MAX_MALLOC_ALIGN and libc spec

[Almaz Mingaleev]

That's exactly what I've meant, thank you!

On Fri, 14 Nov 2025 at 10:51, Maurizio Cimadamore <
maurizio.cimadamore at oracle.com> wrote:

> Hi,
> I think this very topic is being discussed here?
>
> https://github.com/openjdk/jdk/pull/28235
>
> Cheers
> Maurizio
>
> On 14/11/2025 10:42, Almaz Mingaleev wrote:
> > Hello,
> >
> > SegmentFactories defines MAX_MALLOC_ALIGN variable and sets it to 16
> > on LP64 architectures. However, section 7.24.4.1 of the libc spec only
> > says
> >
> > > The pointer returned if the allocation succeeds is suitably aligned
> > so that it can be
> > > assigned to a pointer to any type of object with a fundamental
> alignment
> > > requirement and size less than or equal to the size requested
> >
> > (https://open-std.org/JTC1/SC22/WG14/www/docs/n3467.pdf#page=397)
> >
> > The largest type defined by the spec seems to be “long double” and
> > there is only “Recommended practice” for it:
> >
> > > The long double type should match the ISO/IEC 60559 binary128
> > format, else
> > > an ISO/IEC 60559 binary64-extended format.
> >
> > Do I understand correctly that the currently used value of 16 is
> > relying on existing implementations and that the current
> > implementation might misbehave if a malloc call for a size of 8 bytes
> > returns an 8 bytes aligned address, which is allowed by the spec?
> >
> > Also, shouldn’t the name be MIN_MALLOC_ALIGN?
> >
> > Thanks,
> > Almaz
>
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