I'm working with the example for phidgets bridge, and I'm well confused on how to use the input argument void *userPtr for the function that goes in Phidget_set_OnAttach_Handler.

I'm assuming that it's a way for me to pass in a pointer to get the data out of the function, but all the example code has this set as 'NULL'. How does one actually make use of that argument?

It could be anything (a value, a pointer, whatever), just make sure to cast it to whatever you sent in. I use it to call functions in C++ objects. Have a look at my recently posted http://phidgets.com/phorum/viewtopic.php?f=42&t=5598, and see line 38 and 43 of RFID-phidgets.cpp

thanks! That makes so much more sense now. I'm still fairly new to c++, so the pointer/class notation probably messed my understanding. Always good to see an example of use