[Xenomai-help] Re: [Socketcan-users] Select on a RTCAN socket

[Wolfgang Grandegger <wg@domain.hid>]

Wolfgang Grandegger wrote:
> Jan Kiszka wrote:
>> Hi all,
>>
>> Wolfgang Grandegger wrote:
>>> Hi Daniel,
>>>
>>> Daniel Schnell wrote:
>>>> Hi,
>>>>
>>>>  
>>>>
>>>> I wanted to use select() on a socket to find out how many messages are 
>>>> available. I have opened this socket to access the RTCAN sockets under 
>>>> Xenomai. According to /usr/xenomai/lib/posix.wrappers select is not 
>>>> "supported" by the realtime kernel. Accordingly it doesn't work. If i 
>>>> start the according task under SCHED_FIFO, the task simply hangs when 
>>>> initiating select(), in SCHED_OTHER it is simply returning: bad file 
>>>> descriptor.
>>> select is not supported and cannot be used, indeed.
>> And select is tricky to provide under hard-RT constraints.
>>
>>>> Any ideas ? Is there any other way to find out, how many messages are 
>>>> waiting ?
>>> Select returns the sockets which have data but it does not tell you how 
>>> many messages are waiting. You could use separate threads to read from 
>>> sockets.
>>>
>>> BTW: this mail is off-topic on the socketcan MLs.
>> Mmh, do you mean it the other way around?
> 
> Well, I think this is a RTDM/Xenomai related questions and sending it to 
> xenomai-help is OK. CAN related questions should go to the 
> socketcan-users maling-list. Am I wrong?
> 
>> Anyway, is there a mechanism of the standard socket API the obtain the
>> current incoming queue length? Don't think so actually, but it wouldn't
>> be the first time I'm blind.
> 
> Not that I know of. For normal file I/O I remember vague some ioctl() or 
> fnctl(). Does POSIX define a function for this purpose?

Got it! In Linux it can be done with ioctl FIONREAD, which returns the 
number of bytes that are immediately available to be read on a file 
descriptor. It could be implemented, if useful.

Wolfgang.