如何从Objective-C将字符串传递给C函数并获取值？

I have an Objective-C view controller class, from which I am trying to call a straight-C (not Objective-C) function. I want to pass in a string variable by reference, set its value inside the C function, and then back in my view controller I want to convert this to a normal NSString object.I have an Objective-C view controller class, fr