os, syscall: implement ReadAt for unix

Windows will take more work, so test is skipped there.

src/os/os_test.go

@@ -0,0 +1,48 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package os_test
+
+import (
+	"io"
+	. "os"
+	"runtime"
+	"testing"
+)
+
+// localTmp returns a local temporary directory not on NFS.
+func localTmp() string {
+	return TempDir()
+}
+
+func newFile(testName string, t *testing.T) (f *File) {
+	// TODO: use CreateTemp when it lands
+	f, err := OpenFile(TempDir()+"/_Go_"+testName, O_RDWR|O_CREATE, 0644)
+	if err != nil {
+		t.Fatalf("TempFile %s: %s", testName, err)
+	}
+	return
+}
+
+func TestReadAt(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Log("TODO: implement Pread for Windows")
+		return
+	}
+	f := newFile("TestReadAt", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	const data = "hello, world\n"
+	io.WriteString(f, data)
+
+	b := make([]byte, 5)
+	n, err := f.ReadAt(b, 7)
+	if err != nil || n != len(b) {
+		t.Fatalf("ReadAt 7: %d, %v", n, err)
+	}
+	if string(b) != "world" {
+		t.Fatalf("ReadAt 7: have %q want %q", string(b), "world")
+	}
+}