/ null-pointer /

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


generated Wed Jul 23 02:53:41 2003mlg