example.cpp

void onFinish(SomeData *data) {
  callback(data);
  if (data->myData) {
    // "pointer being freed was not allocated"
    // debugger says address is different from before callback
    free(data->myData);
  }
}

void callback(SomeData *data) {
  doStuff(*data);
}

void doStuff(SomeData data) {
  char *buffer = (char*) malloc(numBytes);
  memcpy(buffer, data.myData, numBytes);
  // ...do stuff with buffer...
}