The buttons on the EV3 are mapped as regular keyboard keys


Polling Key States

If you would rather poll the button states instead of using regular keyboard input methods, you can do that via the EVIOCGKEY ioctl on /dev/input/eventX. Key codes and ioctl definitions are in linux/input.h.

Here is an example using python:

#!/usr/bin/env python

import array
import fcntl
import sys

# from linux/input.h

KEY_UP = 103
KEY_DOWN = 108
KEY_LEFT = 105

KEY_MAX = 0x2ff

def EVIOCGKEY(length):
    return 2 << (14+8+8) | length << (8+8) | ord('E') << 8 | 0x18

# end of stuff from linux/input.h

BUF_LEN = (KEY_MAX + 7) / 8

def test_bit(bit, bytes):
    # bit in bytes is 1 when released and 0 when pressed
    return bool(bytes[bit / 8] & (1 << (bit % 8)))

def main():
    buf = array.array('B', [0] * BUF_LEN)
    with open('/dev/input/by-path/platform-gpio-keys.0-event', 'r') as fd:
        ret = fcntl.ioctl(fd, EVIOCGKEY(len(buf)), buf)

    if ret < 0:
        print "ioctl error", ret

    for key in ['UP', 'DOWN', 'LEFT', 'RIGHT', 'ENTER', 'BACKSPACE']:
        key_code = globals()['KEY_' + key]
        key_state = test_bit(key_code, buf) and "pressed" or "released"
        print '%9s : %s' % (key, key_state)

if __name__ == "__main__":

Directly Reading the Event Device

If you want your program to be event driven, you can read the /dev/input/by-path/platform-gpio-keys.0-event file. It will block until a key is pressed. The data is in 16 byte blocks. The first 8 bytes are the time stamp (2 unsigned 64-bit integers, seconds and microseconds), the next 2 bytes are the type (unsigned 16-bit integer), the next 2 bytes are the code (unsigned 16-bit integer) and the last 4 bytes are the value (unsigned 32-bit integer).

Here is an example. It prints out 2 lines each time you press a button on the EV3 and 2 more lines each time you release a button. And of course, press CTRL+C to end.

root@ev3dev:~# hexdump -e \
'"timestamp:%d.%6d""\t""" 1/2 "type:%i""\t"""  1/2 "code:%3i""\t"""  "value:%d\n"' \
< /dev/input/by-path/platform-gpio-keys.0-event 
timestamp:1391366282.119886 type:1  code: 14    value:1
timestamp:1391366282.120100 type:0  code:  0    value:0
timestamp:1391366282.829942 type:1  code: 14    value:0
timestamp:1391366282.830179 type:0  code:  0    value:0
timestamp:1391366284.300099 type:1  code:105    value:1
timestamp:1391366284.300319 type:0  code:  0    value:0
timestamp:1391366284.829989 type:1  code:105    value:0
timestamp:1391366284.830223 type:0  code:  0    value:0
timestamp:1391366286.019866 type:1  code:103    value:1
timestamp:1391366286.020076 type:0  code:  0    value:0
timestamp:1391366286.319967 type:1  code:103    value:0
timestamp:1391366286.320202 type:0  code:  0    value:0
timestamp:1391366287.169925 type:1  code:106    value:1
timestamp:1391366287.169973 type:0  code:  0    value:0
timestamp:1391366287.479994 type:1  code:106    value:0
timestamp:1391366287.480234 type:0  code:  0    value:0
timestamp:1391366288.339882 type:1  code: 28    value:1
timestamp:1391366288.340092 type:0  code:  0    value:0
timestamp:1391366288.699929 type:1  code: 28    value:0
timestamp:1391366288.700291 type:0  code:  0    value:0
timestamp:1391366289.470046 type:1  code:108    value:1
timestamp:1391366289.470263 type:0  code:  0    value:0
timestamp:1391366289.829943 type:1  code:108    value:0
timestamp:1391366289.830176 type:0  code:  0    value:0