diff --git a/sink.go b/sink.go
index fbccc83..119f1c0 100644
--- a/sink.go
+++ b/sink.go
@@ -2,6 +2,7 @@ package uio
 
 type Sink interface {
 	Write(src []byte) (n int, err error)
+	WriteFully(src []byte) error
 }
 
 type BufferedSink interface {
diff --git a/source.go b/source.go
index 393d84b..b03f7c2 100644
--- a/source.go
+++ b/source.go
@@ -2,6 +2,7 @@ package uio
 
 type Source interface {
 	Read(dst []byte) (n int, err error)
+	ReadFully(dst []byte) error
 }
 
 type BufferedSource interface {