[mercury-users] Examples of posix module

[Fergus Henderson]

On 28-Apr-2001, John Eikenberry <jae at zhar.net> wrote:
> 
> Are there any examples of the posix extras module available (besides
> hello.m that comes with it)?

Not that I know of.

> My goal is to figure out how to use posix select to monitor stdin/stdout.
> And while I'd like to figure this out myself from examples and docs, if its
> easier to just show me how to do it I'd appreciate a basic example. I have
> use select before, but only in python. I also have some prolog experience,
> but it was a few years ago and never with select.

I've attached a simple example, which I took from the Linux man page for
select() and converted to Mercury, with some minor adaptions.
This just reads from stdin with a timeout.

The Posix interface is a fairly thin binding, so the interface is
quite similar to the equivalent in C.  A few of the details differ,
but those should hopefully be clear from the Mercury type and mode
declarations.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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% This module shows an example of the use of `select'.

:- module select_example.
:- interface.
:- import_module io.

:- pred main(io__state, io__state).
:- mode main(di, uo) is det.

:- implementation.

:- import_module int, exception, posix, posix__select.

main -->
	new_fdset_ptr(ReadFDSet),
	new_fdset_ptr(WriteFDSet),
	new_fdset_ptr(ExceptFDSet),
	{ StdinFD = 0 },
	fd_set(fd(StdinFD), ReadFDSet),
	{ TimeToWait = timeval(5 /* seconds, + */, 0 /* microseconds */) },
	{ MaxFD = StdinFD },
	posix__select__select(MaxFD + 1, ReadFDSet, WriteFDSet, ExceptFDSet,
		TimeToWait, Result),
	(
		{ Result = ok(NumFDs) },
		(if { NumFDs > 0 } then
			print("Data is available now.\n"),
		        /* fd_isset(StdinFD, ReadFDSet, X) will give X=yes. */
			io__read_line_as_string(StringRes),
			(if { StringRes = ok(String) } then
				print("Data read: "), print(String), nl
			else
				{ throw("wierd, something went wrong") }
			)
		else
			print("No data within five seconds.\n")
		)
	;
		{ Result = error(Errno) },
		print("Error: "), print(Errno), nl
	).

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