Update to gobook@c6d7a22edd03e7738c38d5baa2174ff325d8d863

ch13/unsafeptr/main.go

@@ -0,0 +1,38 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 357.
+
+// Package unsafeptr demonstrates basic use of unsafe.Pointer.
+package main
+
+import (
+	"fmt"
+	"unsafe"
+)
+
+func main() {
+	//!+main
+	var x struct {
+		a bool
+		b int16
+		c []int
+	}
+
+	// equivalent to pb := &x.b
+	pb := (*int16)(unsafe.Pointer(
+		uintptr(unsafe.Pointer(&x)) + unsafe.Offsetof(x.b)))
+	*pb = 42
+
+	fmt.Println(x.b) // "42"
+	//!-main
+}
+
+/*
+//!+wrong
+	// NOTE: subtly incorrect!
+	tmp := uintptr(unsafe.Pointer(&x)) + unsafe.Offsetof(x.b)
+	pb := (*int16)(unsafe.Pointer(tmp))
+	*pb = 42
+//!-wrong
+*/