #  define sem_unlink(name) 0
 #endif