all bits zero?

[Jim Patterson]

In article <12570051 at hpclscu.HP.COM> shankar at hpclscu.HP.COM (Shankar Unni) writes:
>>   how many machines exist where the following does not hold.
>> 
>>    char*, struct FOO* NULL is represented as binary zeros
>
>I know of at least one (albeit only by software convention): on the classic
>HP3000's (16-bit stack machines), a pointer with all-zeros is a perfectly
>valid address

On the "classic" HP 3000 though, there is no "invalid" address. Using
0 for NULL is as good as any choice; all that the compiler-system has
to do to ensure that this is legal is reserve the first word of the
address space (i.e.  make sure that no "valid" object ever goes
there). Putting a string literal "(nil)" there might be nice (it causes
printf() to do nice things with NULL pointers, like the old PDP-11
compilers, apparently).

Note that the ANSI definition of a null pointer constant states simply
that it cannot point to any valid object. So long as the compiler
never allocates data at that address (either static or auto), and so
long as the memory allocation subsystem malloc et. al. don't (except
perhaps for size 0 objects), then this constraint is satisfied.
In fact, the compiler need not actually prevent dereferencing of "null";
the behaviour is undefined according to the standard.

>I must admit, however, that the C compiler on that machine (at least, the
>one that I am familiar with - the CCS C/3000 compiler) leaves NULL as
>"all-zeroes",

No doubt a wise choice given the immense amount of "prior art" regarding
NULL and 0.


-- 
Jim Patterson                              Cognos Incorporated
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