Question

I am trying to use the function signal(int,void(*)(int)) from <csignal> to handle the floating point exception SIGFPE. I’d like to be able to print some useful diagnostics besides just a message saying “Floating point exception” or something to that effect. This means the function I pass as the handler to signal needs access to some of the data in my code. Therein lies the rub.

The function must return void and accept only 1 parameter of type int. I cannot make the handler a member function of my data storage class since then the type would be void(Foo::*)(int) due to the hidden this pointer.

I thought about using lambdas to try and make an anonymous function like this;

void handler(int nSig, Foo data)
{
    // do something
}
// snip
Foo data;
signal(SIGFPE, [&](int nSig)->void{handler(nSig,data);});

however because the lambda captures the variable data from outside the compiler will not let it be cast to a pointer to void(*)(int) (which is a shame as this seems like an ideal use for lambdas).

I could simply make data a global variable which could then be seen in handler but I am loath to do this for obvious reasons.

So my question is thus; what is the best way of mimicking anonymous functions in C++?

Note: I would prefer a native C++ solution and not to have to use boost or equivalent.

Answer 1

There is no such thing as an anonymous function in C (C++ is irrelevant here, as the function must abide by the C calling convention).

The only thing you can do is shiver access globals from the handler, probably global variables (and not constants which would be fine).

I advise making those globals thread local to avoid multithreading issues, but it is still bad in the sense that global variables make for more brittle applications.

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How to ?

Note: as Luc Danton patiently explained to me, a signal may interrupt any non-atomic activity, and thus reading from a global is safe only if it is a lock-free atomic (or a few other things). Unfortunately std::function may not be so, depending on your implementation, I will still leave this code to explain how it could be done providing that std::function accesses are atomic.

It is possible to create a trampoline that will call stateful stuff, isolating thread and allowing re-entrant calls.

typedef std::function<void(int)> SignalHandlerType;

extern thread_local ignalHandlerType SignalHandler;

And we create the following accessor (passed to signal):

void handle_signal(int const i) {
    if (SignalHandler) { SignalHandler(i); }
}

as well as the following RAII setter:

class SignalSetter: boost::noncopyable {
public:
    SignalSetter(int signal, SignalHandlerType&& sh):
        signal(signal), chandler(0), handler(sh)
    {
        chandler = std::signal(signal, &handle_signal<T>);
        swap(SignalHandler, handler);
    }

    ~SignalSetter() {
        std::signal(signal, chandler);
        swap(SignalHandler, handler);
    }

private:
    typedef void(*CHandlerType)(int);

    int signal;
    CHandlerType chandler;
    SignalHandlerType handler;
};

Note: both the global variable and the handle_signal could be private to the SignalSetter class… but since std::signal is not…

Expected usage:

int main(int argc, char* argv[]) {
    SignalSetter setter(SIGFPE, [argc, argv]() {
        std::cout << argc << ": " << argc << std::endl;
    });

    // do what you want.
}