h_id	IN:	Access identifier returned by Hstartwrite
	len	IN:	Length of segment to be written
	data	IN:	Pointer to the data to be written

	Purpose	Writes the next data segment to a specified data element.
	Return value	Returns the length of the segment actually written if successful and FAIL (or -1) otherwise.
	Description	Hwrite begins writing at the current position of the access identifier, writes the specified number of bytes, then moves the access identifier to the position immediately following the last accessed byte. Calling Hwrite with length = 0 results in an error condition. To reposition an access identifier before writing data, use Hseek.
		If the space allocated in the data element is smaller than the length of data, the data is truncated to the length of the data element. Although only one access identifier is allowed per data element, it is possible to interlace writes to more than one data element in a file.