diff --git a/src/pkg/io/utils.go b/src/pkg/io/utils.go
index f3331a20c5..23acfb4ef1 100644
--- a/src/pkg/io/utils.go
+++ b/src/pkg/io/utils.go
@@ -30,8 +30,8 @@ func ReadFile(filename string) ([]byte, os.Error) {
 }
 
 // WriteFile writes data to a file named by filename.
-// If the file does not exist, WriteFile creates it with permissions perm.
-//
+// If the file does not exist, WriteFile creates it with permissions perm;
+// otherwise WriteFile truncates it before writing.
 func WriteFile(filename string, data []byte, perm int) os.Error {
 	f, err := os.Open(filename, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, perm);
 	if err != nil {