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From: Steven Bardwell <SBardwell AT lbmsys DOT com>
To: "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Subject: RE: FD_SETSIZE and sizeof(fd_set)
Date: Thu, 23 Jun 2016 11:36:35 +0000
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> >
> > 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=%d\n", FD_SETSIZE);
> >         fprintf(stdout, "sizeof(fd_set)=%d\n", sizeof(fd_set));
> > }
> >
> > Steve Bardwell
> >
> >
> 
> I don't know if this is still the case, but when I looked into this years ago I
> found that it was not possible to change the size of the fd set in linux, it's
> fixed at 1024 (generally), unless you rebuild the kernel.
> 
> 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.
> 
> I can't speak for the cygwin implementations, but if they offer poll() or,
> better, epoll(), use those.
> 
> -lee

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.

Steve Bardwell