diff --git a/gpsctl.c b/gpsctl.c
index e14c1042..ff1ea6b7 100644
--- a/gpsctl.c
+++ b/gpsctl.c
@@ -4,6 +4,12 @@
  * BSD terms apply: see the file COPYING in the distribution root for details.
  *
  */
+
+/* sys/ipc.h needs _XOPEN_SOURCE, 500 means X/Open 1995 */
+#define _XOPEN_SOURCE 500
+/* pselect() needs _POSIX_C_SOURCE >= 200112L */
+#define _POSIX_C_SOURCE 200112L
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdbool.h>
diff --git a/libgps_shm.c b/libgps_shm.c
index d8e11f75..a9250ca0 100644
--- a/libgps_shm.c
+++ b/libgps_shm.c
@@ -15,7 +15,7 @@ PERMISSIONS
 
 ***************************************************************************/
 
-/* sys/ipc.h needs _XOPEN_SOURCE, 500 mean X/Open 1995 */
+/* sys/ipc.h needs _XOPEN_SOURCE, 500 means X/Open 1995 */
 #define _XOPEN_SOURCE 500
 
 #include <time.h>             /* for time_t */
diff --git a/ntpshmwrite.c b/ntpshmwrite.c
index 9cb43a2c..a9bfa02a 100644
--- a/ntpshmwrite.c
+++ b/ntpshmwrite.c
@@ -5,6 +5,9 @@
  * see the file COPYING in the distribution root for details.
  */
 
+/* sys/ipc.h needs _XOPEN_SOURCE, 500 means X/Open 1995 */
+#define _XOPEN_SOURCE 500
+
 #include <string.h>
 #include <libgen.h>
 #include <stdbool.h>
diff --git a/shmexport.c b/shmexport.c
index 2a0e4166..cef94d77 100644
--- a/shmexport.c
+++ b/shmexport.c
@@ -14,6 +14,10 @@ PERMISSIONS
    BSD terms apply: see the file COPYING in the distribution root for details.
 
 ***************************************************************************/
+
+/* sys/ipc.h needs _XOPEN_SOURCE, 500 means X/Open 1995 */
+#define _XOPEN_SOURCE 500
+
 #include <time.h>            /* for time_t */
 #include "gpsd_config.h"