What data can a HID device receive?

I am designing a USB keyboard with special capabilities. What information can such a HID device receive from the host?

Can I via USB:

• Read data from a form on the screen?
• Find out what OS the user is on?
• Find out if there's been an error message?
• Even 'know' what's going on visually on the screen, i.e. what program is selected or whether the program is windowed or fullscreen?

Thank you!

The device can't get any of this information from a standard driver that the operating system supplies because that would be a security issue. It can receive any information that your own driver or application sends it. There are many ways to communicate with it - your device could present multiple interfaces (which will appear as separate devices), multiple endpoints, or use the control channel. You will definitely need to study the spec, and I also found this tutorial helpful.

I have done something similar and used the control channel to exchange feature data with a Windows application (over the standard Windows driver). On Windows, the API calls are HidD_SetFeature() and HidD_GetFeature().

On the device side, my hardware ran embedded Linux and I used the GadgetFS library to create a user-mode driver - much easier to debug than a kernel driver.