diff --git a/api/apiplan.c b/api/apiplan.c
index b8642a9..c7df958 100644
--- a/api/apiplan.c
+++ b/api/apiplan.c
@@ -20,7 +20,50 @@
 
 #include "api/api.h"
 
-static planner_hook_t before_planner_hook = 0, after_planner_hook = 0;
+#if defined(__WIN32__) || defined(_WIN32) || defined(_WINDOWS)
+/* hack: windef.h defines INT for its own purposes and this causes
+   a conflict with our own INT in ifftw.h.  Divert the windows
+   definition into another name unlikely to cause a conflict */
+#define INT magnus_ab_INTegro_seclorum_nascitur_ordo
+#include <windows.h>
+#include <process.h>
+#include <intrin.h>
+#undef INT
+
+/* windows does not have statically-initialized mutexes---fake a
+   spinlock */
+static volatile LONG planner_mutex = 0;
+
+static void lock_planner_mutex(void)
+{
+     while (InterlockedExchange(&planner_mutex, 1) == 1) {
+          YieldProcessor();
+          Sleep(0);
+     }
+}
+
+static void unlock_planner_mutex(void)
+{
+     LONG old = InterlockedExchange(&planner_mutex, 0);
+     A(old == 1);
+}
+#else
+#include <pthread.h>
+
+static pthread_mutex_t planner_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+static void lock_planner_mutex(void)
+{
+     pthread_mutex_lock(&planner_mutex);
+}
+
+static void unlock_planner_mutex(void)
+{
+     pthread_mutex_unlock(&planner_mutex);
+}
+#endif
+
+static planner_hook_t before_planner_hook = lock_planner_mutex, after_planner_hook = unlock_planner_mutex;
 
 void X(set_planner_hooks)(planner_hook_t before, planner_hook_t after)
 {