Listening key events on system level - c++

Question

I need to write an application, which will listen to some key events and after they occurs, application will do something (this isn't important for this question).
The application will run like a deamon - on background (possible on system tray) and waiting for input.
The question is, how can I listen to key events on system level? I prefer some Unix C solution (priority isn't portability to non-Unix system), but if there is some handy Qt class, why don't use it?
EDIT: Isn't there some way, to tell the operating system something like: "Hi! I am here, wake me up on 'some keyboard event'!"?

Answer 1

qxtglobalshortcut is for shortcuts. Qt offers different ways to handle native events. It is for example QWidget::nativeEvent or QAbstractNativeEventFilter.
But if you want to use system API, then you can try my code. It is code which executes inside separate thread and asynchronously invoke method to notify user when event occurs. Ready for copy-paste, but set name of your keyboard.
#include <QApplication>
#include <QDebug>
#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>
#include <QSystemTrayIcon>
#include <thread>
QSystemTrayIcon *tray;
void handler (int sig)
{
qDebug ("nexiting...(%d)n", sig);
exit (0);
}
void perror_exit (char *error)
{
perror (error);
handler (9);
}
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
tray = new QSystemTrayIcon;
QPixmap px(20,20);
px.fill(Qt::green);
tray->setIcon(QIcon(px));
tray->show();
tray->showMessage("hello","hello",QSystemTrayIcon::Information,1000);
//need this to use invokeMthod
qRegisterMetaType<QSystemTrayIcon::MessageIcon>("QSystemTrayIcon::MessageIcon");
std::thread thread([tray]()
{
struct input_event ev[64];
int fd, rd, value, size = sizeof (struct input_event);
char name[256] = "Unknown";
char *device = NULL;
if ((getuid ()) != 0)
qDebug ("You are not root! This may not work...n");
//my keyboard,set name of yours
device = "/dev/input/by-id/usb-SIGMACHIP_USB_Keyboard-event-kbd";
//Open Device
if ((fd = open (device, O_RDONLY)) == -1)
qDebug ("%s is not a vaild device.n", device);
//Print Device Name
ioctl (fd, EVIOCGNAME (sizeof (name)), name);
qDebug ("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
qDebug ("Code[%d]n", (ev[1].code));
QMetaObject::invokeMethod(tray,"showMessage",Qt::QueuedConnection,Q_ARG(QString,"Was pressed"),Q_ARG(QString,QString::number(ev[1].code)),
Q_ARG(QSystemTrayIcon::MessageIcon,QSystemTrayIcon::Information),Q_ARG(int,500));
}
}
});
qDebug("after thread");
return a.exec();
}
I used code from here, but just changed it to be in Qt manner.
To run programm you must use sudo.
sudo /path/to/exe
#if you want to run it inside qt creator but with sudo
sudo /path/to/qtcreator

Answer 2

It's not Qt (yet) but is somehow connected, there is this class in the Qxt library called qxtglobalshortcut
Here is the link: http://libqxt.bitbucket.org/doc/tip/qxtglobalshortcut.html

Answer 3

Short: You can't do this.
Longer answer: You can, but you have to write an keyboard-driver.
Even with root/Administrator privileges, you need an input-widget which will have OS input focus. (This is to avoid grabbing the input from other input widgets, like password-fields or your chat). - I fully agree ddriver's comment.
If your service has a focused-widget, you can use Widget's text-events or use QObject::installEventFilter
Another point: You can reuse/notify the event within your service but not send to other applications. See notify. If the service handled the keyboard event, no other application will receive the event. And if focus-widget's application already accepted key-event, your service does not get it.
But I do agree: Some OS allow to access the keyboad-devices. (Like Chernobyl's answer). For these, you can implement an own-Keyboard-device driver / handler. Example for Linux embedded is: QWSServer.
Note for users: Using such driver is not save! Please be carefull if you use 3rd party keyboard-driver! For Windows it is highly recomanded only to use Windows-trusted drivers.
Anyway: Implementing an own keyboard-driver can solve Rainbow Tom's issue.