Unix Signal Watcher For Qt

Author: Simon Knopp
Licence: MIT

Summary

When writing a Qt application, as with any application, it is sometimes useful to handle Unix signals. Of course, Qt already incorporates the notion of signals, so it would be nice if Unix signals could be mapped to Qt signals. Then we could write handlers for Unix signals and connect them up in the same way as normal Qt slots.

The class described below does just this. It is heavily based on this example in the Qt documentation, but it encapsulates that functionality in a generic re-usable class.

Interface

The interface is simple: you call watchForSignal() with the signals you're interested in, and connect() your handlers to SIGNAL(unixSignal(int)).

class UnixSignalWatcher : public QObject
{
    Q_OBJECT
public:
    explicit UnixSignalWatcher(QObject *parent = 0);
    ~UnixSignalWatcher();

    void watchForSignal(int signal);

signals:
    void unixSignal(int signal);
};

Example usage

Let's look at an example program.

#include <QCoreApplication>
#include <QDebug>
#include "sigwatch.h"

int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);
    qDebug() << "Hello from process" << QCoreApplication::applicationPid();

    UnixSignalWatcher sigwatch;
    sigwatch.watchForSignal(SIGINT);
    sigwatch.watchForSignal(SIGTERM);
    QObject::connect(&sigwatch, SIGNAL(unixSignal(int)), &app, SLOT(quit()));

    int exitcode = app.exec();
    qDebug() << "Goodbye";
    return exitcode;
}

This simply registers signal handlers for SIGINT and SIGTERM and then idles forever. If you run it (qmake && make && ./sigwatch-demo) you'll see a greeting and the pid of the process:

Hello from process 6811

Press ^C to send SIGINT. The UnixSignalWatcher will handle the signal, which in turn is connected to QCoreApplication::quit(), so the event loop exits and the farewell message is printed.

^CCaught signal: Interrupt 
Goodbye

Similarly, you could use kill to send SIGTERM.

$ ./sigwatch-demo &
Hello from process 6848
$ kill 6848
Caught signal: Terminated
Goodbye

If you send a signal that does not have a handler, though, you won't see the farewell message. For instance:

$ ./sigwatch-demo
Hello from process 6906
$ kill -SIGABRT 6906
Aborted (core dumped)

Compatibility

Tested with Qt 4.6 and 5.2 on Linux.