HIDRAW - Raw Access to USB and Bluetooth Human Interface Devices¶
The hidraw driver provides a raw interface to USB and Bluetooth Human Interface Devices (HIDs). It differs from hiddev in that reports sent and received are not parsed by the HID parser, but are sent to and received from the device unmodified.
Hidraw should be used if the userspace application knows exactly how to communicate with the hardware device, and is able to construct the HID reports manually. This is often the case when making userspace drivers for custom HID devices.
Hidraw is also useful for communicating with non-conformant HID devices which send and receive data in a way that is inconsistent with their report descriptors. Because hiddev parses reports which are sent and received through it, checking them against the device’s report descriptor, such communication with these non-conformant devices is impossible using hiddev. Hidraw is the only alternative, short of writing a custom kernel driver, for these non-conformant devices.
A benefit of hidraw is that its use by userspace applications is independent of the underlying hardware type. Currently, Hidraw is implemented for USB and Bluetooth. In the future, as new hardware bus types are developed which use the HID specification, hidraw will be expanded to add support for these new bus types.
Hidraw uses a dynamic major number, meaning that udev should be relied on to create hidraw device nodes. Udev will typically create the device nodes directly under /dev (eg: /dev/hidraw0). As this location is distribution- and udev rule-dependent, applications should use libudev to locate hidraw devices attached to the system. There is a tutorial on libudev with a working example at:
The HIDRAW API¶
read() will read a queued report received from the HID device. On USB devices, the reports read using read() are the reports sent from the device on the INTERRUPT IN endpoint. By default, read() will block until there is a report available to be read. read() can be made non-blocking, by passing the O_NONBLOCK flag to open(), or by setting the O_NONBLOCK flag using fcntl().
On a device which uses numbered reports, the first byte of the returned data will be the report number; the report data follows, beginning in the second byte. For devices which do not use numbered reports, the report data will begin at the first byte.
The write() function will write a report to the device. For USB devices, if the device has an INTERRUPT OUT endpoint, the report will be sent on that endpoint. If it does not, the report will be sent over the control endpoint, using a SET_REPORT transfer.
The first byte of the buffer passed to write() should be set to the report number. If the device does not use numbered reports, the first byte should be set to 0. The report data itself should begin at the second byte.
Hidraw supports the following ioctls:
- Get Report Descriptor Size
This ioctl will get the size of the device’s report descriptor.
- Get Report Descriptor
This ioctl returns the device’s report descriptor using a hidraw_report_descriptor struct. Make sure to set the size field of the hidraw_report_descriptor struct to the size returned from HIDIOCGRDESCSIZE.
- Get Raw Info
This ioctl will return a hidraw_devinfo struct containing the bus type, the vendor ID (VID), and product ID (PID) of the device. The bus type can be one of:
- BUS_USB - BUS_HIL - BUS_BLUETOOTH - BUS_VIRTUAL
which are defined in uapi/linux/input.h.
- Get Raw Name
This ioctl returns a string containing the vendor and product strings of the device. The returned string is Unicode, UTF-8 encoded.
- Get Physical Address
This ioctl returns a string representing the physical address of the device. For USB devices, the string contains the physical path to the device (the USB controller, hubs, ports, etc). For Bluetooth devices, the string contains the hardware (MAC) address of the device.
- Send a Feature Report
This ioctl will send a feature report to the device. Per the HID specification, feature reports are always sent using the control endpoint. Set the first byte of the supplied buffer to the report number. For devices which do not use numbered reports, set the first byte to 0. The report data begins in the second byte. Make sure to set len accordingly, to one more than the length of the report (to account for the report number).
- Get a Feature Report
This ioctl will request a feature report from the device using the control endpoint. The first byte of the supplied buffer should be set to the report number of the requested report. For devices which do not use numbered reports, set the first byte to 0. The report will be returned starting at the first byte of the buffer (ie: the report number is not returned).