Pointer swizzling

In computer science, pointer swizzling is the conversion of references based on name or position to direct pointer references. It is typically performed during the deserialization (loading) of a relocatable object from disk, such as an executable file or pointer-based data structure. The reverse operation, replacing pointers with position-independent symbols or positions, is sometimes referred to as unswizzling, and is performed during serialization (saving).

Example

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

 record node {

     int data

     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 next time we load it the memory positions the nodes now use may be in use by other data. 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:

 record 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. Alternatives 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.

Methods of unswizzling

There are a potentially unlimited number of forms into which a pointer can be unswizzled, but some of the most popular include:

The offset of the pointed-to object in the file
The index of the pointed-to object in some sequence of records
A unique identifier possessed by the pointed-to object, such as a person's social security number; in databases, all pointers are unswizzled in this manner (see foreign key)

Methods of swizzling

Swizzling in the general case can be complicated. The reference graph of pointers might contain an arbitrary number of cycles; this complicates maintaining a mapping from the old unswizzled values to the new addresses. Associative arrays are useful for maintaining the mapping, while algorithms such as breadth-first search help to traverse the graph, although both of these require extra storage. Various serialization libraries provide general swizzling systems. In many cases, however, swizzling can be performed with simplifying assumptions, such as a tree or list structure of references.

The different types of swizzling are: