[TUHS] Another odd comment in V6

Noel Chiappa jnc at mercury.lcs.mit.edu
Wed Feb 15 02:17:08 AEST 2017


    > From: Random832

    > You could return the address of the last character read, and let the
    > user code do the math.

Yes, but that's still 'design the system call to work with interrupted and
re-started system calls'.

    > If the terminal is in raw/cbreak mode, the user code must handle a
    > "partial" read anyway, so returning five bytes is fine.

As in, if a software interrupt happens after 5 characters are read in, just
terminate the read() call and have it return 5? Yeah, I suppose that would
work.

    > If it's in canonical mode, the system call does not copy characters into
    > the user buffer until they have pressed enter.

I didn't remember that; that TTY code makes my head hurt! I've had to read it
(to add 8-bit input and output), but I can't remember all the complicated
details unless I'm looking at it!


    > Maybe there's some other case other than reading from a terminal that it
    > makes sense for, but I couldn't think of any while writing this post.

As the Bawden paper points out, probably a better example is _output_ to a
slow device, such as a console. If the thing has already printed 5 characters,
you can't ask for them back! :-)

So one can neither i) roll the system call back to make it look like it hasn't
started yet (as one could do, with input, by stuffing the characters back into
the input buffer with kernel ungetc()), or ii) wait for it to complete (since
that will delay delivery of the software interrupt). One can only interrupt
the call (and show that it didn't complete, i.e. an error), or have
re-startability (i.e. argument modification).

	Noel


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