[m-users.] How to get the file descriptor of a stream

[Volker Wysk]

Am Donnerstag, dem 20.07.2023 um 17:01 +1000 schrieb Julien Fischer:
> Hi Volker,

Hi Julien!

> On Thu, 20 Jul 2023, Volker Wysk wrote:
> 
> > I would like to have a predicate which returns the file descriptor, which is
> > used to implement a stream. Such as the fileno(3) C library function. It's
> > for querying the terminal width, of streams that can be connected to a
> > terminal. I want to print (error and normal) messages which are formatted
> > for the terminal's width.
> > 
> > There's stdout_stream and stderr_stream and you can assume that they have
> > the file descriptors 1 and 2. But this isn't foolproof and other streams
> > might be connected to a terminal as well (or isn't it?).
> > 
> > It might, for instance, theoretically happen that stdout_stream is closed
> > and the file descriptor 1 gets reused later. Then we could end up with a fd
> > 1 which doesn't belong to stdout_stream.
> > 
> > I've searched the io and stream libraries, but couldn't find such a
> > predicate.
> 
> For the C backends, it's straightfoward to get the file descriptor for a
> Mercury file stream. Here's how you do it for text streams (binary
> file streams would be very similar).
> 
> :- pred get_text_input_stream_fd(io.text_input_stream::in, int::out,
>       io::di, io::uo) is det.
> 
> :- pragma foreign_proc("C",
>      get_text_input_stream_fd(Stream::in, FD::out, _IO0::di, _IO::uo),
>      [will_not_call_mercury, promise_pure, tabled_for_io],
> "
>      FD = fileno(MR_file(*MR_unwrap_input_stream(Stream)));
> ").
> 
> :- pred get_text_output_stream_fd(io.text_output_stream::in, int::out,
>        io::di, io::uo) is det.
> 
> :- pragma foreign_proc("C",
>      get_text_output_stream_fd(Stream::in, FD::out, _IO0::di, _IO::uo),
>      [will_not_call_mercury, promise_pure, tabled_for_io],
> "
>      FD = fileno(MR_file(*MR_unwrap_output_stream(Stream)));
> ").
> 
> Julien.

Thanks! The "MR_file(*MR_unwrap_input/output_stream(Stream))" part was the
missing piece.

MR_file and MR_unwrap_input/output_stream don't occur in the Language
Reference Manual or the Library Reference Manual. But they're mentioned in a
comment in io.m. 

So the documentation isn't completely complete and you have to resort to
reading the source code, sometimes.

Regards,
Volker
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