diff --git a/src/os/os_test.go b/src/os/os_test.go
index 62173d9bf4..717330e86a 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -115,20 +115,16 @@ func size(name string, t *testing.T) int64 {
 	if err != nil {
 		t.Fatal("open failed:", err)
 	}
-	defer file.Close()
-	var buf [100]byte
-	len := 0
-	for {
-		n, e := file.Read(buf[0:])
-		len += n
-		if e == io.EOF {
-			break
-		}
-		if e != nil {
-			t.Fatal("read failed:", e)
+	defer func() {
+		if err := file.Close(); err != nil {
+			t.Error(err)
 		}
+	}()
+	n, err := io.Copy(io.Discard, file)
+	if err != nil {
+		t.Fatal(err)
 	}
-	return int64(len)
+	return n
 }
 
 func equal(name1, name2 string) (r bool) {