Author: Simon Knopp
Licence: MIT

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.

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); };

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) 

Tested with Qt 4.6 and 5.2 on Linux.