The IO$_WRITEVBLK function transmits data from the specified
    user buffers to an Internet host. Use both p1 and p2 arguments
    to specify a single user buffer. Use the p5 argument to specify
    multiple buffers.
    For connection-oriented protocols, such as TCP, if the socket
    transmit buffer is full, the IO$_WRITEVBLK function is blocked
    until the socket transmit buffer has room for the user data.
    For connectionless-oriented protocols, such as UDP and raw IP,
    the user data is transmitted in one datagram. If the user data
    is greater than the socket's transmit quota, the error code (SS$_
    TOOMUCHDATA) is returned.
    Related Functions
    The equivalent Sockets API functions are send(), sendto(),
    sendmsg(), and write().