[Info-vax] Copying VMS SaveSet Under Windows

[glen herrmannsfeldt]

Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
(snip)
>>> Question: does the tape drive itself signal end of file and end of tape
>>> ? Or just end of tape ?
 
> Both.  On reel to reel tape, there is a reflective marker shortly before 
> the physical end of tape.  The tape drive senses this, ceases forward 
> motion, and signals this to the program.  There are dozens of different 
> types of cartridge tapes but all that I know of are able, somehow, to 
> signal end-of-tape.

For the traditional mainframe IBM drive, the reflective strip is
only used on write.  The drive continues with the current block,
and then signal EOT.  On read, it is ignored.  A common problem is
that on mounting a tape the first operation by the OS is to read
and verify the label.  A READ operation on a blank tape will run
the tape off the reel.  (It doesn't stop at the reflective strip.)

(snip)

> The basic problem with Unix is that it lacks the concept of "records". 
> To Unix, the world is just a string of bytes.  Records are a figment of 
> YOUR imagination and finding and interpreting them is YOUR problem.

The read() system call for tapes knows about blocks.  It will
read one block, up to the given buffer length, and return.

For disks, it doesn't, and will fill the buffer with bytes,
up to EOF.
 
> <snip>

-- glen