diff --git a/mailbox.c b/mailbox.c
index 3f0ece0..f3252eb 100644
--- a/mailbox.c
+++ b/mailbox.c
@@ -33,6 +33,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <sys/mman.h>
 #include <sys/ioctl.h>
 #include <sys/stat.h>
+#include <sys/sysmacros.h>
 
 #include "mailbox.h"