RiscOS way to make non-blocking asynchronous HTTP requests

Hello,

I’m a newbie in RiscOS programming.
I’m writing the chat application. My application will maintain one long-lived HTTP long-poll connection to server to read incoming data.
I’m plan to use cURL library in non-blocking “multi” mode for this.
Is there any good way to receive the data as soon as possible and not make additional CPU load.

I plan to make main event loop like this pseudo-code:

    

    event_set_mask(0);

    while (!quit)

    {

        event_poll(&event_code, &poll_block, 0);

        if (netconn.data_available()) {

            netconn.process_data();

        }

    }

And second question:
Is it true that any long HTTP request in blocking mode (where event_poll is not called) in will block whole system (not just my app)?

You can see what happens if you do while(1) {}, complete stop! Someone may correct me, but wimp_poll is the one that drives wimp multitasking. If you do a simple c program with a loop, and start it from filer , you will see the same thing. (wow post 467 and am user 467! :) )

I’ve never actually needed to sit down and write socket code, but my understanding is that the basic socket swis block by default. However, there are non-blocking versions (I believe you mark the socket as non-blocking before first use). You can read a bit of info here if you search for “non-blocking”: http://www.riscos.com/support/developers/prm/internet.html

I don’t know how your curl-port will work in that context. I’d imagine that unixlib handles that automatically, but I have no experience of that.

Hopefully there will be folks on here that will be able to provide advice on using non-blocking sockets specifically, and how that fits into curl.

You can see what happens if you do while(1) {}, complete stop! Someone may correct me, but wimp_poll is the one that drives wimp multitasking. If you do a simple c program with a loop, and start it from filer , you will see the same thing. (wow post 467 and am user 467!

Yes, but event_poll() is one of the the Wimp libraries’ ways to call Wimp_Poll, though.

But as far as I know this way will make additional load on CPU. Is there better way?

You should be calling Wimp_PollIdle with a suitable delay, rather than straight Wimp_Poll – see your library’s documentation for how to make event_poll() do this. Beyond that, it’s down to you to write an event scheduler to poke curl’s non-blocking interface to fetch a bit more data at suitable intervals.

NetSurf does exactly this, and has a lot of code dedicated to such a scheduler – this is one of the areas where RISC OS doesn’t help you at all, unfortunately.

see your library’s documentation for how to make event_poll() do this.

Just as a guess, as I use DeskLib, but I’d imagine something like…

#define POLL_DELAY 50

event_set_mask(1);

while (!quit)
{
   poll_time = os_read_monotonic_time() + POLL_DELAY;
   event_poll_idle (&event_code, &poll_block, poll_time, NULL );

   if ( netconn.data_available() )
   {
      netconn.process_data();
   }

   // and handle wimp events! :-)
}

This will return with a null event every 50cs unless something else happens in the meantime.

any long HTTP request in blocking mode (where event_poll is not called) in will block whole system

Yes, that’s what blocking means.

You need to specify the socket as non blocking with:

socketioctl(sock_handle, FIONBIO, &flag);

Where flag is an integer set to 1.

Call this between creating the socket and listening or accepting connections. Note that this means you will need to handle “soft” errors such as EINPROGRESS and EWOULDBLOCK, which basically mean “hang on, in doing it!”.

and has a lot of code dedicated to such a scheduler

Indeed. You just get to (usually) come back from Wimp polls periodically. It’s then up to you to decide what to do and how to do it. In Manga, I just look to see if data is available, and if it is I transfer it. Rinse and repeat until done.

Great thanks to all authors!
Now I have more understanding now to make it.

From a very vague memory – isn’t there an internet event you can use to tell you when there’s something on the socket?

Event 19

But note that events are not something you can reasonably use from application code…

A few lines of code in the RMA sets your wimp poll word. Bob’s your uncle.