diff --git a/vendor/golang.org/x/sys/unix/fcntl_darwin.go b/vendor/golang.org/x/sys/unix/fcntl_darwin.go
index 5868a4a..a9911c7 100644
--- a/vendor/golang.org/x/sys/unix/fcntl_darwin.go
+++ b/vendor/golang.org/x/sys/unix/fcntl_darwin.go
@@ -16,3 +16,9 @@
 	_, err := fcntl(int(fd), cmd, int(uintptr(unsafe.Pointer(lk))))
 	return err
 }
+
+// FcntlFstore performs a fcntl syscall for the F_PREALLOCATE command.
+func FcntlFstore(fd uintptr, cmd int, fstore *Fstore_t) error {
+	_, err := fcntl(int(fd), cmd, int(uintptr(unsafe.Pointer(fstore))))
+	return err
+}