Appendix B Aliasing

We have already presented examples of casting a void * pointer to another pointer type, and casting another pointer type to void *.

One common kind of pointer cast is guaranteed safe: casting the value returned by malloc and related functions (see Dynamic Memory Allocation). It is safe because these functions do not save the pointer anywhere else; the only way the program will access the newly allocated memory is via the pointer just returned.

In fact, C allows casting any pointer type to any other pointer type. Using this to access the same place in memory using two different data types is called aliasing.

Aliasing is necessary in some programs that do sophisticated memory management, such as GNU Emacs, but most C programs don’t need to do aliasing. When it isn’t needed, stay away from it! To do aliasing correctly requires following the rules stated below. Otherwise, the aliasing may result in malfunctions when the program runs.

The rest of this appendix explains the pitfalls and rules of aliasing.