[mercury-users] Examples of posix module

[John Eikenberry]

Great. Thanks for the example Fergus. I think I have a handle on it now and
can start asking questions on things other than the posix binding. ;)

Fergus Henderson wrote:

> 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.
> 

> % 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
> 	).
> 

> 
>       int
>        main(void)
>        {
>            fd_set rfds;
>            struct timeval tv;
>            int retval;
> 
>            /* Watch stdin (fd 0) to see when it has input. */
>            FD_ZERO(&rfds);
>            FD_SET(0, &rfds);
>            /* Wait up to five seconds. */
>            tv.tv_sec = 5;
>            tv.tv_usec = 0;
> 
>            retval = select(1, &rfds, NULL, NULL, &tv);
>            /* Don't rely on the value of tv now! */
> 
>            if (retval)
>                printf("Data is available now.\n");
>                /* FD_ISSET(0, &rfds) will be true. */
>            else
>                printf("No data within five seconds.\n");
> 
>            exit(0);
>        }
> 
> 


-- 

John Eikenberry
[jae at zhar.net - http://zhar.net]
______________________________________________________________
"A society that will trade a little liberty for a little order
 will deserve neither and lose both."
                                          --B. Franklin
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