Null Versus Aligned Pointer
Use of 0 to represent a null pointer is extremely common, with many programming languages (such as Ada) explicitly relying on this behavior. In theory, other values in an operating system-reserved block of logical memory could be used to tag conditions other than a null pointer, but these uses appear to be rare, perhaps because they are at best non-portable. It is generally accepted practice in software design that if a special pointer value distinct from null (such as a sentinel in certain data structures) is needed, the programmer should explicitly provide for it.
Taking advantage of the alignment of pointers provides more flexibility than null pointers/sentinels because it allows pointers to be tagged with information about the type of data pointed to, conditions under which it may be accessed, or other similar information about the pointer's use. This information can be provided along with every valid pointer. In contrast, null pointers/sentinels provide only a finite number of tagged values distinct from valid pointers.
In a tagged architecture, a number of bits in every word of memory are reserved to act as a tag. Tagged architectures, such as the Lisp machines, often have hardware support for interpreting and processing tagged pointers.
GNU libc malloc always returns 8-byte aligned memory addresses. Larger alignment values can be obtained with posix_memalign.
Read more about this topic: Tagged Pointer
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