Grab Raw Keyboard Input from Event Device Node (/dev/input/event)

May

Posted by Derek@TheDailyLinux in Programming » 12 Comments »

Grab Raw Keyboard Input from Event Device Node (/dev/input/event)

The following is a quick c program that will capture raw keyboard data from the event device node such as /dev/input/event1. Simply compile and run with the specific device node as an argument. ie. ./keyboard_key_capture /dev/input/event1.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <dirent.h>
#include <linux/input.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/select.h>
#include <sys/time.h>
#include <termios.h>
#include <signal.h>

void handler (int sig)
{
  printf ("nexiting...(%d)n", sig);
  exit (0);
}

void perror_exit (char *error)
{
  perror (error);
  handler (9);
}

int main (int argc, char *argv[])
{
  struct input_event ev[64];
  int fd, rd, value, size = sizeof (struct input_event);
  char name[256] = "Unknown";
  char *device = NULL;

  //Setup check
  if (argv[1] == NULL){
      printf("Please specify (on the command line) the path to the dev event interface devicen");
      exit (0);
    }

  if ((getuid ()) != 0)
    printf ("You are not root! This may not work...n");

  if (argc > 1)
    device = argv[1];

  //Open Device
  if ((fd = open (device, O_RDONLY)) == -1)
    printf ("%s is not a vaild device.n", device);

  //Print Device Name
  ioctl (fd, EVIOCGNAME (sizeof (name)), name);
  printf ("Reading From : %s (%s)n", device, name);

  while (1){
      if ((rd = read (fd, ev, size * 64)) < size)
          perror_exit ("read()");      

      value = ev[0].value;

      if (value != ' ' && ev[1].value == 1 && ev[1].type == 1){ // Only read the key press event
	   printf ("Code[%d]n", (ev[1].code));
      }
  }

  return 0;
}

Here is an example output from running the above command. Notice that Code[] is printed before the key that was pressed.

# ./keyb_key_cap_x86 /dev/input/by-id/usb-Dell_Dell_USB_Keyboard-event-kbd
Reading From : /dev/input/by-id/usb-Dell_Dell_USB_Keyboard-event-kbd (Dell Dell USB Keyboard)
Code[30]
aCode[48]
bCode[46]
cCode[32]
dCode[18]
eCode[33]
fCode[34]
gCode[35]
hCode[23]
iCode[36]
jCode[37]
kCode[38]
lCode[50]
mCode[49]
nCode[24]
oCode[25]
pCode[16]
qCode[19]
rCode[31]
sCode[20]
tCode[22]
uCode[47]
vCode[17]
wCode[45]
xCode[21]
yCode[44]
zCode[2]
1Code[3]
2Code[4]
3Code[5]
4Code[6]
5Code[7]
6Code[8]
7Code[9]
8Code[10]
9Code[11]
0

This is only an example program that I picked up from work. I hope it will be of use to somebody out there so it can quickly help you get started with your project.

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There's 12 Comments So Far

Adam Marchetti
August 12th, 2010 at 9:54 am

What license are you posting this under?

I’m including it in an keyboard monitoring app I wrote (nealy verbatim), and you
never specified a license.
Derek@TheDailyLinux
August 12th, 2010 at 10:57 am

Good question. Anything I post on this site is open game to everybody. I wouldn’t say there is any license, but if I were to give it one I suppose it would fall under the BSD License which allows everyone to use and redistribute the program as they wish.

Just don’t use it for evil.
Adam Marchetti
August 12th, 2010 at 2:29 pm

“Just don’t use it for evil. ”

Decide yourself

The project is at http://github.com/adamnew123456/kbmon

It is (in it’s slightly modified form) in “src/kbd-demo.c”
Helmik
December 22nd, 2010 at 4:25 am

Hi, thank you very much for this program! I just wonder why did you skip ev[0]? it was not working for me. I have modified it in a such way:

instead of this code:
if (value != ‘ ‘ && ev[1].value == 1 && ev[1].type == 1){ // Only read the key press event
printf (“Code[%d]n”, (ev[1].code));
}

i used this:
if (ev[0].value == 1 && ev[0].type == EV_KEY) // Only read the key press event
printf (“Code[%d]n”, (ev[0].code));
Derek@TheDailyLinux
December 22nd, 2010 at 8:05 am

It was more of a throw-it-together program for testing purposes. I’m happy to hear that 1) you’ve found it useful and 2) you have brought an improvement to the table! Thank you!
Michael Gardner
December 27th, 2010 at 3:37 am

Thank you for posting this. I’ve been working on the same thing for about a day now and while I can read the output from event(x) I was having a difficult time figuring out its format, especially since I haven’t had much practice with UTF. After digging through the Linux kernel device drivers I got lucky finding your included headers and got pointed in the right direction. Thanks a ton!
matt
February 9th, 2011 at 1:22 pm

is it possible to have the ((rd = read (fd, ev, size * 64)) < size) non blocking (if no key is pressed) ?
matt
February 9th, 2011 at 1:53 pm

hum found sorry
O_RDONLY | O_NONBLOCK
dima
February 15th, 2011 at 5:03 am

Thank you for this good example!
mathio
July 18th, 2011 at 6:27 am

Hi all,
I have a problem with reading USB keyboard event file. I can read this file using this program but the problem is that some characters that I type are read multiple times(2 or 3 times). Like I type on my keyboard some number, for example code(73) and my application reads this thing often 2 times, or 3 times. I’ve been looking for this all over the Internet and I didn’t find anything useable to solve this problem yet. Could anyone help me with this?
Or could anyone explain me if the keyboard event file is somehow buffered and if there is any way to clean this file after every reading from this file? Maybe this could solve my problem that some characters are read multiple – 2 or 3 times.
Devesh Yamparala
October 24th, 2011 at 6:40 am

Thanks man. It was really helpful