diff --git a/Misc/NEWS b/Misc/NEWS
index c2c1c336e75..efd4343af6b 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -124,6 +124,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #9586: Redefine SEM_FAILED on MacOSX to keep compiler happy.
+
 - Issue 10527: make multiprocessing use poll() instead of select() if available.
 
 - Issue #16688: Fix backreferences did make case-insensitive regex fail on
diff --git a/Modules/_multiprocessing/semaphore.c b/Modules/_multiprocessing/semaphore.c
index 8ea92f2b588..2c2b020e3bd 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