Several sockets on one interface in Python

Several sockets on one interface in Python

[Joachim Jaus]

Hi all,
this is my first email to this list, therefore I want start with a big 
"thank you" to all the people involved in the Linux CAN socket 
development, which has opened up a lot of interesting possibilities for 
sensor and machine control for me and my colleagues.

We would like to bind several sockets with different filter settings to 
one single CAN interface within the same application. Is that generally 
a safe thing to do (both in C and in Python)?
Is there a way to set filters on a CAN socket in Python similar to the 
implementation in C?

Background:
We are implementing a machine control system with several sensors and 
actuators communicating over the same CAN bus using the CAN-open protocol.
There are several control loops running simultaneously, each controlled 
by a python sub-process.
It would be convenient if each subprocess had several sockets, with each 
socket receiving only messages sent by a certain CAN-open node or a 
group of CAN-open nodes. That way, we could most easily receive all 
relevant messages (SDOs, PDOs, ...) from a specific node or group of nodes.

Best regards,
Joachim

Re: Several sockets on one interface in Python

[Oliver Hartkopp]

Hi Joachim,

On 31.05.2013 11:55, Joachim Jaus wrote:
> Hi all,
> this is my first email to this list, therefore I want start with a big "thank
> you" to all the people involved in the Linux CAN socket development, which has
> opened up a lot of interesting possibilities for sensor and machine control
> for me and my colleagues.

Great! Mind-expansion without drugs but only through SocketCAN :-)

> 
> We would like to bind several sockets with different filter settings to one
> single CAN interface within the same application. Is that generally a safe
> thing to do (both in C and in Python)?

Yes, it's a safe thing to do.

E.g. the candump programm does it in this way.

You may create multiple sockets having each multiple filters.

The only potential drawback that you need to put a select() statement around
it, to manage to read from all created sockets/filedescriptors.

> Is there a way to set filters on a CAN socket in Python similar to the
> implementation in C?

I don't know exactly.
But "CAN_RAW_FILTER" is defined in the Python patch:

http://bugs.python.org/file23234/socketcan_v5.patch

http://libbits.wordpress.com/2012/05/22/socketcan-support-in-python/

so you can hope that the filters are configurable in some way.

Perhaps Yegor knows more about this.

Best ragards,
Oliver

Re: Several sockets on one interface in Python

[Yegor Yefremov]

On Fri, May 31, 2013 at 6:38 PM, Oliver Hartkopp <socketcan@hartkopp.net> wrote:
> Hi Joachim,
>
> On 31.05.2013 11:55, Joachim Jaus wrote:
>> Hi all,
>> this is my first email to this list, therefore I want start with a big "thank
>> you" to all the people involved in the Linux CAN socket development, which has
>> opened up a lot of interesting possibilities for sensor and machine control
>> for me and my colleagues.
>
> Great! Mind-expansion without drugs but only through SocketCAN :-)
>
>>
>> We would like to bind several sockets with different filter settings to one
>> single CAN interface within the same application. Is that generally a safe
>> thing to do (both in C and in Python)?
>
> Yes, it's a safe thing to do.
>
> E.g. the candump programm does it in this way.
>
> You may create multiple sockets having each multiple filters.
>
> The only potential drawback that you need to put a select() statement around
> it, to manage to read from all created sockets/filedescriptors.
>
>> Is there a way to set filters on a CAN socket in Python similar to the
>> implementation in C?
>
> I don't know exactly.
> But "CAN_RAW_FILTER" is defined in the Python patch:
>
> http://bugs.python.org/file23234/socketcan_v5.patch
>
> http://libbits.wordpress.com/2012/05/22/socketcan-support-in-python/
>
> so you can hope that the filters are configurable in some way.
>
> Perhaps Yegor knows more about this.

No. Never tried filtering. Was just curious, how Python support feels
like, tried the example and wrote the blog entry. Joachim, please try
and share your experience. I'll then include this into our wiki:
http://elinux.org/Python_Can

You can also try to ask devs involved in this patch
creation/reviewing: http://bugs.python.org/issue10141

Yegor