Pointer Swizzling - Examples

Examples

For example, suppose we have the following linked list data structure:

struct node { int data; struct node *next; };

We can easily create a linked list data structure in memory using such an object, but when we attempt to save it to disk we run into trouble. Directly saving the pointer values won't work on most architectures, because the nodes will almost certainly be loaded into different memory positions. One way of dealing with this is to assign a unique id number to each node and then unswizzle the pointers by turning them into a field indicating the id number of the next node:

struct node_saved { int data; int id_number; int id_number_of_next_node; };

We can save these records to disk in any order, and no information will be lost. Other options include saving the file offset of the next node or a number indicating its position in the sequence of saved records.

When we go to load these nodes, however, we quickly discover that attempting to find a node based on its number is cumbersome and inefficient. We'd like our original data structure back so we can simply follow next pointers to traverse the list. To do this, we perform pointer swizzling, finding the address of each node and turning the id_number_of_next_node fields back into direct pointers to the right node.