atexit() called after setup()

[cider101]

First of all, there is no atexit() definition provieded by the runtime-environment which gives you, in case of "stack-based" singleton implementation - undefined references while linking.
class Singleton {
static Signleton& instance() { static Signleton theInstance; return theInstance; }
};
Secondly, the atexit is called after leaving setup() - which if implemented correctly - will just delete the instance and leave you with invalid memory!

[matthijskooijman]

I'm not sure what you mean here. I don't see any atexit function call in your code. Normally, atexit can be called to register function that should be called before the program terminates. In Arduino, the main function never returns, so this will never happen.

You also mention undefined references while linking and invalid memory, which are two completely separate issues. I suspect you just have a problem in your code that you're misinterpreting as an Arduino bug.

Could you paste a complete (but as small as possible) sketch that displays the problem and describe what happens and what you had expected to happen?

[cider101]

ok...as i promised a more verbose explanation ;)

if a singleton is definded in the follwoing way:

class Singleton {
    static Signleton& instance() { 
         static Signleton theInstance;
         return theInstance; 
         }

     void sayHello() {  Serial.println("Hello World"); }
};

void setup() {
    Singleton::instance().sayHello();
}
void loop() {
    Singleton::instance().sayHello();
}

the linker will complain about an undefined/unresolved symbol for atexit(). If a correct implementation of atexit is provided in the user code (to fix the unresovled symbol)

int atexit(void (*func)()) {
    func();
    return 0;
}

the code will link, but at atexit is beeing called after the completion of the setup method. If func() is executed, it will invoke the destructor of the singleton-class and leaving you behind with an invalid reference in the loop() or on the next invovaction of instance().

[matthijskooijman]

Right. Your implementation of atexit is wrong: the function pointer passed to atexit should be called just before program termination, but you call it right away, which explains why things get broken.

I'm surpised that the compiler actually inserts calls to atexit - you'd expect that it can more efficiently call the destructors without using a function pointer and (thus) an indirect function call. However, thinking about this more, I think that the static variable you declared will not be initialized at startup, but only when the instance function is first called. Since it is not guaranteed this ever happens, the compiler can't call the destructor unconditionally at shutdown, so I suspect this is why it uses the atexit function. I'm just guessing here, though.

In any case, if you supply an empty atexit function, I think that would be more correct (since the main loop never terminates, so it's ok to never call the functions passed to atexit) and I think it would solve your problem. Now I wonder if the Arduino environment should be providing this dummy atexit...

[matthijskooijman]

Oh, one more thing: If you paste code, make sure to indent it at least 4 spaces or a tab, or put triple backticks (```) before and after the block of code, to get a fixed-width font.

[cider101]

i just checked the c/c++ reference again... oops...you are right about the wrong implementation - sorry, my bad - seems i forgot to many thing within the last 5years i didn't use c++ ;)

yes, the static variable won't be initialized until the first call of instance(). One could provide an different, pointer based implementation - but this would produce more overhead

instance() {
    if(_ptr == NULL) {
         _ptr = new Singleton();
    }
    return *_ptr;
}

I "solved" the problem as you said with an empty implementation of atexit since atexit() doesn't make sense in the Arduino-Environment anyway. But as you suggested, an empty default implementation would be nice - maybe with an attribute weak, so in the unlikely case it's needed one can provide a different implementation

[cider101]

thanks for the hint with the 4 spaces !

[matthijskooijman]

Perhaps you could supply a pullrequest that adds a weak and empty atexit() function? Perhaps proposing this on the developers mailing list (see the contact us page on arduino.cc) first is a good idea, though.

[cmaglie]

Fixed with 1bbcb2f
To be released in Arduino IDE 1.5.8