diff --git a/src/getentropy_bsd.c b/src/getentropy_bsd.c
index 705f65b..551b646 100644
--- a/src/getentropy_bsd.c
+++ b/src/getentropy_bsd.c
@@ -26,6 +26,8 @@
 #include <errno.h>
 #include <stddef.h>
 
+int	getentropy(void *buf, size_t len);
+
 /*
  * Derived from lib/libc/gen/arc4random.c from FreeBSD.
  */