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Date: Wed, 23 Aug 2006 19:21:47 +0200
From: Jan Kiszka <jan.kiszka@domain.hid>
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Subject: [Xenomai-help] Re: [Socketcan-users] Select on a RTCAN socket
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To: Wolfgang Grandegger <wg@domain.hid>
Cc: Haukur Hafsteinsson <haukur@domain.hid>, xenomai-help <xenomai@xenomai.org>, socketcan-users@domain.hid, Daniel Schnell <danielsch@domain.hid>

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Hi all,

Wolfgang Grandegger wrote:
> Hi Daniel,
>=20
> Daniel Schnell wrote:
>> Hi,
>>
>> =20
>>
>> I wanted to use select() on a socket to find out how many messages are=
=20
>> available. I have opened this socket to access the RTCAN sockets under=
=20
>> Xenomai. According to /usr/xenomai/lib/posix.wrappers select is not=20
>> "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=20
>> descriptor.
>=20
> select is not supported and cannot be used, indeed.

And select is tricky to provide under hard-RT constraints.

>=20
>> Any ideas ? Is there any other way to find out, how many messages are =

>> waiting ?
>=20
> Select returns the sockets which have data but it does not tell you how=
=20
> many messages are waiting. You could use separate threads to read from =

> sockets.
>=20
> BTW: this mail is off-topic on the socketcan MLs.

Mmh, do you mean it the other way around?

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.

Jan


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