[libhid-discuss] finding multile devices

[Warren Jasper]

Yes, that is exactly what I want to do, write a custom matcher 
function.  However, please look at line 86 in hid_opening.c

        usb_claim_interface(usbdev_h, hidif->interface);

in the function hid_find_usb_device.  The subroutine is traversing the 
usbbuses and usbdevices looking for a device to compare
(send to hid_compare_usb_device).   There is no check on the return to 
usb_claim_interface().  If the interface is already claimed
by a previous call to that device, why would I want to proceed?  Now the 
matcher function does not know if that device/interface
has been previously claimed.  This is ONLY a problem for the case I am 
interested in, where there are multiple devices of
the same type, otherwise the vendor and product won't match.  Should 
this line not read:

   if (usb_claim_interface(usbdev_h, hidif->interface)) continue;   // 
returns 0 on success

Now I am "more" certain that devices that need to be matched have NOT 
been matched previously.

Warren


Charles Lepple wrote:
> [please keep the list CC'd.]
>
> On Jan 12, 2008, at 10:48 AM, Warren Jasper wrote:
>
>> I think you missed my point.  When searching the bus for a match, 
>> there should be a helper function that returns either:
>>
>> 1. ALL the devices that match (could be more than one)
>
> Most people are using one device at a time. You might have an argument 
> for an extra function that returns a list of opened device handles, 
> though.
>
>> 2. Subsequent calls need to return matched devices that have not been 
>> open.
>
> I understand exactly what you're looking for here, and what I was 
> saying is that checking with the hid_is_opened() function can only 
> prove that something in that process (or thread, really) has opened 
> that interface. That won't catch the device/interface being opened by 
> another library, program or thread.
>
> That said, my suggestion about keeping a list inside your code would 
> work. The custom matcher function doesn't have to match against serial 
> numbers - it can also check against a list of devices that your code 
> has previously opened. The reason why I am suggesting this before 
> changing libhid code is that it means you don't have to wait for a new 
> release of libhid to get this functionality, and you can test this 
> idea to make sure there isn't something wrong with my reasoning.
>