[Xorp-users] select timeout question

[Pavlin Radoslavov]

Ben Greear <greearb at candelatech.com> wrote:

> If you pass a timeout of 0 to selector::wait_and_dispatch, it causes select to wait 'forever'.
> Is that on purpose or a bug?  If you want to wait for zero time in select, pass a time-val
> with all zero values (not 0, which is actually treated as NULL).
> 
>      if (timeout == 0 || *timeout == TimeVal::MAXIMUM()) {
> 	n = ::select(_maxfd + 1,
> 		     &testfds[SEL_RD_IDX],
> 		     &testfds[SEL_WR_IDX],
> 		     &testfds[SEL_EX_IDX],
> 		     0);
>      } else {

I believe it is on purpose.
Part of the logic behind it is probably because select(2) with
"timeout" argument of NULL also blocks forever.
One extreme scenario that comes to mind is, say, if there are no
timeout events scheduled. In that case you want to select() until a
file descriptor is ready (for reading/writing/etc).

Strictly speaking, the "timeout" argument doesn't need to be a
pointer; instead, it could be pass-by-reference argument.
In fact, the more I think, the more I believe it should be
pass-by-reference. Then, if some code really wants to select(2)
without timeout, the caller should use TimeVal::MAXIMUM() as an
argument.

And yes, TimeVal::ZERO() is used typically to schedule a timer that
will expire immediately.

Thanks,
Pavlin

> 
> -- 
> Ben Greear <greearb at candelatech.com>
> Candela Technologies Inc  http://www.candelatech.com
> 
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