diff --git a/src/os/file.go b/src/os/file.go
index 9b7863e9b6b84a06d181b848357892f9aee2c854..228777c67772f347b47f2eaae47ac0166de8b399 100644
--- a/src/os/file.go
+++ b/src/os/file.go
@@ -73,7 +73,7 @@ const (
 	O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
 	O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist.
 	O_SYNC   int = syscall.O_SYNC   // open for synchronous I/O.
-	O_TRUNC  int = syscall.O_TRUNC  // if possible, truncate file when opened.
+	O_TRUNC  int = syscall.O_TRUNC  // truncate regular writable file when opened.
 )
 
 // Seek whence values.