text: // Copyright 2011 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 aix darwin dragonfly freebsd linux netbsd openbsd solaris
  
  // Socket control messages
  
  package unix
  
  import (
  	"runtime"
  	"unsafe"
  )
  
  // Round the length of a raw sockaddr up to align it properly.
  func cmsgAlignOf(salen int) int {
  	salign := SizeofPtr
  
  	switch runtime.GOOS {
  	case "aix":
  		// There is no alignment on AIX.
  		salign = 1
  	case "darwin", "dragonfly", "solaris", "illumos":
  		// NOTE: It seems like 64-bit Darwin, DragonFly BSD,
  		// illumos, and Solaris kernels still require 32-bit
  		// aligned access to network subsystem.
  		if SizeofPtr == 8 {
  			salign = 4
  		}
  	case "netbsd", "openbsd":
  		// NetBSD and OpenBSD armv7 require 64-bit alignment.
  		if runtime.GOARCH == "arm" {
  			salign = 8
  		}
  	}
  
  	return (salen + salign - 1) & ^(salign - 1)
  }
  
  // CmsgLen returns the value to store in the Len field of the Cmsghdr
  // structure, taking into account any necessary alignment.
  func CmsgLen(datalen int) int {
  	return cmsgAlignOf(SizeofCmsghdr) + datalen
  }
  
  // CmsgSpace returns the number of bytes an ancillary element with
  // payload of the passed data length occupies.
  func CmsgSpace(datalen int) int {
  	return cmsgAlignOf(SizeofCmsghdr) + cmsgAlignOf(datalen)
  }
  
  func cmsgData(h *Cmsghdr) unsafe.Pointer {
  	return unsafe.Pointer(uintptr(unsafe.Pointer(h)) + uintptr(cmsgAlignOf(SizeofCmsghdr)))
  }
  
  // SocketControlMessage represents a socket control message.
  type SocketControlMessage struct {
  	Header Cmsghdr
  	Data   []byte
  }
  
  // ParseSocketControlMessage parses b as an array of socket control
  // messages.
  func ParseSocketControlMessage(b []byte) ([]SocketControlMessage, error) {
  	var msgs []SocketControlMessage
  	i := 0
  	for i+CmsgLen(0) <= len(b) {
  		h, dbuf, err := socketControlMessageHeaderAndData(b[i:])
  		if err != nil {
  			return nil, err
  		}
  		m := SocketControlMessage{Header: *h, Data: dbuf}
  		msgs = append(msgs, m)
  		i += cmsgAlignOf(int(h.Len))
  	}
  	return msgs, nil
  }
  
  func socketControlMessageHeaderAndData(b []byte) (*Cmsghdr, []byte, error) {
  	h := (*Cmsghdr)(unsafe.Pointer(&b[0]))
  	if h.Len < SizeofCmsghdr || uint64(h.Len) > uint64(len(b)) {
  		return nil, nil, EINVAL
  	}
  	return h, b[cmsgAlignOf(SizeofCmsghdr):h.Len], nil
  }
  
  // UnixRights encodes a set of open file descriptors into a socket
  // control message for sending to another process.
  func UnixRights(fds ...int) []byte {
  	datalen := len(fds) * 4
  	b := make([]byte, CmsgSpace(datalen))
  	h := (*Cmsghdr)(unsafe.Pointer(&b[0]))
  	h.Level = SOL_SOCKET
  	h.Type = SCM_RIGHTS
  	h.SetLen(CmsgLen(datalen))
  	data := cmsgData(h)
  	for _, fd := range fds {
  		*(*int32)(data) = int32(fd)
  		data = unsafe.Pointer(uintptr(data) + 4)
  	}
  	return b
  }
  
  // ParseUnixRights decodes a socket control message that contains an
  // integer array of open file descriptors from another process.
  func ParseUnixRights(m *SocketControlMessage) ([]int, error) {
  	if m.Header.Level != SOL_SOCKET {
  		return nil, EINVAL
  	}
  	if m.Header.Type != SCM_RIGHTS {
  		return nil, EINVAL
  	}
  	fds := make([]int, len(m.Data)>>2)
  	for i, j := 0, 0; i < len(m.Data); i += 4 {
  		fds[j] = int(*(*int32)(unsafe.Pointer(&m.Data[i])))
  		j++
  	}
  	return fds, nil
  }
  