unix: add FcntlInt

Fixes golang/go#24649
Fixes golang/go#24677

Change-Id: I3faa74ab68e093a097c3f2a8ec7c054431bdfb3f
Reviewed-on: https://go-review.googlesource.com/104736
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/fcntl.go

@@ -0,0 +1,28 @@
+// 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.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
+package unix
+
+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
+
+// FcntlInt performs a fcntl syscall on fd with the provided command and argument.
+func FcntlInt(fd uintptr, cmd, arg int) (int, error) {
+	valptr, _, err := Syscall(fcntl64Syscall, fd, uintptr(cmd), uintptr(arg))
+	return int(valptr), err
+}
+
+// FcntlFlock performs a fcntl syscall for the F_GETLK, F_SETLK or F_SETLKW command.
+func FcntlFlock(fd uintptr, cmd int, lk *Flock_t) error {
+	_, _, errno := Syscall(fcntl64Syscall, fd, uintptr(cmd), uintptr(unsafe.Pointer(lk)))
+	if errno == 0 {
+		return nil
+	}
+	return errno
+}