GNU bug report logs - #30066
'get-bytevector-some' returns only 1 byte from unbuffered ports

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From: ludo <at> gnu.org (Ludovic Courtès)
To: Andy Wingo <wingo <at> igalia.com>
Cc: Mark H Weaver <mhw <at> netris.org>, 30066 <at> debbugs.gnu.org
Subject: bug#30066: 'get-bytevector-some' returns only 1 byte from unbuffered ports
Date: Sat, 13 Jan 2018 21:53:34 +0100

Hey,

Andy Wingo <wingo <at> igalia.com> skribis:

> On Fri 12 Jan 2018 11:15, ludo <at> gnu.org (Ludovic Courtès) writes:
>
>> Andy Wingo <wingo <at> igalia.com> skribis:
>>
>>> On Thu 11 Jan 2018 22:55, Mark H Weaver <mhw <at> netris.org> writes:
>>
>> [...]
>>
>>>>>> Out of curiosity, is there a reason why you're using an unbuffered port
>>>>>> in your use case?
>>>>>
>>>>> It’s to implement redirect à la socat:
>>>>>
>>>>>   https://git.savannah.gnu.org/cgit/guix.git/commit/?id=17af5d51de7c40756a4a39d336f81681de2ba447
>>>>
>>>> Why is an unbuffered port being used here?  Can we change it to a
>>>> buffered port?
>>>
>>> This was also a question I had!  If you make it a buffered port at 4096
>>> bytes (for example), then get-bytevector-some works exactly like you
>>> want it to, no?
>>
>> It might work, but that’s more by chance no?
>
> No, it is reliable.  get-bytevector-some on a buffered port must either
> return all the buffered bytes or perform exactly one read (up to the
> buffer size) and either return those bytes or EOF.  As far as I
> understand, that is exactly what you want.

Indeed, that works well, thanks!  So, after all, problem solved?

I think the confusion for me comes from the fact that we don’t have a
FILE*/fd distinction like in C.  It’s as if we were always using FILE*
in the sense that I’m never sure what’s going to happen or whether a
particular behavior can be relied on.

Thank you,
Ludo’.

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