unix: add ioctl wrappers to get and set RTC time on Linux

Add IoctlGetRTCTime and IoctlSetRTCTime to get and set the RTC time via
the RTC_RD_TIME and RTC_SET_TIME ioctls.

Change-Id: I63190026be39746599c2cd7116603d4a1d5bd695
Reviewed-on: https://go-review.googlesource.com/c/160397
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux.go

@@ -14,6 +14,7 @@ package unix
 import (
 	"encoding/binary"
 	"net"
+	"runtime"
 	"syscall"
 	"unsafe"
 )
@@ -80,6 +81,12 @@ func ioctlSetTermios(fd int, req uint, value *Termios) error {
 	return ioctl(fd, req, uintptr(unsafe.Pointer(value)))
 }
 
+func IoctlSetRTCTime(fd int, value *RTCTime) error {
+	err := ioctl(fd, RTC_SET_TIME, uintptr(unsafe.Pointer(value)))
+	runtime.KeepAlive(value)
+	return err
+}
+
 // IoctlGetInt performs an ioctl operation which gets an integer value
 // from fd, using the specified request number.
 func IoctlGetInt(fd int, req uint) (int, error) {
@@ -100,6 +107,12 @@ func IoctlGetTermios(fd int, req uint) (*Termios, error) {
 	return &value, err
 }
 
+func IoctlGetRTCTime(fd int) (*RTCTime, error) {
+	var value RTCTime
+	err := ioctl(fd, RTC_RD_TIME, uintptr(unsafe.Pointer(&value)))
+	return &value, err
+}
+
 //sys	Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flags int) (err error)
 
 func Link(oldpath string, newpath string) (err error) {

unix/syscall_linux_test.go

@@ -32,6 +32,21 @@ func TestIoctlGetInt(t *testing.T) {
 	t.Logf("%d bits of entropy available", v)
 }
 
+func TestIoctlGetRTCTime(t *testing.T) {
+	f, err := os.Open("/dev/rtc0")
+	if err != nil {
+		t.Skipf("skipping test, %v", err)
+	}
+	defer f.Close()
+
+	v, err := unix.IoctlGetRTCTime(int(f.Fd()))
+	if err != nil {
+		t.Fatalf("failed to perform ioctl: %v", err)
+	}
+
+	t.Logf("RTC time: %04d-%02d-%02d %02d:%02d:%02d", v.Year+1900, v.Mon+1, v.Mday, v.Hour, v.Min, v.Sec)
+}
+
 func TestPpoll(t *testing.T) {
 	if runtime.GOOS == "android" {
 		t.Skip("mkfifo syscall is not available on android, skipping test")