The upper level amanda code (including some of the other tape_xxx 
routines) calls the following routines which implement a virtual
tape table:
    int tape_access(filename, mode) 
	Acts like access(2) on possibly virtual devices
    int tape_open(filename, mode)
	Acts like open(2) on possibly virtual devices
    int tape_stat(filename, buf) 
	Acts like stat(2) on possibly virtual devices
    int tapefd_close(tapefd)
	Acts like close(2) on possibly virtual devices
    int tapefd_fsf(tapefd, count)
	Forward skips the (possibly virtual) device.
    int tapefd_read(tapefd, buffer, count)
	Reads a block from the (possibly virtual) device.
    int tapefd_rewind(tapefd)
	Reads a block from a (possibly virtual) device.
    void tapefd_resetofs(tapefd)
	Uses lseek() tricks to reset the write/read offset counter on
	a virtual tape device.
    int tapefd_unload(tapefd)
	Unloads the media from a (possibly virtual) device.
    int tapefd_status(tapefd) 
	prints status of the (possibly virtual) device to standard output.
    int tapefd_weof(tapefd, count)
	writes a filemark/moves to the next file on a device for writing.
    int tapefd_write(tapefd, buffer, count)
	writes a block of data to a (possibly virtual) device.

For a tape type xxx, the following routines must be provided, and 
entered into the table "vtable" in tape-src/tapeio.c:
    int xxx_tape_access(filename, mode) 
    int xxx_tape_open(filename, mode)
    int xxx_tape_stat(filename, buf) 
    int xxx_tapefd_close(xxx_tapefd)
    int xxx_tapefd_fsf(xxx_tapefd, count)
    int xxx_tapefd_read(xxx_tapefd, buffer, count)
    int xxx_tapefd_rewind(xxx_tapefd)
    void xxx_tapefd_resetofs(xxx_tapefd)
    int xxx_tapefd_unload(xxx_tapefd)
    int xxx_tapefd_status(xxx_tapefd) 
    int xxx_tapefd_weof(xxx_tapefd, count)
    int xxx_tapefd_write(xxx_tapefd, buffer, count)

Along with a prefix string which will identify the type.  The initial
vtape layer has two types "plain" and "rait" (Redundant Array of
Inexpensive Tapes) but we hope to add a "rmt" (remote tape client),
and possibly "dvd" types soon.