Malloc tests

[Joel Sherrill]

On Thu, Jan 6, 2022 at 2:55 PM Gedare Bloom <gedare at rtems.org> wrote:
>
> On Tue, Jan 4, 2022 at 6:10 PM zack leung <zakthertemsdev at gmail.com> wrote:
> >
> > Helllo  ,
> > I'm working on a patch for malloc_get_usable size right now so far i have
> > this test case for malloc, I just make sure that the value is null and i
> > just malloc an int and then  i make a call to the function malloc_usable
> > size and then i compare it like this.
> >
> > static void test_malloc_usablesize( void ){
> >    int * a = malloc(sizeof(int));
> >    rtems_test_assert((int) malloc_usable_size(a) == 12);
> >    free (a);
> >
> >    int * b = NULL;
> >    rtems_test_assert( 0 == malloc_usable_size(b));
> >    free(b);
> >
> >
> > }
> > Is there a good amount of storage to allocate? Also I heard someone made a
>
> I think that this test case is quite brittle. The "usable size" will
> depend on the implementation details of malloc, and the conditions of
> the heap when the allocation request gets made. I don't have better
> ideas however for how to test the correctness of the usable_size
> function. Maybe there are certain sequences of malloc/free calls that
> can be relied upon to give you deterministic sizes of allocations?

My suggestion in private chat was that you can depend on the
malloc heap being initialized with CPU_HEAP_ALIGNMENT. That's
what that macro is specifically for. It is used in
include/rtems/score/wkspaceinit*.h
and has been since the dawn of time.

Then the size for a valid pointer from malloc() should be between
the allocated size and the next number on a CPU_HEAP_ALIGNMENT
boundary. I think the exact number is actually something like this:

expected = alloc_size;
mod = alloc_size % CPU_HEAP_ALIGMENT;
expected += mod;

Adding a helper function in the test to compute the expected
size allocated and validate the return may be wise if multiple
size allocations are going to be tested.


> > formatter for rtems source files.  Can someone tell me how to use it and if
> > it is on the main branch?
>
> This was part of a gsoc project last year. We haven't quite made the
> switch over to it and the associated coding style I think, but you can
> find the code via
> https://devel.rtems.org/wiki/GSoC/2021#StudentsSummerofCodeTrackingTable
> the student was Meh Mbeh Ida Delphine
>
>
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