[Haskell-cafe] Re: I/O interface

Ferenc Wagner wferi at afavant.elte.hu
Wed Jan 12 06:16:14 EST 2005

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

> Marcin 'Qrczak' Kowalczyk wrote:
>> Ben Rudiak-Gould <Benjamin.Rudiak-Gould at cl.cam.ac.uk> writes:
>>> fileRead can be implemented in terms of OS primitives,
>> Only if they already support reading from a fixed offset (like pread).
>> I'm not sure if we can rely on something like this being always
>> available, or whether it should be emulated using lseek which is safe
>> only as long as we are the only process using the given open file.
> First of all, I don't think any OS shares file pointers between
> processes. Otherwise it would be practically impossible to safely use an
> inherited filehandle via any API.  Different threads using the same
> filehandle do share a file pointer (which is a major nuisance in my
> experience, because of the lack of an atomic seek- read/write), but a
> Posix fork duplicates the file pointer along with all other state. I can't
> believe I'm wrong about this, but someone please correct me if I am.

This may be what you wrote, but let me still put it:

dup()-ed filehandles share a common file position.
Handles straight from open() have independent file positions.
fork() duplicates filehandles and the child inherits those
=> the child process shares the file position with its parent.
Threads or processes doesn't make the difference; dup() or
open() does.

This is my interpretation of the docs, I didn't test it... :)

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