status.i4.v = ntv_receive_request([receive_buffr.i2a.r],
					  [event_flag.i1.v], [ast_func.f.r])

	Unlock the receive buffer of a RUM-connected process or (first
	time called) connect to Receive Unsolicited Messages (requests
	and USM's).  Can optionally change the address or length of the
	receive buffer, the event flag set when the receive buffer is
	filled or the address of the user request AST routine.

	IMPORTANT:  If it is possible that more than one handle exists
	for the task, it is necessary to ensure that the correct handle
	is selected prior to any non-AST call to 'ntv_receive_request'.
	This is done by calling ntv_handle.

	[receive_buffr]	new receive buffer.  Default is to retain the the
			current receive buffer.

	[buffer_length]	new length of the (new) receive buffer (in words).
			Default is to retain the current length.

	[event_flag]	event flag to be set when a request/USM is received.
			If explicitly defaulted (or 0), no event flag is used.
			If implicitly defaulted (by a short argument list),
			current event flag number is unchanged.

	[ast_func]	user AST routine called when a request/USM is received.
			If explicitly defaulted (or 0), then no AST routine is
			called.  If implicitly defaulted (by a short argument
			list), then the current AST routine address is not
			changed.  The AST parameter is a pointer to a structure
			of type UAB_REQUEST.

	status	 VMS completion condition code.

	This function returns status values as follows:

	SS__NORMAL	success
	SS__EXQUOTA	AST quota exceeded, cannot declare network AST
	SS__ILLEFC	illegal event flag number
	SS__INSFARG	insufficient number of arguments (on first call)
	SS__INSFMEM	insufficient VMS memory to declare network AST
	SS__INTERLOCK	RCB is locked for modification
	SS__NONEXPR	non-existant network process
	SS__SSFAIL	XSC block does not exist
	SS__UNASEFC	process not associated with event flag cluster
			with the specified event flag number
	SS__VECINUSE	UPP_GL_ACTV.CB longword vector in use (non-zero)
	NET__NOBUFR	no required receive buffer address was specified,
			or the length is 0 (when doing RUM-connect).
	NET__NOCTLBLK	no control blocks free
	NET__NOTCNCT	not connected to network
	NET__RUMLSTFUL	cannot connect as RUM as RUM task list full
	NET__RUMNAMEXI	cannot connect as RUM as task name is already
			in use by another RUM task

	This function requires the following include files:

	acnet_h, ssdef_h

	Related functions:

	ntv_wait_request, ntv_check_request, ntv_send_reply,
	net_set_option, ntv_handle