func (m ControlMessage) MarshalHeader(lvl, typ, dataLen int) error {
	if len(m) < controlHeaderLen() {
		return errors.New("short message")
	}
	h := (*cmsghdr)(unsafe.Pointer(&m[0]))
	h.set(controlMessageLen(dataLen), lvl, typ)
	return nil
}
MarshalHeader marshals the header fields of the control message at
the head on m.
package socket  
import (
	"errors"
	"net"
	"unsafe"
)
type Option struct {
	Level int  
	Name  int  
	Len   int  
}
func (o *Option) Get(c *Conn, b []byte) (int, error) {
	if o.Name < 1 || o.Len < 1 {
		return 0, errors.New("invalid option")
	}
	if len(b) < o.Len {
		return 0, errors.New("short buffer")
	}
	return o.get(c, b)
}
func (o *Option) GetInt(c *Conn) (int, error) {
	if o.Len != 1 && o.Len != 4 {
		return 0, errors.New("invalid option")
	}
	var b []byte
	var bb [4]byte
	if o.Len == 1 {
		b = bb[:1]
	} else {
		b = bb[:4]
	}
	n, err := o.get(c, b)
	if err != nil {
		return 0, err
	}
	if n != o.Len {
		return 0, errors.New("invalid option length")
	}
	if o.Len == 1 {
		return int(b[0]), nil
	}
	return int(NativeEndian.Uint32(b[:4])), nil
}
func (o *Option) Set(c *Conn, b []byte) error {
	if o.Name < 1 || o.Len < 1 {
		return errors.New("invalid option")
	}
	if len(b) < o.Len {
		return errors.New("short buffer")
	}
	return o.set(c, b)
}
func (o *Option) SetInt(c *Conn, v int) error {
	if o.Len != 1 && o.Len != 4 {
		return errors.New("invalid option")
	}
	var b []byte
	if o.Len == 1 {
		b = []byte{byte(v)}
	} else {
		var bb [4]byte
		NativeEndian.PutUint32(bb[:o.Len], uint32(v))
		b = bb[:4]
	}
	return o.set(c, b)
}
func controlHeaderLen() int {
	return roundup(sizeofCmsghdr)
}
func controlMessageLen(dataLen int) int {
	return roundup(sizeofCmsghdr) + dataLen
}
func ControlMessageSpace(dataLen int) int {
	return roundup(sizeofCmsghdr) + roundup(dataLen)
}
type ControlMessage []byte
func (m ControlMessage) Data(dataLen int) []byte {
	l := controlHeaderLen()
	if len(m) < l || len(m) < l+dataLen {
		return nil
	}
	return m[l : l+dataLen]
}
func (m ControlMessage) Next(dataLen int) ControlMessage {
	l := ControlMessageSpace(dataLen)
	if len(m) < l {
		return nil
	}
	return m[l:]
}