[Haskell-cafe] Re: I/O interface

Marcin 'Qrczak' Kowalczyk qrczak at knm.org.pl
Wed Jan 12 05:38:13 EST 2005

Ben Rudiak-Gould <Benjamin.Rudiak-Gould at cl.cam.ac.uk> writes:

> First of all, I don't think any OS shares file pointers between
> processes.

Unix does.

It's because shared files are usually stdin/stdout/stderr (I mean
that they are visible as stdin/stdout/stderr, rather than about their
nature as terminals - they may be regular files), which are usually
accessed sequentially without seeking. Quite often they are not
seekable at all (terminals or pipes), which means that they behave as
if the file pointer was always positioned at the end (for writing) or
beginning (for reading) and shared. If they are seekable, the position
is shared so you can redirect I/O to a process running subprograms.

> Otherwise it would be practically impossible to safely use an
> inherited filehandle via any API.

Pipes are not seekable and always behave as if the position is shared.
It doesn't make them impossible to safely inherit.

They are inherited on fork because they are anonymous objects, so
it's the only way to connect them; after fork most programs close the
reading end in one of the processes and the writing end in the other.

It's rare that two processes read from the same pipe or write to the
same pipe. If they do, one of them is usually a helper program started
by the other, and the other waits for the helper to finish.

If you want two processes to access the same file indepenently, pass a
file name and open it twice.

> The file interface in this library is only used for files, which are
> always seekable (by definition).

What do you mean by files? What you get from open() is not always
seekable, because it's not always a regular file. The name may refer
to a device (whether it's seekable depends on the particular device;
block devices are seekable but most character devices are not, e.g.
/dev/ttyS0, /dev/lp0 - serial and parallel ports) or to a named pipe
(not seekable).

   __("<         Marcin Kowalczyk
   \__/       qrczak at knm.org.pl
    ^^     http://qrnik.knm.org.pl/~qrczak/

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