NULL need not be all bits 0

[Werner Koch]

"D. Hugh Redelmeier" <hugh@mimosa.com> writes:

> It is a little known fact that the representation of a NULL of a
> particular pointer type may not have all bits 0.  So clearing a chunk

I have never seen such an architecure but I am usually aware of this.
AfAIK, the standard says that (char*)0 will be evaluated as false in 
an expression and storing a 0 into a char* will result in storing
the machines representation of a NULL-pointer.  This ends up that we
don't have to care about it when writing C code ...

> gnupg-1.0.0/mpi/mpi-mpow.c" line 90:
>     G = m_alloc_clear( (1<<k) * sizeof *G );

But sure, you are right here - in this case the compiler can't know
that we are going to store 0s in some pointers.

> The code assumes that this initializes an array of MPI pointers to
> NULL.  This works on almost all machines, but is not guaranteed to
> work by the C standard.

Okay, so we better do:

       G = m_alloc( (1<<k) * sizeof *G );
       for(i=0; i < (1<<k); i++ )
       	   g[i] = NULL;  /* or 0 */

or use a special m_alloc_clear_array_of_type_G().  I some vendor
builds such a machine he probably has the resources to fic most
software :-)

You will probably find out that I also assume that a byte has 8 bits
;-)


   Werner


-- 
Werner Koch at guug.de           www.gnupg.org           keyid 621CC013