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Re: Reading data from a file descriptor

From: tomas
Subject: Re: Reading data from a file descriptor
Date: Mon, 16 Nov 2015 14:02:49 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Hash: SHA1

On Fri, Nov 13, 2015 at 10:51:58AM -0500, Mark H Weaver wrote:
> Jan Synáček <address@hidden> writes:
> > On Sun, Nov 8, 2015 at 12:49 AM, Andreas Rottmann <address@hidden>
> > wrote:
> >
> >     Also note that if there's no requirement to actually implement
> >     this in
> >     C, there's `fdes->inport' and `fdes->outport' on the Scheme level,
> >     so
> >     something like the following would be analogous to the C example
> >     code
> >     posted:
> >     
> >     (import (ice-9 binary-ports))
> >     
> >     (define (process-fd fd)
> >     (let ((port (fdes->inport fd)))
> >     (display "read: ")
> >     (display (get-bytevector-n port 100))
> >     (display "\n")))
> >     
> >     (process-fd (acquire-valid-fd))
> >     
> >
> > This is something very similar that I ended up with. Just instead of
> > get-byte-vector, I used read-string!/partial.
> I would advise against using 'read-string!/partial' or any of the
> procedures in (ice-9 rw).  This is a vestigial module from Guile 1.8
> when strings were arrays of bytes, which they no longer are.  We should
> probably mark them as deprecated.
> For one thing, when we switch to using UTF-8 as the internal string
> encoding, it will not be possible to keep 'read-string!/partial'
> efficient.  It will necessarily have to do an encoding conversion.
> In Guile 2+, I would advise using byte vectors when working with binary
> data.  Portions of these can be converted to strings with a given
> encoding if desired.  I might be able to give better advice if I knew
> more about what you are doing here.


what Jan is after (and what I'd like to have too) is something
akin to Unix read(2) with O_NONBLOCK: provide a buffer, request
(up to) N bytes from the file (descriptor) and get an answer
(with possibly less bytes).

I tried that a while ago and was surprised that I had to resort
to (character) strings, with all the downsides you mention. Something
like that for byte vectors would be awesome. Either it exists (and
neither Jan nor me have succeeded in finding it) or it doesn't.

I'll go have a look later to see whether my recollection is accurate.

- -- tomás
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