HIDRAW - Raw Access to USB and Bluetooth Human Interface Devices¶
The hidraw driver provides a raw interface to USB and Bluetooth HumanInterface Devices (HIDs). It differs from hiddev in that reports sent andreceived are not parsed by the HID parser, but are sent to and received fromthe device unmodified.
Hidraw should be used if the userspace application knows exactly how tocommunicate with the hardware device, and is able to construct the HIDreports manually. This is often the case when making userspace drivers forcustom HID devices.
Hidraw is also useful for communicating with non-conformant HID deviceswhich send and receive data in a way that is inconsistent with their reportdescriptors. Because hiddev parses reports which are sent and receivedthrough it, checking them against the device’s report descriptor, suchcommunication with these non-conformant devices is impossible using hiddev.Hidraw is the only alternative, short of writing a custom kernel driver, forthese non-conformant devices.
A benefit of hidraw is that its use by userspace applications is independentof the underlying hardware type. Currently, Hidraw is implemented for USBand Bluetooth. In the future, as new hardware bus types are developed whichuse the HID specification, hidraw will be expanded to add support for thesenew bus types.
Hidraw uses a dynamic major number, meaning that udev should be relied on tocreate hidraw device nodes. Udev will typically create the device nodesdirectly under /dev (eg: /dev/hidraw0). As this location is distribution-and udev rule-dependent, applications should use libudev to locate hidrawdevices attached to the system. There is a tutorial on libudev with aworking example at:
The HIDRAW API¶
read()¶
read() will read a queued report received from the HID device. On USBdevices, the reports read using read() are the reports sent from the deviceon the INTERRUPT IN endpoint. By default, read() will block until there isa report available to be read. read() can be made non-blocking, by passingthe O_NONBLOCK flag to open(), or by setting the O_NONBLOCK flag usingfcntl().
On a device which uses numbered reports, the first byte of the returned datawill be the report number; the report data follows, beginning in the secondbyte. For devices which do not use numbered reports, the report datawill begin at the first byte.
write()¶
The write() function will write a report to the device. For USB devices, ifthe device has an INTERRUPT OUT endpoint, the report will be sent on thatendpoint. 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 reportnumber. If the device does not use numbered reports, the first byte shouldbe set to 0. The report data itself should begin at the second byte.
ioctl()¶
Hidraw supports the following ioctls:
- HIDIOCGRDESCSIZE:
- Get Report Descriptor Size
This ioctl will get the size of the device’s report descriptor.
- HIDIOCGRDESC:
- Get Report Descriptor
This ioctl returns the device’s report descriptor using ahidraw_report_descriptor struct. Make sure to set the size field of thehidraw_report_descriptor struct to the size returned from HIDIOCGRDESCSIZE.
- HIDIOCGRAWINFO:
- Get Raw Info
This ioctl will return a hidraw_devinfo struct containing the bus type, thevendor ID (VID), and product ID (PID) of the device. The bus type can be oneof:
- BUS_USB- BUS_HIL- BUS_BLUETOOTH- BUS_VIRTUAL
which are defined in uapi/linux/input.h.
- HIDIOCGRAWNAME(len):
- Get Raw Name
This ioctl returns a string containing the vendor and product strings ofthe device. The returned string is Unicode, UTF-8 encoded.
- HIDIOCGRAWPHYS(len):
- 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 (theUSB controller, hubs, ports, etc). For Bluetooth devices, the stringcontains the hardware (MAC) address of the device.
- HIDIOCSFEATURE(len):
- Send a Feature Report
This ioctl will send a feature report to the device. Per the HIDspecification, feature reports are always sent using the control endpoint.Set the first byte of the supplied buffer to the report number. For deviceswhich do not use numbered reports, set the first byte to 0. The report databegins in the second byte. Make sure to set len accordingly, to one morethan the length of the report (to account for the report number).
- HIDIOCGFEATURE(len):
- Get a Feature Report
This ioctl will request a feature report from the device using the controlendpoint. The first byte of the supplied buffer should be set to the reportnumber of the requested report. For devices which do not use numberedreports, set the first byte to 0. The report will be returned starting atthe first byte of the buffer (ie: the report number is not returned).
Example¶
In samples/, find hid-example.c, which shows examples of read(), write(),and all the ioctls for hidraw. The code may be used by anyone for anypurpose, and can serve as a starting point for developing applications usinghidraw.
Document by:
Alan Ott <alan@signal11.us>, Signal 11 Software