syscall: add Flock_t.Lock method

Fixes #7059

R=golang-codereviews, iant, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/53470043

src/pkg/syscall/consistency_unix_test.go

@@ -37,8 +37,8 @@ func _() {
 	_ = syscall.Flock_t{
 		Type:   int16(0),
 		Whence: int16(0),
-		Start:  0,
-		Len:    0,
+		Start:  int64(0),
+		Len:    int64(0),
 		Pid:    int32(0),
 	}
 }

src/pkg/syscall/flock.go

@@ -0,0 +1,22 @@
+// +build linux darwin freebsd openbsd netbsd dragonfly
+
+// Copyright 2014 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 syscall
+
+import "unsafe"
+
+// fcntl64Syscall is usually SYS_FCNTL, but is overridden on 32-bit Linux
+// systems by flock_linux_32bit.go to be SYS_FCNTL64.
+var fcntl64Syscall uintptr = SYS_FCNTL
+
+// Lock performs a fcntl syscall for F_GETLK, F_SETLK or F_SETLKW commands.
+func (lk *Flock_t) Lock(fd uintptr, cmd int) error {
+	_, _, errno := Syscall(fcntl64Syscall, fd, uintptr(cmd), uintptr(unsafe.Pointer(lk)))
+	if errno == 0 {
+		return nil
+	}
+	return errno
+}

src/pkg/syscall/flock_linux_32bit.go

@@ -0,0 +1,13 @@
+// +build linux,386 linux,arm
+
+// Copyright 2014 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 syscall
+
+func init() {
+	// On 32-bit Linux systems, the fcntl syscall that matches Go's
+	// Flock_t type is SYS_FCNTL64, not SYS_FCNTL.
+	fcntl64Syscall = SYS_FCNTL64
+}