[PATCH] Nano-malloc: Fix for unwanted external heap fragmentation

[Ola Olsson]

Hi again,

I am sorry but I have no idea what picolibc is and I haven't seen any
errata/todo or anything in newlib indicating that this issue will be fixed.
I am just a simple programmer; I pulled latest and greatest of newlib, used
"git log" and inspected the last 7 commits of nano-mallocr.c and then I was
back in year 2017...Hence, I pretty much came to the conclusion that this
issue won't be fixed unless I fix it myself.

Please tell me if there is anything I can do or if something is expected
from me regarding the patch before it's getting merged.

Btw, there are some cleanup I would like to do in this file as well, mostly
regarding comments and tabs->spaces but I guess it's okay to send a new
patch for that.

On Mon, Apr 26, 2021 at 5:31 PM Keith Packard <keithp@keithp.com> wrote:

> Ola Olsson <ola1olsson@gmail.com> writes:
>
> > The solution for this problem is to check if the last
> > item in the free list is adjacent to sbrk(0). If it is,
> > as it is in this case, we can just ask sbrk for the
> > remainder of what is needed. In this case 1 byte.
>
> I've already implemented both of these suggestions in the nano malloc
> version included as part of picolibc. This version has some significant
> additional work which fixes issues with memalign (does anyone use this
> on embedded systems?).
>
>
> https://github.com/picolibc/picolibc/blob/main/newlib/libc/stdlib/nano-mallocr.c
>
> Growing the chunk adjacent to the brk is useful for both malloc and
> realloc as this allows the common pattern of a buffer which grows
> incrementally to be realloc'd in place.
>
> I was hoping to get some time to push this upstream to newlib as you're
> quite correct that having these two changes is very useful on limited
> memory devices. However, I have been redirected away from this work.
>
> There are also a couple of tests included with picolibc that uncovered
> the issues with the original implementation:
>
> https://github.com/picolibc/picolibc/blob/main/test/malloc.c
> https://github.com/picolibc/picolibc/blob/main/test/malloc_stress.c
>
> --
> -keith
>