diff --git a/Misc/NEWS b/Misc/NEWS
index 9e04951..a7e2534 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -189,6 +189,8 @@
 Library
 -------
 
+- Issue #9586: Redefine SEM_FAILED on MacOSX to keep compiler happy.
+
 - Issue 10527: make multiprocessing use poll() instead of select() if available.
 
 - Issue #16485: Fix file descriptor not being closed if file header patching
diff --git a/Modules/_multiprocessing/semaphore.c b/Modules/_multiprocessing/semaphore.c
index 1f37d6a..1ee224d 100644
--- a/Modules/_multiprocessing/semaphore.c
+++ b/Modules/_multiprocessing/semaphore.c
@@ -197,6 +197,13 @@
 #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