Problem with semantics?

Problem with semantics?

[Shay Goikhman]

Dear Linux maintainers,

 I'm doing :

      setsockopt(s,  SO_RCVTIMEO, t1 );                  // set time-out
t1 on socket while block receiving on it
      select(,,, &fd_set_including(s), .., &errs, t2);      // block till
receive or time-out  t 2 jointly on a set of sockets

Apparently, I could no find reference on the coupled behavior of the two
above statements in Linux documentation.
As I understand the blocking semantics, I would expect  that  if t1<t2 ,
select should return after t1 with the descriptor 's' in 'errs' if 's' does
not become readable in the t1 interval.

It is not so in life -- select ignores t1 altogether.

Do you have some enlightening knowledge on the matter?
Thanks,
-Shay

Re: Problem with semantics?

[Andi Kleen]

Shay Goikhman <GOIKHMAN@il.ibm.com> writes:

> Dear Linux maintainers,
> 
>  I'm doing :
> 
>       setsockopt(s,  SO_RCVTIMEO, t1 );                  // set time-out
> t1 on socket while block receiving on it
>       select(,,, &fd_set_including(s), .., &errs, t2);      // block till
> receive or time-out  t 2 jointly on a set of sockets
> 
> Apparently, I could no find reference on the coupled behavior of the two
> above statements in Linux documentation.
> As I understand the blocking semantics, I would expect  that  if t1<t2 ,
> select should return after t1 with the descriptor 's' in 'errs' if 's' does
> not become readable in the t1 interval.
> 
> It is not so in life -- select ignores t1 altogether.
> 
> Do you have some enlightening knowledge on the matter?

RCVTIMEO only applies to recvmsg et.al., similar to SNDTIMEO only
apply to sendmsg etc. But select/poll only report events, they
do not actually send or receive by themselves.

Michael, perhaps you can clarify that in the manpages

-Andi

Re: Problem with semantics?

[Michael Kerrisk]

Hi Andi,

Andi Kleen wrote:
> Shay Goikhman <GOIKHMAN@il.ibm.com> writes:
> 
>> Dear Linux maintainers,
>>
>>  I'm doing :
>>
>>       setsockopt(s,  SO_RCVTIMEO, t1 );                  // set time-out
>> t1 on socket while block receiving on it
>>       select(,,, &fd_set_including(s), .., &errs, t2);      // block till
>> receive or time-out  t 2 jointly on a set of sockets
>>
>> Apparently, I could no find reference on the coupled behavior of the two
>> above statements in Linux documentation.
>> As I understand the blocking semantics, I would expect  that  if t1<t2 ,
>> select should return after t1 with the descriptor 's' in 'errs' if 's' does
>> not become readable in the t1 interval.
>>
>> It is not so in life -- select ignores t1 altogether.
>>
>> Do you have some enlightening knowledge on the matter?
> 
> RCVTIMEO only applies to recvmsg et.al., similar to SNDTIMEO only
> apply to sendmsg etc. But select/poll only report events, they
> do not actually send or receive by themselves.
> 
> Michael, perhaps you can clarify that in the manpages

I added the following to sockets.7:

              Timeouts have
              effect   for  socket  I/O  calls  (read(2),  recv(2),
              recvfrom(2),    recvmsg(2),    write(2),     send(2),
              sendto(2),  sendmsg(2));  timeouts have no effect for
              select(2), poll(2), epoll_wait(2), etc.

The change will be in man-pages-2.65.

Thanks for your note.

Cheers,

Michael

-- 
Michael Kerrisk
maintainer of Linux man pages Sections 2, 3, 4, 5, and 7

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