Re: [TUHS] Systematic approach to command-line interfaces

[jnc@mercury.lcs.mit.edu (Noel Chiappa)]

    > From: Larry McVoy

    > If you read(2) a page and mmap()ed it and then did a write(2) to the
    > page, the mapped page is the same physical memory as the write()ed
    > page. Zero coherency issues.

Now I'm confused; read() and write() semantically include a copy operation
(so there are then two copies of that data chunk, and possible consistency
issues between them), and the copied item is not necessarily page-sized (so
you can't ensure consistency between the original+copy by mapping it in). So
when one does a read(file, &buffer, 1), one gets a _copy of just that byte_
in the process' address space (and similar for write()).

Yes, there's no coherency issue between the contents of an mmap()'d page, and
the system's idea of what's in that page of the file, but that's a
_different_ coherency issue.

Or am I confused?


PS:
    > From: "Greg A. Woods"

    > I now struggle with liking the the Unix concept of "everything is a
    > file" -- especially with respect to actual data files.  Multics also got
    > it right to use single-level storage -- that's the right abstraction

Oh, one other thing that SLS breaks, for data files, is the whole Unix 'pipe'
abstraction, which is at the heart of the whole Unix tools paradigm. So no
more 'cmd | wc' et al. And since SLS doesn't have the 'make a copy'
semantics of pipe output, it would be hard to trivially work around it.

Yes, one could build up a similar framework, but each command would have to
specify an input file and an output file (no more 'standard in' and 'out'),
and then the command interpreter would have to i) take command A's output file
and feed it to command B, and ii) delete A's output file when the whole works
was done. Yes, the user could do it manually, but compare:

  cmd aaa | wc

and

  cmd aaa bbb
  wc bbb
  rm bbb

If bbb is huge, one might run out of room, but with today's 'light my cigar
with disk blocks' life, not a problem - but it would involve more disk
traffic, as bbb would have to be written out in its entirety, not just have a
mall piece kept in the disk cache as with a pipe.

	Noel