[libhid-discuss] Missing reports in hid_interrupt_read()

[Charles Lepple]

On Mar 31, 2011, at 7:27 AM, Jacques Dirac wrote:

> - Is there some routine in libhid (or libusb) that is fetching the
> reports from the IN-report-buffer of the device and saves them in a
> 'buffer' where hid_interrupt_read() is reading from? In my case it
> looks like that buffer is overwritten.

hid_interrupt_read() is simply a thin wrapper around libusb's  
usb_interrupt_read() function. It merely dereferences libhid's  
HIDInterface pointer to get the libusb handle, and prints a few error  
messages as necessary.

http://libhid.alioth.debian.org/doc/hid__exchange_8c-source.html#l00332

> - If so, can this be disabled or influenced?
> - If not, what else can be going on here?

Is it possible that your code is expecting a certain length for the  
reply, and you are discarding the packet if the return code from  
hid_interrupt_read() is non-zero?

If not, you may have to look deeper in the stack. libusb 0.1 generally  
doesn't buffer things, either (although I admit I haven't looked at  
all the platform-specific code lately). There are often kernel-level  
debugging facilities available - with recent Linux 2.6 kernels and  
development versions of libpcap, you can even capture USB traffic to  
be displayed in Wireshark. Unfortunately, that might not pinpoint  
where the extra IN request is coming from.

FreeBSD prior to 8.0 seems to continuously poll interrupt endpoints at  
the interval specified in the USB descriptor, and it buffers the  
results with marginal success. (I wouldn't recommend developing for  
the pre-8.0 USB stack.)