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X-HELO: calimero.vinschen.de
Date: Thu, 23 Jun 2016 13:52:49 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: FD_SETSIZE and sizeof(fd_set)
Message-ID: <20160623115249.GF15373@calimero.vinschen.de>
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On Jun 23 11:36, Steven Bardwell wrote:
> > >
> > > Here is a "program" that shows the issue I am worried about. It is so=
 simple
> > that I must be overlooking something really obvious:
> > >
> > > #include <stdio.h>
> > > #undef FD_SETSIZE
> > > #define FD_SETSIZE 256
> > > #include <sys/types.h>
> > > #include <sys/select.h>
> > >
> > > main()
> > > {
> > >         fd_set rfds;
> > >         fprintf(stdout, "FD_SETSIZE=3D%d\n", FD_SETSIZE);
> > >         fprintf(stdout, "sizeof(fd_set)=3D%d\n", sizeof(fd_set));
> > > }
> > >
> > > Steve Bardwell
> > >
> > >
> >=20
> > I don't know if this is still the case, but when I looked into this yea=
rs ago I
> > found that it was not possible to change the size of the fd set in linu=
x, it's
> > fixed at 1024 (generally), unless you rebuild the kernel.
> >=20
> > Secondly, in the windows api, their version of an fd_set is more like a=
 poll()
> > implementation, you can fake out any size you want since the size of the
> > array is the first entry.
> >=20
> > I can't speak for the cygwin implementations, but if they offer poll() =
or,
> > better, epoll(), use those.
> >=20
> > -lee
>=20
> For what it's worth, this 'program' works as expected in SUA. The size
> of the fd_set changes depending on the value of FD_SETSIZE.

In my case it prints 'sizeof(fd_set)=3D8', which is correct.  For
historical reasons and an ill-advised compatibility with old cruft,
fd_set is an array of bit per descriptor.  8 * 8 =3D 256.


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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