Re: [PATCH] compat: Add another rudimentary poll() emulation

From: Marko Kreen <markokr@gmail.com>
Date: May 27, 2010, 13:29 UTC
Message-ID: <AANLkTil_dqbXChKpMJ_ZFTuNrF8tQRwJd5j4SkLOaPaJ@mail.gmail.com>
In-Reply-To: <AANLkTikezlVaX8ARkRw8kEk9wL9RL_5I6X3vK83nzFUl@mail.gmail.com>

On 5/27/10, Erik Faye-Lund <kusmabite@googlemail.com> wrote:

Show 29 quoted lines

> On Thu, May 27, 2010 at 1:00 PM, Erik Faye-Lund
>  <kusmabite@googlemail.com> wrote:
>  > On Thu, May 27, 2010 at 12:10 PM, Jonathan Nieder <jrnieder@gmail.com> wrote:
>  >> Implement the subset of poll() semantics needed by git in terms of
>  >> select(), for use by the Interix port.  Inspired by commit 6ed807f
>  >> (Windows: A rudimentary poll() emulation, 2007-12-01).
>  >>
>  >
>  > A possible problem with this approach is that the maximum number of
>  > file descriptors poll can handle limited by RLIMIT_NOFILE, whereas the
>  > maximum number of file descriptors select can handle is limited by
>  > FD_SETSIZE.
>  >
>  > I don't think this is a big problem in reality, though - both values
>  > seem to be pretty high in most implementations. And IIRC git-daemon is
>  > the only one who needs more than 2, and it doesn't even check
>  > RLIMIT_NOFILE.
>  >
>
>
> To be clear: I think this strategy is the best option (at least for
>  non-Windows, where select() might be our only option).
>
>  But perhaps you should include a check along the lines of this:
>
>  if (nfds > FD_SETSIZE)
>         return errno = EINVAL, error("poll: nfds must be below %d", FD_SETSIZE);
>
>  Just so we can know when the code fails :)

Well, per your own FD_SET example, the FD_SETSIZE on windows means different thing than FD_SETSIZE on old-style bitmap-based select() implementation.

On Unix, it's max fd number + 1, on windows it's max count.

Thus on windows the nfds can safely be larger than FD_SETSIZE, assuming the actual count is smaller.

RLIMIT_NOFILE - I don't see why it is relevant here, as you get error on open() when you cross the limit. So how can you pass more than that meny fds to select()/poll()?

-- 
marko