Format of /dev/input/event*

What is the “format” of the character devices located in /dev/input/event*?

In other words, how can I decode the character stream? A Python example would be greatly appreciated.

Answers:

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Method 1

A simple and raw reader can be just done using:

#!/usr/bin/python
import struct
import time
import sys

infile_path = "/dev/input/event" + (sys.argv[1] if len(sys.argv) > 1 else "0")

"""
FORMAT represents the format used by linux kernel input event struct
See https://github.com/torvalds/linux/blob/v5.5-rc5/include/uapi/linux/input.h#L28
Stands for: long int, long int, unsigned short, unsigned short, unsigned int
"""
FORMAT = 'llHHI'
EVENT_SIZE = struct.calcsize(FORMAT)

#open file in binary mode
in_file = open(infile_path, "rb")

event = in_file.read(EVENT_SIZE)

while event:
    (tv_sec, tv_usec, type, code, value) = struct.unpack(FORMAT, event)

    if type != 0 or code != 0 or value != 0:
        print("Event type %u, code %u, value %u at %d.%d" % 
            (type, code, value, tv_sec, tv_usec))
    else:
        # Events with code, type and value == 0 are "separator" events
        print("===========================================")

    event = in_file.read(EVENT_SIZE)

in_file.close()

Method 2

The format is described in the Documentation/input/input.txt file in the Linux source. Basically, you read structs of the following form from the file:

struct input_event {
    struct timeval time;
    unsigned short type;
    unsigned short code;
    unsigned int value;
};

type and code are values defined in linux/input.h. For example,
type might be EV_REL for relative moment of a mouse, or EV_KEY for
a keypress, and code is the keycode, or REL_X or ABS_X for a
mouse.

Method 3

Right here in the Input.py module. You’ll also need the event.py module.

Method 4

The python-evdev package provides bindings to the event device interface. A short usage example would be:

from evdev import InputDevice
from select import select

dev = InputDevice('/dev/input/event1')

while True:
   r,w,x = select([dev], [], [])
   for event in dev.read():
       print(event)

# event at 1337427573.061822, code 01, type 02, val 01
# event at 1337427573.061846, code 00, type 00, val 00

Keep in mind that, unlike the very convenient, purely Pythonic modules mentioned so far, evdev contains C extensions. Building them requires having your Python development and kernel headers installed.

Method 5

The data is in the form of input_event structures; see http://www.thelinuxdaily.com/2010/05/grab-raw-keyboard-input-from-event-device-node-devinputevent/ for a C example. The struct definition is at (for example) http://www.cs.fsu.edu/~baker/devices/lxr/http/source/linux/include/linux/input.h?v=2.6.11.8. Note that you will need to use a bunch of ioctl calls to get information on the device before reading from it.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0