How can I retrieve wLength of a hid device in linux?

Question

I want to retrieve wLength (that can be specified by a hid device). The clear answer maybe is to send a ctrl signal to device. But I tried it by the following code:

struct usbfs_ctrltransfer ctrl = {
    .bmRequestType = LIBUSB_ENDPOINT_IN,
    .bRequest = LIBUSB_REQUEST_GET_CONFIGURATION,
    .wValue = 0,
    .wIndex = 0,
    .wLength = 1,
    ...
}
....
r = ioctl(fd, 0, &ctrl);
....

The result of this code was just an error value (I think that was -1!). I reloaded the hid module (kernel module) in debug mode(modprobe hid debug=100 --> don't panic for this large debug level!). In this case the hid print out the true value of the wLength.

/build/buildd/linux-3.13.0/drivers/hid/usbhid/hid-core.c: submitting ctrl urb: Get_Report wValue=0x0100 wIndex=0x0001 wLength=64

I track it in Linux kernel source code and I found that this information are printed in usb_get_intfdata. In summary I wonder to know if there is an equal function in userland or not?

Answer 1

The answer to this question is using udev. ‌By reading special attribute, called bmAttributes, you can find the I/O's actual length. Using below code to read it:

 ....
 udev_device_get_sysattr_value(dev, "bmAttributes")
 ....