Is it OK to poll() a device file descriptor

[Omer Zak]

Why not to use a socket (Unix socket or TCP/IP socket), push your
"0123456789" in one side and retrieve the same from the other side?

You'll need different code to open/close the socket, but writing/reading
will look the same.

--- Omer


On Wed, 2013-06-19 at 09:01 +0300, Elazar Leibovich wrote:
> I think I didn't explain myself correctly, so let me give a different
> example.
> 
> Let's make a file descriptor that counts to 9.
> 
> 
> I.e, we want to emulate the behavior:
> 
> 
>     int fd = make_counter_fd();
>     assert(write(fd, buf, 100) == 11);
>     assert(strcmp(buf, "0123456789") == 0);
> 
> 
> Let me describe a very simple version of the main poll loop:
> 
> 
>     struct poll_cb {
>         int fd;
>         int (*read_cb)(int fd);
>         int (*write_cb)(int fd);
>     };
>     while (poll(&pfd, nr, -1) != -1) {
>         for (int i=0; i < nr; i++) {
>             if (pfd[i].revent |= POLLIN)
>                 poll_cbs[i].read_cb(pfd[i].fd);
>             if (pfd[i].revent |= POLLOUT) 
>                 poll_cbs[i].write_cb(pfd[i].fd);
> 
>             if (pfd[i].revent |= POLLHUP)
>                 pfds[i].fd = -1; // don't poll again
>         }
>     }
> 
> 
> Now, in order to emulate the 0-9 file descriptor, we'll associate to a
> "/dev/zero" a read callback function that looks roughly like
> 
> 
>     int written = 0;
>     char buf[10];
>     int zero_to_nine_cb(int fd) {
>         for (int i=0; i< 10; i++) buf[i] = '0' + i;
>         close(fd); // note, we never touched the fd
>     }
> 
> 
> But I have to use a file descriptor! Otherwise poll will have no
> reason to call my callback.
> On Jun 19, 2013 7:47 AM, "Shachar Shemesh" <shachar at shemesh.biz>
> wrote:
>         On 18/06/13 22:16, Elazar Leibovich wrote:
>         
>         > I'm using it as a fake "always non-blocking" file
>         > descriptor. 
>         > 
>         > 
>         > My main libevent-like poll loop looks like:
>         > 
>         > 
>         >     poll(fds)
>         >     for fd in fds:
>         >        if fd.revents | POLLIN:
>         >            fd.read_callback()
>         >        if fd.revents | POLLOUT:
>         >            fd.write_callback()
>         > 
>         > 
>         > Now let's say I want a fake filedescriptor that always reads
>         > 'z's (a sleepy fd).
>         Why? What you just did was to turn the whole thing into a
>         non-sleeping loop. If that's the case, simply call poll with a
>         zero timeout, so it won't sleep, and call your callback at the
>         end of each loop. No need to artificially introduce another
>         file descriptor into the mix.
>         
>         Mind you, I still don't understand WHY you'd want such a
>         thing. This code will, by definition, consume 100% CPU all the
>         time.

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