[FFmpeg-devel] [PATCH 3/8] avutil/mem: Add av_fast_realloc_array()

[Tomas Härdin]

ons 2022-07-06 klockan 16:46 +0200 skrev Andreas Rheinhardt:
> Tomas Härdin:
> > tis 2022-07-05 klockan 22:26 +0200 skrev Andreas Rheinhardt:
> > > From: Andreas Rheinhardt <andres.rheinhardt at outlook.com>
> > > 
> > > This is an array-equivalent of av_fast_realloc(). Its advantages
> > > compared to using av_fast_realloc() for allocating arrays are as
> > > follows:
> > > 
> > > a) It performs its own overflow checks for the multiplication
> > > that is
> > > implicit in array allocations. (And it only needs to perform
> > > these
> > > checks (as well as the multiplication itself) in case the array
> > > needs
> > > to
> > > be reallocated.)
> > > b) It allows to limit the number of elements to an upper bound
> > > given
> > > by the caller. This allows to restrict the number of allocated
> > > elements
> > > to fit into an int and therefore makes this function usable with
> > > counters of this type. It can also be used to avoid overflow
> > > checks
> > > in
> > > the caller: E.g. setting it to UINT_MAX - 1 elements makes it
> > > safe to
> > > increase the desired number of elements in steps of one. And it
> > > avoids
> > > overallocations in situations where one already has an upper
> > > bound.
> > > c) av_fast_realloc_array() will always allocate in multiples of
> > > array
> > > elements; no memory is wasted with partial elements.
> > > d) By returning an int, av_fast_realloc_array() can distinguish
> > > between
> > > ordinary allocation failures (meriting AVERROR(ENOMEM)) and
> > > failures
> > > because of allocation limits (by returning AVERROR(ERANGE)).
> > > e) It is no longer possible for the user to accidentally lose the
> > > pointer by using ptr = av_fast_realloc(ptr, ...).
> > 
> > If you add an option for clearing the newly allocated memory then
> > this
> > could work for my av_fast_recalloc() use case in the jpeg2000
> > decoder.
> > Or we could have two functions.
> > 
> 
> I'd prefer it if the zeroing function were a wrapper around the
> non-zeroing function.

That should work, the change in allocated entries can be detected

> 
> > Small bikeshed: since the function takes a pointer to a pointer as
> > argument, av_fast_realloc_arrayp() might be a better name. I had in
> > mind to similarly rename av_fast_recalloc() to av_fast_recallocp().
> > 
> > 
> > > +
> > > +    nb = min_nb + (min_nb + 14) / 16;
> > 
> > Not +15? Or +0?
> 
> "av_fast_realloc_array() instead allocates nb + (nb + 14) / 16.
> Rounding
> up is done in order not to reallocate in steps of one if the current
> number is < 16; adding 14 instead of 15 has the effect of only
> allocating one element if one element is desired."

Wups, I actually read that part of the commit message then forgot about
it. I blame sickness

/Tomas