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

[Wolfgang Grandegger <wg@domain.hid>]

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?

Wolfgang.