status.i4.v = fm_write_disable([vun.i2.r])

	This routine allows the programmer to disable writing to all files.
	A message will instead be written to a log file.  If "vun" is
	specified, only that connection will be disabled.

	[vun]		virtual unit number on which the file was opened
			(default is NULL)

	This function returns status values as follows:

	CBS_NO_SUCH		no such file exists
	TRUE			writing was previously enabled
	FALSE			writing was not previously enabled

	This function requires the following include files:

	cnsparam_h, cbslib_h, acnet_errors_h

	Related functions:

	fm_write_enable, fm_is_writing_enabled, fm_write(_c)

	C/C++ usage:

	short	vun;
	int	status;

	status = fm_write_disable(&vun);