sysv IPC and shminfo ABI

Alfred Perlstein alfred at
Tue Feb 12 05:25:52 UTC 2008

* Christian S.J. Peron <csjp at> [080211 18:00] wrote:
> All,
> A while ago, I committed a patch which changed what was a kernel private
> structure 'shminfo'.  In order to access this structure, you must define
> _KERNEL which suggests to me we should not be making guarantees about it's
> size.
> See /usr/src/sys/shm.h

... yeah but...

> So I guess I have a few questions:
> (1) Is struct shminfo considered a sys V standard structure that has a
>     predefined size that should be maintained across all architectures?

No, that's not required, what is required is backwards compat.

> (2) Is this an ABI breakage that we care about?

Yes, if it's breaking people, then yes.  for ia32 in particular.

> (1) Back the change out now before any damage is done (assuming we care)
> (2) Apply this safety patch to RELENG_7 and RELENG_7_0

Honestly I was going to suggest all this cruft to "get it right", but
this patch looks like the most simple, most correct thing.  I think
it should go in.

> (3) Leave things as they are, as we are going to be getting support for large
>     shared memory allocations in the future.  (This is why I asked if the
>     structure supposed to be a standard size).

Afaik there's no "standard size" in the API (I think shmctl->shminfo is
pretty unix specific) but it pays to keep the backward compat.

> I apologize for this, I understand this is probably the last thing we need
> before the release, but I just want to make sure that we are doing the right
> thing.



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