Using C to simulate a keystroke in linux with the uinput library

Question

I've been using the following C code to try to simulate keystrokes on a CentOS 6.0 machine:

#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#inlcude <linux/input.h>
#include <linux/uinput.h>
#include <sys/time.h>

static int fd = -1;
struct uinput_user_dev uidev;
struct input_event event;

int main()
{
    int i;
    fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK);

    memset(&uidev, 0, sizeof(uidev));

    snrpintf(uidev.name, UINPUT_MAX_NAME_SIZE, "uinput-kbd");
    uidev.id.version = 1;
    uidev.id.vendor = 0x1;
    uidev.id.product = 0x1;
    uidev.id.bustype = BUS_USB;

    ioctl(fd, UI_SET_EVBIT, EV_KEY);

    for(i = 0; i < 256; i++)
    {
        ioctl(fd, UI_SET_KEYBIT, i);
    }

    ioctl(fd, UI_SET_EVBIT, EV_SYN);

    write(fd, &uidev, sizeof(uidev));

    ioctl(fd, UI_DEV_CREATE));

    memset(&event, 0, sizeof(event));
    gettimeofday(&event.time, NULL);
    event.type = EV_KEY;
    event.code = KEY_1;
    event.value = 1;
    write(fd, &event, sizeof(event));

    event.type = EV_SYN;
    event.code = SYN_REPORT;
    event.value = 0;
    write(fd, &event, sizeof(event));

    memset(&event, 0, sizeof(event));
    gettimeofday(&event.time, NULL);
    event.type = EV_KEY;
    event.code = KEY_1;
    event.value = 0;
    write(fd, &event, sizeof(event));

    event.type = EV_SYN;
    event.code = SYN_REPORT;
    event.value = 0;
    write(fd, &event, sizeof(event));

    ioctl(fd, UI_DEV_DESTROY);
    close(fd);

    return 0;
}

If I'm correct, this code should create a virtual input device on the machine and then press the "1" key on that device. And when I execute the code, it seems to run without any issues (I haven't include the code that checks to make sure the device is being created and the keystrokes are being written, etc, in my example code, because it would have gotten way too long), but I can't see any sign of the actual keystroke.

My impression was that if I run this from a terminal window while logged directly into the machine, I should see a "1" character appear on the terminal window that I'm running it from. And if I log into the machine via ssh and run it that way, the keystroke should register on the machine rather than the ssh session. But I'm not getting anything in either situation.

Am I misunderstanding the purpose of this code? Have I done it wrong? Or is the more that I need to add to properly simulate a keystroke?

Answer 1

Wow, I was having the same issue as you just a few minutes ago, but now I don't have the issue anymore, and according to evtest it seems to work. I don't know what I did that resolved the problem unfortunately.

Here's the steps I used to troubleshoot, hopefully that at least gets you somewhere:

• Verify that dmesg reports input device when connected.

Example:

[Fri Jul 22 22:38:55 2022] input: Custom Tourbox TBG_H Driver as /devices/virtual/input/input25

• Verify that program can write to /dev/uinput file. Can be done via checking return value of write() call (the example code you listed doesn't).

If you have any issues with the above, double-check permissions. ( I did the hack of running my program as root and then changing the permission on the /dev/uinput file to be world-writable, via:

$ chmod +0666 /dev/uinput       

This probably isn't recommended though, it would probably be better to create a group and then add your user to that group )

• Verify that evtest recognizes your new driver.

Run:

$ sudo evtest

And you should see your device (probably at the bottom):

/dev/input/event24: Custom Tourbox TBG_H Driver

• Verify that evtest recognizes that your device has registered an event handler for the keys you wish to use. In my case, I need to simulate key presses for 'a', so I made sure to verify that key was listed as one of the possible event types:

This is what the example output looked like:

Input driver version is 1.0.1
Input device ID: bus 0x3 vendor 0x483 product 0xbeef version 0x0
Input device name: "Custom Tourbox TBG_H Driver"
Supported events:
  Event type 0 (EV_SYN)
  Event type 1 (EV_KEY)
    Event code 30 (KEY_A)

From there, all I did was press the key and it worked.

Answer 2

Here now I found why this can't work, I spend two days finding the problem. Luckily and finally, I found it in linux kernel 5.60 source example which is in chapter 7.4. Here is the picture .Solution

the document said that we'd better wait for some time so that there is a space for userspace to detect event, because creating a device node in kernel state will spend lots of time compared to sending a event to fd, hopes that the answer is not too late.

Here is the link 7.Uinput-module