Question about socket timeouts

[Adrian Chadd]

Yes! Please file a PR!



-adrian



On 19 August 2013 12:33, Vitja Makarov <vitja.makarov at gmail.com> wrote:

> Hi!
>
> Recently I was playing with small socket timeouts. setsockopt(2)
> SO_RCVTIMEO and found a problem with it: if timeout is small enough
> read(2) may return before timeout is actually expired.
>
> I was unable to reproduce this on linux box.
>
> I found that kernel uses a timer with 1/HZ precision so it converts
> time in microseconds to ticks that's ok linux does it as well. The
> problem is in details: freebsd uses floor() approach while linux uses
> ceil():
>
> from FreeBSD's sys/kern/uipc_socket.c:
> val = (u_long)(tv.tv_sec * hz) + tv.tv_usec / tick;
> if (val == 0 && tv.tv_usec != 0)
>      val = 1; /* at least one tick if tv > 0 */
>
> from Linux's net/core/sock.c:
> *timeo_p = tv.tv_sec*HZ + (tv.tv_usec+(1000000/HZ-1))/(1000000/HZ);
>
> So, for instance, we have a freebsd system running with kern.hz set
> 100 and set receive timeout to 25ms that is converted to 2 ticks which
> is 20ms. In my test program read(2) returns with EAGAIN set in
> 0.019ms.
>
> So the question is: is that a problem or not?
>
> --
> vitja.
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