unixsignal.cpp

#include "unixsignal.h"

#include <signal.h>
#include <sys/signalfd.h>
#include <unistd.h>

#include <QSocketNotifier>

UnixSignal::UnixSignal(int signal, QObject *parent) :
	QObject(parent)
{
	sigset_t signal_set;
	// Mask only the signal number passed as argument.
	sigemptyset(&signal_set);
	sigaddset(&signal_set, signal);

	// SIG_BLOCK means the calling thread will block the already blocked signals
	// plus the ones (one actually) we are passing in our signal set.
	// SIG_SETMASK would clear other signals, AIUI.
	// Note though: "The use of sigprocmask() is unspecified in a multithreaded
	// process; see pthread_sigmask(3)."
	if (pthread_sigmask(SIG_BLOCK, &signal_set, NULL) != 0) {
		qFatal("Could not mask signal %d: %s", signal, strerror(errno));
	}

	// -1 -> create a new file descriptor instead of reusing an existing one.
	// 0  -> flags to the call. Could be SFD_NONBLOCK or SFD_CLOEXEC.
	descriptor = signalfd(-1, &signal_set, 0);

	if (descriptor < 0) {
		qFatal("Invalid descriptor from signalfd: %s", strerror(errno));
	}

	QSocketNotifier *sn = new QSocketNotifier(descriptor, QSocketNotifier::Read, this);
	connect(sn, &QSocketNotifier::activated, this, &UnixSignal::handleSignal);
}

void UnixSignal::handleSignal()
{
	struct signalfd_siginfo info;
	::read(descriptor, &info, sizeof(info));
	emit triggered();
}

unixsignal.h

#ifndef UNIXSIGNAL_H
#define UNIXSIGNAL_H

#include <QObject>

class UnixSignal : public QObject
{
	Q_OBJECT
public:
	explicit UnixSignal(int signal, QObject *parent = 0);

signals:
	void triggered();

private slots:
	void handleSignal();

private:
	int descriptor;

};

#endif // UNIXSIGNAL_H