The getsockopt function gets information about the value of
option optname at level level for socket socket.
The option value is stored in a buffer that optval points to.
Before the call, you should supply in *optlen-ptr the
size of this buffer; on return, it contains the number of bytes of
information actually stored in the buffer.
Most options interpret the optval buffer as a single int
value.
The actual return value of getsockopt is 0 on success
and -1 on failure. The following errno error conditions
are defined:
EBADF- The socket argument is not a valid file descriptor.
ENOTSOCK- The descriptor socket is not a socket.
ENOPROTOOPT- The optname doesn't make sense for the given level.
