On Wed, 11 Jan 2023 10:40:06 -0000 (UTC)
Kaz Kylheku <864-117-4973@kylheku.com> wrote:
>On 2023-01-09, Nicolas George <nicolas$george@salle-s.org> wrote:
>> Kaz Kylheku , dans le message <20230109030226.641@kylheku.com>, a
>> écrit :
>>> However, if some trailing part of your request is sitting in a buffer,
>>> such that lp is waiting for that last bit before replying, then you have
>>> a deadlock.
>>
>> I would guess it is not enough: lp is probably waiting for EOF, and there is
>> no way to close only half of a popen stream.
>
>That's a great observation.
>
>Now the "r+" mode is not portable to begin with.
>
>The POSIX model of standard I/O streams is such that a stream has one
>underlying descriptor, accessible via fileno.
>
>Suppose an implementation implements a bidirectional popen using only
>one descriptor, such that the descriptor returned by fileno is all there
>is; that object handles both data transfer directions.
>
>Such an implementation is quite possibly using the socketpair function
>to creae the bidirectional descriptor.
>
>And so if socketpair is being used, then it should be possible to use
>shutdown on the descriptor to do a half-close.
>
>As in:
>
> // write data to lp ...
>
> // send everything to socket
> fflush(pipe_stream);
>
> // close in writing direction
> if (shutdown(fileno(pipe_stream), SHUT_WR) != 0)
> {
> // didn't work: not a socket?
> }
>
>It's worth a try.

Tried, didn't work. feof() was always true and fgets() returned garbage. :(

On 2023-01-11, Geoff Clare <geoff@clare.See-My-Signature.invalid> wrote:
> Kaz Kylheku wrote:
>
>> 2. The bidirectional pipe is actually a socket pair.
>
> This would not conform to POSIX, as fstat() would show S_ISSOCK(st_mode)
> true instead of S_ISFIFO(st_mode).

Sure, but the behavior "r+" in popen is unspecified.

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