Tape records

Richard <legalize at xmission.com> wrote: Good question. And people have some interesting answers as well. :-) A tape block is simply a bunch of bytes. The point is that it is a spcific number of bytes. A single read or write will process one tape block. This is important. It means that doing two consequtive writes for n bytes will not produce the same result as one write for 2*n bytes. It also means that if you request to read n bytes, and the tape block is shorter, you will not get data from the next tape block in addition to the current, but instead the read will return less data. Assuming the block is n bytes long, and you request a read for x number of bytes. If x > n you will get the whole block, but no more than the block. If x < n you will only get x bytes from the block, while the rest will be skipped. The next read will read from the next block anyway. Each block is physically separated from other blocks by a inter-block gap. This is meta-data that is not received as a part of a read, but which can be inferred from the number of bytes actually read, unless you get a data overrun error, in which case the block was larger than the read allowed for. In addition to this, you also have something called file marks. When you read and the tape position is at a file mark, you usually get a return value indicating that you read 0 bytes. This is a hint that it was a file mark you passed. Convention then have it that two consequtive file marks indicates the logical end of tape. However, ANSI-formatted tapes look and work slightly different, but that is all still implemented on top of the concept of tape blocks and file marks. Those two details are defined by hardware. This means that a tape can never properly behave like a stream of bytes. As I've said, Unix have a way of trying to make tapes look like a stream of bytes, but noone uses that, since it's not useful, nor meaningful. Even in Unix, you can control these things correctly from a program. That's what ioctls are for. Johnny -- Johnny Billquist || "I'm on a bus || on a psychedelic trip email: bqt at softjar.se || Reading murder books pdp is alive! || tryin' to stay hip" - B. Idol