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X-HELO: calimero.vinschen.de
Date: Thu, 23 Jun 2016 14:06:53 +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: <20160623120653.GC5996@calimero.vinschen.de>
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On Jun 23 14:01, Corinna Vinschen wrote:
> On Jun 23 13:55, Corinna Vinschen wrote:
> > On Jun 23 13:52, Corinna Vinschen wrote:
> > > 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 th=
is years ago I
> > > > > found that it was not possible to change the size of the fd set i=
n linux, 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 p=
oll() or,
> > > > > better, epoll(), use those.
> > > > >=20
> > > > > -lee
> > > >=20
> > > > For what it's worth, this 'program' works as expected in SUA. The s=
ize
> > > > of the fd_set changes depending on the value of FD_SETSIZE.
> > >=20
> > > 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.
> >=20
> > Now that's embarassing.  I guess I shgould go to primary school again.
> >=20
> > Hang on, I 'll have a nother look while I'nm trying to get the red out =
of
> > my face...
>=20
> Oh yeah, the inclusion of stdio.h also includes sys/select.h so the
> first (default) definition of FD_SETSIZE to 64 rules.  In theory it
> shouldn't do that so we might have to change it in the newlib headers,

Oh no, we don't have to.  sys/select.h is included via sys/types.h
*iff* __BSD_VISIBLE is set.  __BSD_VISIBLE is set by default unless
you set some feature macro like _POSIX_C_SOURCE.  Have a look into
<sys/features.h> for a description.


Corinna

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

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