Re: [9fans] manual suggestions and upas/fs bug

["Douglas A. Gwyn" <DAGwyn@null.net>]

David Swasey wrote:
> The malloc(2) page says:
>         The call realloc(0, size) means the same as `malloc(size)'.
>         Further, the call realloc(ptr, 0) means the same as
>         `free(ptr)'.
> I suggest you clarify the meaning of realloc(0,0).

If the intention is to be standard-conforming, then realloc(0,x)
is the same as malloc(x) for all x.  malloc(0) is required to
return either a null pointer or a valid heap pointer (which a
strictly conforming program is not allowed to dereference).

realloc(p,0) cannot be identical to free(p) since the former
returns a value and the latter does not.  Presumably it acts
as free(p) then does not succeed in allocating a 0-sized new
object, thus returns a null pointer.

free(0) is required to be a no-op.

> diff /n/dump/2002/0916/sys/src/cmd/upas/fs/mbox.c mbox.c
> 1358c1358,1362
> <       p = realloc(p, n);
> ---
> >       if(n==0){
> >               free(p);
> >               p = malloc(n);
> >       } else
> >               p = realloc(p, n);

p = realloc(p,n) is almost always a mistake.  If the allocation
fails, the previous pointer value in p gets replaced by a null
pointer value, so you lose the handle to the data that did not
get extended (or, rarely, shrunk).