Re: A note about testing EXEC_BACKEND on recent Linuxen

Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > FYI, the shared memory address was originally relocatable by using
> > offsets to address it from the child, but now that we use fork() on
> > Unix, it isn't an issue, and Win32 seems to be OK.
>
> In the worst case we could go back to using offsets everywhere, but I'm
> really reluctant to do that for reasons of code clarity and reliability.
> The main problem with it is that you have to explicitly cast to and from
> the correct pointer type, which is not only ugly but completely defeats
> any chance of the compiler catching wrong-type errors.

As I remember, the offsets also had a performance and/or storage impact
we don't want either.

> What I am seeing on Fedora 4 (I suppose it's common to most recent Linux
> versions) is that the system preferentially maps the shared memory
> segment just below the stack, which is good from the point of view of
> preserving maximum address space for the heap, but not so great if stack
> size changes or the stack moves a bit. It'd be possible to get a little
> more flexibility by requesting a non-default attach location for the
> postmaster's initial shmat call. I'm not interested in getting into
> that if we don't absolutely have to, but it might be the best answer if
> we find ourselves seeing similar issues on specific platforms (ie
> Windows) in the future.

Agreed.

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