How to pass-by-reference from Swift to C library - c

Question

I am creating a cross-platform library for Windows and macOS that is written in C, and I need to pass a variable by reference. How do I define the function in C so that it can receive a variable passed by reference, and how do I call the function in Swift?
For example, if I were to define a function in my C library like:
int sayBye(const char* greetingString){
greetingString = "bye";
return 0;
}
and call it in Swift like:
var greeting: String = "hello"
let responseCode = sayBye(greeting)
print(greeting)
I want the print statement to output "bye".
Constraints: "bye" cannot be returned in the return statement.
In short, I want to modify the contents of the string "greeting" after it has been passed by reference to a C-function.
Note: I'm not sure if, in the code above, I am defining the C function correctly or if I am calling it correctly in swift, and I am open to recommendations.

Answer 1

Found something that worked.
In the C library:
int sayBye(const char** greetingString){
*greetingString = "bye";
return 0;
}
Called in Swift:
var greeting = ("hello" as NSString).utf8String
let responseCode = sayBye(&greeting)
let stringFromPointer = String.init(cString: greeting!)
print(stringFromPointer)