Shrinking with realloc

[david.f.prosser]

In article <26328 at shemp.CS.UCLA.EDU> signup at CS.UCLA.EDU writes:
>If realloc is used to shrink a chunk of allocated storage, is it guaranteed
>by the standard that the storage won't be moved?  I couldn't find any positive
>or negative statement about the matter.
>
>I.e., after
>	char *p, *q;
>	q = p = (char *) malloc(10);
>	(void) realloc(p,5);
>is it valid to dereference q?

Short answer: "no".

Long answer: Having such a requirement on realloc would overly constrain
its implementation, quite possibly causing wasted memory.  This is because
the fancier malloc/realloc/free implementations have special pools for
certain sized (typically small) blocks, and crossing such a size boundary
would necessitate copying the data.  Also, copying of a small amount of
data is not too costly when the overall space usage "wins".

Dave Prosser	...not an official X3J11 answer...