Atomically modifies a pointer to point to a new structure.
The Structure oldstr_ptr is pointing to is freed and newstr is
taken ownership over.
Either newstr and the value pointed to by oldstr_ptr may be null.
It is a programming error if both newstr and the value pointed to by oldstr_ptr refer to the same, non-%NULL
structure.
| oldstr_ptr |
pointer to a place of a Structure to take |
| newstr |
a new Structure |
|
|