Cancels all pending I/O requests on a specified channel.
    Related Functions
    The equivalent Sockets API function is close().
    Format
      SYS$CANCEL  chan
    C Prototype
      int sys$cancel  (unsigned short int chan);
    Returns
      OpenVMS usage:cond_value
      type:         longword (unsigned)
      access:       write only
      mechanism:    by value
    Longword condition value. All system services return (by
    immediate value) a condition value in R0. Condition values that
    can be returned by this service are listed under Condition Values
    Returned.