From: Paul Brannan <paul@...>
Date: 2001-12-14T00:14:53+09:00
Subject: [ruby-talk:28415] Re: [RCR] IO.select should return array for timeout.

On Thu, Dec 13, 2001 at 04:03:14PM +0900, Yukihiro Matsumoto wrote:
> But now I think it's arguable.  If IO.select returned [[],[],[]] for
> timeout, most routines could handle timeout well.  I'm still not sure,
> so I put this as an RCR to call for discussion.

How would you propose that a select() loop look?

Currently, I write something like this:

  loop do
    if (fds = select([fd1], [fd2, fd3], nil, 10)) then
      for fd in fds[0]
        # Handle read fds
      end
      for fd in fds[1]
        # Handle write fds
      end
    else
      # Timeout
    end
  end

whereas with the new solution, I would have to do:

  loop do
    fds = select([fd1], [fd2, fd3], nil, 10)
    if fds[0].size == 0 and fds[1].size == 0 and fds[2].size == 0 then
      # Timeout
    else
      for fd in fds[0]
        # Handle read fds
      end
      for fd in fds[1]
        # Handle write fds
      end
    end
  end

Note the extra test for size on all three sub-arrays.  I suppose I could
use the test:

  if fds[0].join.size == 0 then
    # Timeout
  else
    # ...
  end

But this is not entirely clear, and is probably even slower.  Contrast
this with a C example (just for kicks):

  for(;;) {
    FD_ZERO(&r_fds); FD_SET(fd1, &r_fds);
    FD_ZERO(&w_fds); FD_SET(fd2, &w_fds); FD_SET(fd3, &w_fds);
    tv.tv_sec = 10; tv.tv_usec = 0;
    retval = select(
        ((fd1 > fd2) ? MAX(fd1, fd3) : MAX(fd2, fd3)),
        &r_fds, &w_fds, NULL, &tv);
    if(retval > 0) {
      if(FD_ISSET(fd1, &r_fds)) {
        /* Do something */
      }
      if(FD_ISSET(fd2, &w_fds)) {
        /* Do something */
      }
      if(FD_ISSET(fd3, &w_fds)) {
        /* Do something */
      }
    } else if(retval == 0) {
      /* Timeout */
    } else {
      /* Error */
    }
  }

In C, I check for timeout by checking the return value of select(), and
I check each fd to see if it is ready (I know no portable way to iterate
over the fds in the set without using a wrapper, like ACE).  So
currently Ruby is mostly consistent with C for timeout, and more
convenient than C for checking if fds are ready.  I like the way it is.

Also, why not post this RCR on the Rubygarden?

Paul