Consider the waitpid function:
lib C
fun waitpid(pid : Int32, status_ptr : Int32*, options : Int32) : Int32
end
The documentation of the function says:
The status information from the child process is stored in the object
that status_ptr points to, unless status_ptr is a null pointer.
We can use this function like this:
pid = ...
options = ...
status_ptr = uninitialized Int32
C.waitpid(pid, pointerof(status_ptr), options)
In this way we pass a pointer of status_ptr to the function for it to fill its value.
There's a simpler way to write the above by using an out parameter:
pid = ...
options = ...
C.waitpid(pid, out status_ptr, options)
The compiler will automatically declare a status_ptr variable of type Int32, because the argument is an Int32*.
This will work for any type, as long as the argument is a pointer of that type (and, of course, as long as the function does fill the value the pointer is pointing to).