Re: (NULL == 0) ?

[Richard Coleman <coleman@math.gatech.edu>]

> >What Unix has a NULL that isn't all-bits-zero?
> 
> I'm not sure about Unices, but the C FAQ does list more than one system.

I would be willing to bet money it's a non-unix box.  Just think of
the tremendous amount of unix code that uses the fact that calloc (in
our case zcalloc) returns memory that is all zeros (and hence is
NULL).  Since zsh only works on unix boxes, I think zeroing out memory
with memset is ok.

> >                                                That would break
> >a tremendous amount of code.  There all lots of places in zsh
> >that assume this and use memset.  I don't think we should change
> >this.
> 
> I haven't noticed any other places that use memset for this purpose.  I
> only noticed this one because I was looking for this particular bug.  I
> think if there are other places that do this we should fix them because
> they are broken.  GCC can probably optimise looped assignments to be
> almost as good as memset on systems where NULL is all-bits-zero.

I'm no expert on compilers, but I would be surprised if gcc could
optimize such a loop to be as fast as memset.  Usually memset is
hand optimized assembler code.

The function `newhashtable' uses zcalloc to allocate hashtable memory
and the function `emptyhashtable' uses memset to set this same memory
to zero when it is emptied.  If assuming NULL was equal to 0 would
crash a machine, then these two pieces of code would have done it a
long time ago.  The hashtable code gets executed a lot.

rc