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Okay just got round to making the changes, it now appears the linker can't resolve the 3 new functions. I just created the new public functions added the relevant code to get the pointer and call the non-static function.

The linker says it's an undefined reference to the functions by the looks of it from when they're set as the callbacks.

Let me guess, the functions themselves are defined in a .cpp file but the calls to jack_add_callback_foo() are in a .c file?

If so you need to mark the callback functions (both in the header and the .cpp file) as being extern "C".

Peter