print trace of signals/slots called

Question

Is there a way to print the signals and slots being called?

I'm experiencing a weird deadlock in Qt which happens only in particular conditions across multiple threads, and I'd like to know the order of signals/slots being called.

Of course, for slots, I write the method body, and in the worst scenario, I can manually add a print out of the method. But the body of signals is generated automatically, so it is not possible, unless I write a custom moc, which seems like an overkill for this task...

Answer 1

If one leverages the built-in hooks, it's possible to automatically instrument all signals, and all slots connected using the Qt 4 connect syntax. Unfortunately, QtPrivate::QSlotObject doesn't implement these hooks: slots connected using the Qt 5 syntax need to be instrumented manually (e.g. by connecting a functor to them, or adding code to them).

Signal notifications can be relied on for connected signals. Objects with no signals, and some signals of objects with other connections, will not be reported. This is presumably what you want.

Thus:

// https://github.com/KubaO/stackoverflown/tree/master/questions/signal-spy-39597233
#include <QtCore>
#include <private/qobject_p.h>

int signalToMethodIndex(const QMetaObject * mo, int signal)
{
    Q_ASSERT(signal >= 0);
    for (int i = 0; i < mo->methodCount(); ++i) {
        if (mo->method(i).methodType() == QMetaMethod::Signal) {
            if (signal == 0) return i;
            -- signal;
        }
    }
    return -1;
}

class Spy {
    static QThreadStorage<bool> entered;
    static void signalBegin(QObject *caller, int signalIndex, void **) {
        if (entered.localData()) return;
        QScopedValueRollback<bool> roll{entered.localData(), true};
        auto index = signalToMethodIndex(caller->metaObject(), signalIndex);
        if (index >= 0)
            qDebug() << "SIGNAL" << caller << caller->metaObject()->method(index).methodSignature();
    }
    static void slotBegin(QObject *caller, int index, void **) {
        if (entered.localData()) return;
        QScopedValueRollback<bool> roll{entered.localData(), true};
        qDebug() << "SLOT" << caller << caller->metaObject()->method(index).methodSignature();
    }
public:
   static void start() {
       QSignalSpyCallbackSet set{&signalBegin, &slotBegin, nullptr, nullptr};
       qt_signal_spy_callback_set = set;
   }
};
QThreadStorage<bool> Spy::entered;


struct Class : QObject {
    Q_SIGNAL void aSignal();
    Q_SLOT void aSlot() { qDebug() << "slot"; }
    Q_OBJECT
};

int main(int argc, char ** argv) {
    Spy::start();
    QCoreApplication app{argc, argv};
    Class obj;
    QObject::connect(&obj, SIGNAL(aSignal()), &obj, SLOT(aSlot()));
    obj.setObjectName("obj");
    emit obj.aSignal();
}
#include "main.moc"

Output:

SIGNAL Class(0x7fff51901af0, name = "obj") "objectNameChanged(QString)"
SIGNAL Class(0x7fff51901af0, name = "obj") "aSignal()"
SLOT Class(0x7fff51901af0, name = "obj") "aSlot()"
slot