Automating keystrokes via evdev

In a previous post I talked about how to capture keys out from under the X11 windowing system by reading from /dev/input/eventX. These character devices can also be useful to generate input simulating keyboard activity.

I circled back to this topic after having to automate user keyboard activity. I've accomplished similar tasks in the past with a tool named xdotool - unfortunately, in this case I did not have the luxury of being able to install software. The remainder of this post highlights the differences between consuming and producing events. (By the way, If you have the need to automate X actions I highly suggest looking at what xdotool can do for you.)

Consuming events is the easier of the two tasks: you simply read open the device and read events into the following structure:

/* See: /usr/include/linux/input.h */
struct input_event {
    struct timeval time;
    __u16 type;
    __u16 code;
    __s32 value;
};      

Filter input with type == 1 and read the code to get the key and value to get the event (eg. press, release).

To produce a compliant event the process is a little more complicated since the input needs to be synchronized. For each event there are three distinct sets of data that are required: setup (EV_MSC); the event (EV_KEY); and event synchronize (EV_SYN). In addition to that, certain events are captured over time so this is a stateful process. An example of this is pressing Ctrl-L; the control key is held down while another key is pressed and then released.

The easiest way I found to initially grok the protocol is to capture all events while there is keyboard activity and see what the output looks like. Obviously, to produce fully compliant input you should consult API documentation or source code.

An example of automatically entering a URL in the Chrome browser (Ctrl-L [URL]) would require the following inputs (the type, code, and value members of struct input_event). The input goes to the focused window (the standard behavior for X) so you need to place focus on the Chrome window for the following example.

4,  4, 29  # Setup  
1, 29,  1  # Press Ctrl key
0,  0,  0  # Sync
4,  4, 29  # Setup  
1, 29,  2  # Ctrl (value == 2 -> autorepeat)
0,  0,  0  # Sync
4,  4, 38
1, 38,  1  # Press 'L' key
0,  0,  0
4,  4, 38
1, 38,  0  # Release 'L' key
0,  0,  0
4,  4, 29
1, 29,  0  # Release Ctrl key
0,  0,  0
  
# and so on for the URL string 

4,  4, 28
1, 28,  1  # Press Enter key
0,  0,  0
4,  4, 28
1, 28,  0  # Release Enter key
0,  0,  0