Re: [PATCH 2/2] Use fixed-size integers for .idx file I/O

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Jan 18, 2007, 15:15 UTC
Message-ID: <Pine.LNX.4.63.0701181612440.22628@wbgn013.biozentrum.uni-wuerzburg.de>
In-Reply-To: <45AF8A86.7070101@fs.ei.tum.de>

Hi,

On Thu, 18 Jan 2007, Simon 'corecode' Schubert wrote:

Show 11 quoted lines

> Morten Welinder wrote:
> > > -                       return ntohl(*((unsigned int *) ((char *) index +
> > > (24 * mi))));
> > > +                       return ntohl(*((uint32_t *)((char *)index + (24 *
> > > mi))));
> > 
> > Is that pointer gymnastics guaranteed to work?  I.e., how do we know
> > that we can access an uint32_t (or unsigned) at such an address?
> 
> if index is always aligned to a 4-byte boundary, this is safe.  apart from
> that, the problem already existed.

index is assigned from p->index_base, which comes from check_packed_git_idx(), and there it comes from an xmmap(). AFAICT mmap() (and for NO_MMAP, malloc()) _always_ return aligned pointers, so there is no problem, as long as the alignment step divides 24.

Ciao, Dscho