Re: Select with device and stdin not working

[Ben Greear <greearb@candelatech.com>]

"M. Tavasti" wrote:
> 
> I found this problem first time in 2.2 kernels, when doing own device
> driver. Then it was not an issue for me, and I suspected it's my
> fault. Now, with 2.4 again I tried to solve problem, but I can't find
> my way out of this, and looks like there in-kernel drivers which have
> same symptoms.
> 
> Here program where I get problems:
> 
> int fd;
> fd_set rfds;
> 
> fd = open("/dev/random", O_RDWR );
> 
> while(1) {
>         FD_ZERO(&rfds);
>         FD_SET(fd,&rfds);
>         FD_SET(fileno(stdin),&rfds);
>         if( select(fd+1, &rfds, NULL, NULL, NULL  ) > 0) {
>                 fprintf(stderr,"Select\n");
>                 fflush(stderr);
>                 if(FD_ISSET(fd,&rfds)) {
>                  .......
>                 } else if(FD_ISSET(fileno(stdin),&rfds) ) {
>                  ......
>                 }
>         }
> }
> 
> Select is working fine for device (in this example /dev/random) or
> stdin. But for both, not. When entering something to stdin, it's not
> sure select will return.
> 
> I haven't tested is this problem present in all devices, but at least
> /dev/random is infected. And if problem lies only in some of the
> drivers, it would be nice to know which driver haves decent
> implementation of poll and get others updated.

I also saw what I believe is a problem with select, but I haven't had
time to poke around the kernel to try to understand it.  In my case,
the problem is that when I have raw sockets reading ethernet frames,
select will not return immediately when there is a packet ready.  When
it does return, due to timeout, or another FD in the set being ready
for action, the FD is set correctly though.  My hack/kludge/work-around
was to sleep for a max of 5ms, but that is not a clean, or efficient
solution.  Another person sent private email to me claiming that he
saw the same problem, so it's likely I'm not insane on this issue :)

So, I do think there may be problems with select, but I'm not sure if it
relates to your problem or not.


Ben

> 
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> 
> --
> M. Tavasti /  tawz@nic.fi  /   +358-40-5078254
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-- 
Ben Greear <greearb@candelatech.com>          <Ben_Greear@excite.com>
President of Candela Technologies Inc      http://www.candelatech.com
ScryMUD:  http://scry.wanfear.com     http://scry.wanfear.com/~greear