vun.i2.v = fm_get_vun(file_name.i1a.r, [,write_access.i4.v]

	This routine allows the application program to read the
	virtual unit number assigned to a file via a prior open.
	A returned value of CBS_NOTOPN indicates that the requested
	file has not been successfully opened.

	file_name	Filesharing file name excluding the prefix
			of "rsx$fshare_applicat_" (must be null terminated)
	[write_access]	read/write access of the desired file
			(FMNGR_READ_ACCESS -> read access only,
			 FMNGR_WRITE_ACCESS -> write access,
			 FMNGR_DEFAULT_ARG -> don't care (default))
	[access_mode]	record access of the desired file
			(FMNGR_DIRECT -> direct (random by record number),
			 FMNGR_SEQUENTIAL -> sequential,
			 FMNGR_APPEND -> append (sequential),
			 FMNGR_KEYED -> keyed,
			 FMNGR_DEFAULT_ARG -> don't care (default))

	This function returns status values as follows:

	CBS_NOTOPN		file is not open
	CBS_DUPREQ		file has been opened more than once and
				this request is not specific enough
	otherwise		desired virtual unit number

	This function requires the following include files:

	cnsparam_h, cbslib_h, acnet_errors_h

	Related functions:

	fm_open(_c), fm_open_io(_c), fm_open_read(_c), fm_open_write(_c)

	C/C++ usage:

	static const char	file_name[] = "controls_misc:my_file.dat";
	short	vun;
	int	write_access = FMNGR_DEFAULT_ARG;
	int	access_mode = FMNGR_DEFAULT_ARG;

	vun = fm_get_vun(file_name,write_access,access_mode);