VRPN C++ code compiles on Linux but not Windows - c++

Question

I've built a VRPN client on Linux. It's based on this: http://www.vrgeeks.org/vrpn/tutorial---use-vrpn
Here's some of the code:
vrpn_Analog_Remote * analog = NULL;
vrpn_Button_Remote * button = NULL;
vrpn_Tracker_Remote * tracker = NULL;
// Things happen...
analog = new vrpn_Analog_Remote("pathToAnalog");
analog->register_change_handler(NULL, handleAnalog);
button = new vrpn_Button_Remote("pathToButton");
button->register_change_handler(NULL, handleButton);
tracker = new vrpn_Tracker_Remote("pathToTracker");
tracker->register_change_handler(NULL, handleTracker);
Here are the callbacks refered to in this code:
void handleAnalog(void * userData, const vrpn_ANALOGCB a) {
// Do stuff...
}
void handleButton(void * userData, const vrpn_BUTTONCB b) {
// Do stuff...
}
void handleTracker(void * userData, const vrpn_TRACKERCB t) {
// Do stuff...
}
And here is where all of these references to VRPN are defined:
https://github.com/vrpn/vrpn/blob/master/vrpn_Analog.h#L168
https://github.com/vrpn/vrpn/blob/master/vrpn_Button.h#L225
https://github.com/vrpn/vrpn/blob/master/vrpn_Tracker.h#L284
These compile without even a warning on Linux and can actually be used. Everything worked as expected. All the types here seem to satisfy the compiler, g++.
But on Windows, whether I use Visual Studio 2015 or MinGW's g++, I get this for the first two callback registrations:
invalid conversion from 'void (*)(void*, vrpn_ANALOGCB) {aka void (*)(void*, _vrpn_ANALOGCB)}' to 'vrpn_ANALOGCHANGEHANDLER {aka
void (__attribute__((__stdcall__)) *)(void*, _vrpn_ANALOGCB)}' [-fpermissive]
invalid conversion from 'void (*)(void*, vrpn_BUTTONCB) {aka void (*)(void*, _vrpn_BUTTONCB)}' to 'vrpn_BUTTONCHANGEHANDLER {aka
void (__attribute__((__stdcall__)) *)(void*, _vrpn_BUTTONCB)}' [-fpermissive]
And for the last one, I get a different error:
call of overloaded 'register_change_handler(NULL, void (&)(void*, vrpn_TRACKERCB))' is
ambiguous
Now that I'm typing this, I'm thinking maybe VRPN was compiled differently on Windows and that's why the compiler now has a problem with my code. But I'm very lost as to what to do.

Answer 1

Try declaring your callbacks like this:
void VRPN_CALLBACK handleAnalog(void * userData, const vrpn_ANALOGCB a) {
// Do stuff...
}
For linux, the VRPN_CALLBACK define is empty, so you did not notice any problems there. For windows, the VRPN library devs decided that they expect a callback that adheres to the __stdcall calling-convention. Thus you have to declare your function accordingly, and the most painless+portable way is to use the very same VRPN_CALLBACK define that they provide.
Clues to this came from your links to the code at github:
[vrpn_Analog.h]
typedef void(VRPN_CALLBACK *vrpn_ANALOGCHANGEHANDLER)(void *userdata,
const vrpn_ANALOGCB info);
and the callback define is made here:
[vrpn_Configure.h]
#ifdef _WIN32 // [ ...
#define VRPN_CALLBACK __stdcall
#else // ... ] WIN32 [
#define VRPN_CALLBACK
#endif // ] not WIN32