title: null pointer
description:
 Many Forth programs use 0 as the null pointer.

 Nevertheless, the standard neither recommends nor prohibits it.

 The word ALLOCATE is allowed to allocate memory at 0 and return 0
 as the address of the alocated memory area.

 In practice, the word ALLOT may allocate memory at 0 only at the
 first invocation, because addresses are unsigned. OTOH, a really
 terrific system may begin to allocate memory at 0 at any moment
 when contiguity of allocated space is allowed to break
 (See 3.3.3.2 Contiguous regions).

 On most platforms, the memory at 0 has some specific purpose and
 is never allocated by ALLOT or ALLOCATE.
 It is easy to define an unique value that may be used as the
 end-of-chain marker, for example,
 CREATE END-OF-CHAIN
 but if two or more packages define two or more end-of-chain markers,
 maintaining will become a nightmare.

 Another advice have been to leave NULL for code that interacts
 with Win32, and use a different name, say, NIL.
 OTOH, having both NULL and NIL in the same system is no good either.

 The overal impression is that if this issue was resolved, this would
 create more problems than there is now.

 So the only recommendation is:

Recommendation:
 ALLOCATE must never return zero as the address of allocated
 memory. The reason is that many existing programs use 0
 as the end-of-chain mark.

Page-written-by:
 mlg