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

- From: Marko Kreen \<hidden\>
- Date: 2016-06-15 22:48:52 
- List: git
- Page: https://ratatoskr.run/git/2016/06/7465694
- Thread: https://ratatoskr.run/git/2016/06/7465679/t.md

```text
On 5/27/10, Erik Faye-Lund [ref] wrote:
> On Thu, May 27, 2010 at 1:39 PM, Marko Kreen [ref] wrote:
>  > On 5/27/10, Erik Faye-Lund [ref] wrote:
>  >> On Thu, May 27, 2010 at 12:10 PM, Jonathan Nieder [ref] 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.
>  >>
>  >>  If we decide to go this route, perhaps it'd make sense to change to
>  >>  this code for Windows also? Our Windows-implementation of poll() has
>  >>  some annoying limitations...
>  >
>  > Example of poll() compat without FD_SETSIZE limit:
>  >
>  >  http://github.com/markokr/plproxy-dev/blob/master/src/poll_compat.c
>  >
>
>
> How does this code convince FD_SET() that the buffer has increased? It
>  looks to me like it depends on a specific FD_SET() implementation...
>  For instance, Windows' FD_SET() implementation is like this:
>
>  #define FD_SET(fd, set) do { \
>     if (((fd_set FAR *)(set))->fd_count < FD_SETSIZE) \
>         ((fd_set FAR *)(set))->fd_array[((fd_set FAR
>  *)(set))->fd_count++]=(fd);\
>  } while(0)
>
>  ...so unless another set is passed in, it won't add any more fds once
>  fd_count reaches FD_SETSIZE.
>
>  Also, FD_SETSIZE is 64 on Windows. IIRC it's 1024 on Linux, so it is
>  much more likely that we encounter this issue on Windows than on
>  Linux, at least ;)

Hm, good catch.  Seems such compat poll() cannot be done without
OS-specific hacks.

Do you know perhaps what other OS-es have non-bitmap fd_set?

-- 
marko
```