Re: [PATCH 1/9] Define a structure for object IDs.

On Sun, May 4, 2014 at 1:07 PM, Michael Haggerty [off-list ref] wrote:

On 05/03/2014 10:12 PM, brian m. carlson wrote:

quoted

Many places throughout the code use "unsigned char [20]" to store object IDs
(SHA-1 values).  This leads to lots of hardcoded numbers throughout the
codebase.  It also leads to confusion about the purposes of a buffer.

Introduce a structure for object IDs.  This allows us to obtain the benefits
of compile-time checking for misuse.  The structure is expected to remain
the same size and have the same alignment requirements on all known
platforms, compared to the array of unsigned char.

Please clarify whether you plan to rely on all platforms having "the
same size and alignment constraints" for correctness, or whether that
observation of the status quo is only meant to reassure us that this
change won't cause memory to be wasted on padding.

It's not just about wasted padding. Some structs, like
ondisk_cache_entry, reflect on-disk format. Padding means breakage.
But I don't think this will be a big issue because we can detect if
padding happens, abort to force the user to complain, and deal with it
on case-by-case basis.
-- 
Duy