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Date: Thu, 22 May 1997 15:06:05 -0700
From: Cabal95 <cabal95@dal.net>
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CC: john@starfire.mn.org
Subject: Re: bin/3622: gethostbyname fails for file descriptors above 255
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john@starfire.mn.org wrote:
> Number:         3622
> Category:       bin
> Synopsis:       gethostbyname fails for file descriptors above 255
> 
> Description:
> 
> gethostbyname() fails for a perfectly good domain name once a program
> already has file descriptors 0-255 open.  I have not yet tracked
> this down to find if it is specific to gethostbyname, or if it
> may be the underlying infrastructure, or possibly even into the kernel
> (in which case the category specified for this report will be wrong).

After compiling a debug version of FreeBSD 2.2.1-Release libc.a 3.0 and
running your test program below through gdb 4.16, the specific cause of
this problem appears to be caused by the following code in
/usr/src/lib/libc/net/res_send.c(I'm sorry, line # not available)
        s = socket(PF_INET, SOCK_DGRAM, 0);
        ...
        if (s+1 > FD_SETSIZE) {
                Perror(stderr, "res_send: too many files");
                _res_close();
                goto next_ns;
        }

When compiling the libs, FD_SETSIZE on most(all?) systems is 256,
defined in /usr/include/sys/types.h.

> Fix:

One way I fixed this on my system was to recompile libc.a with
FD_SETSIZE defined to a higher value.  I used 1024 and have not had any
of the described problems since doing that.

Another way was to go into the /usr/src/lib/libc/net/res_send.c, take
out:
        if (s+1 > FD_SETSIZE) {
                Perror(stderr, "res_send: too many files");
                _res_close();
                goto next_ns;
        }
and change:
        select(s+1, &dsmask, (fd_set *)NULL, (fd_set *)NULL, &timeout)
to:
        select(1, &dsmask, (fd_set *)NULL, (fd_set *)NULL, &timeout)

This also works, but there may be problems i'm unaware of, since
select() appears to be an internal kernel function, and I don't have the
kernel source at this time.

Although, from my programming experience on linux, you can call select()
with FD's greater than FD_SETSIZE, just not a group of FD's who's total
number is greater than FD_SETSIZE.  But perhaps this is different under
FreeBSD?

Cabal95/Daniel