Re: [suse-programming-e] socket/select differences between glibc 2.1 and 2.2

8 Feb 2003

      Hi Jerry,

Thanks for your quick response.

As I said, I know it's not a meaningful thing
to necessarily do. And indeed I should have said the behaviour is
correct once the stream is properly attached.

I just happen to be working on a piece of legacy code that used
the assumption that the select would block and I'm trying to determine
if the assumption was ever valid ( you say not - you think behaviour should be
undefined). So I guess my question is really:

'In this scenario, is the behaviour of select() undefined?'

Many thanks

Andy

On Fri, 7 Feb 2003, Jerry Feldman wrote:
...
Having done my share of using select (mostly on commercial Unix
systems), I think the only issue that I can see here is that your file
descriptor is not really attached to a stream via one of the appropriate
functions, such as listen() or connect(). I would think that the
behavior is undefined. Why it would be different on the two libraries, I
don't know.
--
Jerry Feldman 
Boston Linux and Unix user group
http://www.blu.org PGP key id:C5061EA9
PGP Key fingerprint:053C 73EC 3AC1 5C44 3E14 9245 FB00 3ED5 C506 1EA9
On Fri, 7 Feb 2003 18:54:18 +0000 (GMT)
Andrew Cheadle  wrote:
...
Hi,
Can anyone please explain the following behaviour. Consider the
following:
int main( int argc, char **argv )
{
  int s;
  fd_set              dread;
  fd_set              dwrite;
  struct timeval      to;
to.tv_sec = 3;
  to.tv_usec = 0;
s = socket( AF_INET, SOCK_STREAM, 0 );
FD_ZERO(&dread);
  FD_ZERO(&dwrite);
  FD_SET(s, &dread);
if (select(s + 1, &dread, &dwrite, (fd_set *) 0, &to) < 0)
  {
    fprintf(stderr, "Error in select(): errno=%d\n", errno);
    return errno;
  }
return 0;
}
On a (Suse i386 linux) glibc-2.1.3-27 machine this blocks for the
timeout of 3 seconds and the 'dread' fd_set is cleared and the select
returns 0.
On a (Suse i386 linux) glibc-2.2.2-67 machine this returns immediately
and the 'dread' fd_set is left untouched and the select returns a
value of 1.
The program above is a toy that reproduces my problem, while calling
select immediately on 's' may not be a meaningful thing to do, I just
want to know what the correct semantics are and why they have changed.
Cheers
Andy
*********************************************************************
*  Andrew Cheadle                    email:  a.cheadle@doc.ic.ac.uk *
*  Department of Computing           http://www.doc.ic.ac.uk/~amc4/ *
*  Imperial College                                                 *
*  University of London                                             *
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*  Andrew Cheadle                    email:  a.cheadle@doc.ic.ac.uk *
*  Department of Computing           http://www.doc.ic.ac.uk/~amc4/ *
*  Imperial College                                                 *
*  University of London                                             *
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