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

[Volker Wysk]

Hi.

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.

Cheers,
Volker
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