Re: NULL passed as an argument to memcmp() in parse_func.c

It appears that, according to the standard, passing NULL to memcmp is
undefined behavior, even if the count is 0. See
http://stackoverflow.com/questions/16362925/can-i-pass-a-null-pointer-to-memcmp
for C99 and C++ standard references. I didn't see a good reference for C89
but I find it almost impossible to believe it was changed from defined to
undefined behavior between C89 and C99.

On Mon, Jun 22, 2015 at 2:31 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Mon, Jun 22, 2015 at 2:55 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> If I recall that code correctly, the assumption was that if the third
> >> argument is zero then memcmp() must not fetch any bytes (not should not,
> >> but MUST not) and therefore it doesn't matter if we pass a NULL. Are
> >> you seeing any observable problem here, and if so what is it?
>
> > I dunno, this seems like playing with fire to me. A null-test would
> > be pretty cheap insurance.
>
> A null test would be a pretty cheap way of masking a bug in that logic,
> if we ever introduced one; to wit, that it would cause a call with
> argtypes==NULL to match anything.
>
> Possibly saner is
>
> if (nargs == 0 ||
> memcmp(argtypes, best_candidate->args, nargs * sizeof(Oid)) == 0)
> break;
>
> I remain unconvinced that this is necessary, though. It looks a *whole*
> lot like the guards we have against old Solaris' bsearch-of-zero-entries
> bug. I maintain that what glibc has done is exactly to introduce a bug
> for the zero-entries case, and that Piotr ought to complain to them
> about it. At the very least, if you commit this please annotate it
> as working around a memcmp bug.
>
> regards, tom lane
>
>
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