Re: in-memory representation of NULL pointers?

[Eric Blake]

On 04/23/2010 11:27 AM, Simon Josefsson wrote:
> Not gnulib specific, but related to our coding style:
> 
> Does POSIX somewhere guarantee that the in-memory representation of NULL
> pointers is 0?  I know that C89 doesn't make that guarantee, and that
> some historic systems used non-0 memory values to represent NULL, but
> I'm hoping that this is not permitted today by some standard.

I think POSIX currently sticks by the same weasel-wording as C99, and
allows a weirdnix system where the in-memory representation of NULL is
not all 0 bits.

> 
> I believe there is a bunch of places in gnulib which uses memset(P, 0,
> sizeof(P)) to initialize structures containing pointers, which wouldn't
> be OK if this is not the case.

However, GNU Coding Standards states that we can assume that all
platforms worth porting to obey the industry convention that NULL maps
to all 0 bits, so even if POSIX doesn't guarantee it, gnulib is safe
using the idiom.

-- 
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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