Hi Arnaldo, Ping! Cheers, Michael On Wed, Apr 30, 2014 at 3:59 PM, Michael Kerrisk (man-pages) <mtk.manpages@xxxxxxxxx> wrote: > Arnaldo, > > I raised this issue somewhat more than a year ago, here: > http://thread.gmane.org/gmane.linux.man/3477 > but got no reply from you. (Chris Friesen in that thread agreed > that there is a problem though.) > > Here, a slightly revised version of that mail, since I've just bumper > into a related problem in a different context... > > As part of his attempt to better document the recvmmsg() syscall that > you added in commit a2e2725541fad72416326798c2d7fa4dafb7d337, Elie de > Brauwer alerted to me to some strangeness in the timeout behavior of > the syscall. I suspect there's a bug that needs fixing, as detailed > below. > > AFAICT, the timeout argument was added to this syscall as a result of > the discussion here: > http://markmail.org/message/m5l2ap4hiiimut6k#query:+page:1+mid:m5l2ap4hiiimut6k+state:results > (20-21 May 2009, "[RFC 1/2] net: Introduce recvmmsg...") > > If I understand correctly, the *intended* purpose of the timeout > argument is to set a limit on how long to wait for additional > datagrams after the arrival of an initial datagram. However, the > syscall behaves in quite a different way. Instead, it potentially > blocks forever, regardless of the timeout. The way the timeout seems > to work is as follows: > > 1. The timeout, T, is armed on receipt of first diagram, starting at time X. > 2. After each further datagram is received, a check is made if we have > reached time X+T. If we have reached that time, then the syscall > returns. > > Since the timeout is only checked after the arrival of each datagram, > we can have scenarios like the following: > > 0. Assume a timeout of 10 seconds, and that vlen is 5. > 1. First datagram arrives at time X. > 2. Second datagram arrives at time X+2 secs > 3. No more datagrams arrive. > > In this case, the call blocks forever. Is that intended behavior? > (Basically, if up to vlen-1 datagrams arrive before X+T, but then no > more datagrams arrive, the call will remain blocked forever.) If it's > intended behavior, could you elaborate the use case, since it would be > good to add that to the man page. If not, a fix seems to be needed, > since otherwise, it's hard to see how the recvmmsg() timeout argument > can sanely be used. > > Thanks, > > Michael > > -- > Michael Kerrisk > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ > Linux/UNIX System Programming Training: http://man7.org/training/ -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
