Paolo Mantegazza wrote: > M. Koehrer wrote: >> Hi Dennis, >> >> I have just detected the very same effect. At least with RTAI-3.3cv it is >> not working. >> In rtnet's rt_udp_recvmsg() >> the timeout will be set to -1 if the flag MSG_DONTWAIT is used. >> >> The call to rtdm_sem_timeddown() with this timeout value fails as the >> resulting RTAI >> function _sem_wait_timed rejects a negative timeout => it returns >> -EWOULDBLOCK immediately >> without checking the semaphores value. >> RTDM in RTAI seems not to handle a polling semaphore check correctly. >> That means the only possible way (without changing rtdm) is to it the way >> you have done it: >> Using a extreme small timeout value (=1). >> >> By looking at the code I saw same strange things: >> In rtnet, the time type nanosecs_t is a uint64, however in RTAI's rtdm the >> timetype is a signed int64. >> That could lead to confusion... >> >> To fix the MSG_DONTWAIT issue I have detected, the code can be modified to >> set the timeout >> to 1 (instead of -1) whenever MSG_DONTWAIT is specified. >> > > Negative timeouts produce -EWOULDBLOCK in the original implementation > of RTDM also. The difference is that there the sem count check is > anticipated and the count simply decremented with immediate return if it > is greater than zero. RTAI checks for -EWOULDBLOCK before and so it does > not call its timed sem_wait, where the sem count will be decremented, > withoutn any timeout, if it is greater then zero. So it will behave as > expected. > > Clearly the solution woulfd be to use 0, but it cannot, as zero > indicates an infinite delay in RTDM. So one (nanosec) has to be the > compromise. Notice it will never produce any delay, either because sem > count is greater than zero or because it is too low a value and RTAI > will timeout immediately anyhow, as it is never worth to have to > reschedule for a single nanosec. >
rtdm_sem_timeddown: "This function tries to decrement the given semphore's value if it is positive on entry. If not, the caller is blocked unless non-blocking operation was selected." I read my one text here, so I may interpret it implicitly as I want it to be. But to me it sounds like: "Try to get the sem first, then block or fail." Please consider fixing RTAI's RTDM layer, e.g. by mapping timeouts < 0 to timeout = 1. It's the intention of RTDM to keep the semantic consistent over all platforms. Thanks, Jan
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