How to tell when a 'virtual' file changes, or when data moves through a /dev/ file?

Daravon · 08-14-2012, 03:40 PM

I'm making an embedded linux thing and I want to detect simple digital input to detect an event. I thought it would be cool just to use a USB mouse. All I have to do is poll for mouse clicks.

I can't find a nice simple commandline program for polling mouse button states, so I figured I could just watch /dev/mouse. If I cat /dev/mouse, and I move the mouse around or click it, my screen fills up with mojibake, so I know there is data coming in. I just don't know how to actually use this to poll the mouse with perl or bash, because /dev/mouse is not a normal file.

I want to look at /dev/mouse and detect when any data comes through it. I don't care what the data is.

I don't know much about pipes, but would this work: I could make a named pipe and redirect its output to a temp file. Then, when I run my perl program, I can just test for the existence of the temporary file. If it exists, there has been a mouse click. Then I can just rm the temp file with my perl program or bash and poll again. The temp file will be recreated whenever the mouse is clicked. Would this work? Is there a better way?

sundialsvcs · 08-14-2012, 04:28 PM

You really can't expect to do it that way. The window manager is going to be dealing with those mouse clicks, and your software can register with the WM (whichever one it may be) to receive them.

Daravon · 08-14-2012, 08:10 PM

It's already working, in that I can cat /dev/mouse and get the data. I just need a programmatic way to register when data comes through, and I can't think of a smart way to do that, even using scratch files.

If I can direct /dev/mouse to a file, I can just test the file for length. If it's nonzero, there has been a mouse click. Then I can rm the file and re-touch it.

But there has no be a correct way to simply poll the mouse button at the kernel driver or libusb level.

NevemTeve · 08-14-2012, 10:48 PM

Test if select/poll works on /dev/mouse.

Daravon · 08-15-2012, 08:26 AM

I might try select/poll yet, but right now, this works!

Code:

#include <stdio.h>
main(){
    printf("%s", "Hello World\n");
    int dat;
    FILE *mouse;
    mouse = fopen("/dev/input/mice", "r");
    //mouse = fopen("temp", "rb");
    dat = fgetc(mouse);
    printf("%i", dat);
    return (fclose(mouse));
}

This is pretty much what I need, since I just need to detect any activity on the mouse.
~

schneidz · 08-15-2012, 08:55 AM

would this help:

Code:

cat /dev/mouse | od # man od

i do this sometimes to examine the output of /dev/dsp.