A little more detail would be helpful....I'm assuming you want to pass a NULL pointer to a library function. Putting portability aside for a moment, most platforms use a value of 0 for a NULL pointer. You can do that by configuring the Call Library function with a numeric parameter, pass by value. Just wire a 0 to it in your VI. Of course, you didn't say what you're trying to do with it either, so treat this as a simple case of passing a NULL in and nothing else.
