diff --git a/src/ck.h b/src/ck.h
index 3c896789..4639ae89 100644
--- a/src/ck.h
+++ b/src/ck.h
@@ -14,6 +14,10 @@
 #endif
 #include "first.h"
 #ifdef __FreeBSD__
+#ifndef _RSIZE_T_DEFINED /* expecting __EXT1_VISIBLE 1 and _RSIZE_T_DEFINED */
+#define _RSIZE_T_DEFINED
+typedef size_t rsize_t;
+#endif
 #include <errno.h>
 #endif