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Date: Wed, 6 Jul 2022 16:15:58 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Typo in <sys/select.h>?
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On Jul  6 15:01, Jon Turney wrote:
> On 06/07/2022 08:42, Corinna Vinschen wrote:
> > On Jul  5 17:51, Ken Brown wrote:
> > > On 7/5/2022 10:13 AM, Lavrentiev, Anton (NIH/NLM/NCBI) [C] via Cygwin wrote:
> > 
> > I guess we can change FD_SETSIZE to 1024 as on Linux, albeit this has no
> > real meaning on Cygwin.  On Linux, select(2) is really only capable to
> > handle file descriptors numbers up to descriptor number 1023, but Cygwin
> > doesn't have this problem.  FD_SETSIZE == 64 was only something to save
> > space.  The bigger FD_SETSIZE, the bigger are the default fd_sets,
> > something you don't want on small targets.
> > 
> > So, yeah, something like
> > 
> >    #ifndef FD_SETSIZE
> >    # ifdef __CYGWIN__
> >    #  define FD_SETSIZE      1024
> >    # else
> >    #  define FD_SETSIZE      64
> >    # endif
> >    #endif
> 
> Remember that 64 is MAXIMUM_WAIT_OBJECTS for WaitForMultipleObjects(), the
> underlying Win32 API used to implement select(), so using more than 64 hits
> some complex code to work around that...

This isn't what FD_SETSIZE is about.  FD_SETSIZE does *NOT* define the
maximum count of fd's in an fd_set.

It defines the maximum fd number usable in an fd_set.

So if FD_SETSIZE is defined low enough:

  #define FD_SETSIZE 3
  #include <sys/select.h>

  /* Only fd's 0, 1, and 2 will be allowed in this fd_set */
  fd_set set;

  FD_ZERO (&set);
  /* This will probaly set fd to 3 */
  fd = open ("foo", O_RDONLY); 
  /* So this will (hopefully) fail */
  FD_SET (fd, &set);


Corinna

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