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 <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: > > On Mon, Jun 22, 2015 at 2:55 PM, Tom Lane <t...@sss.pgh.pa.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 > > > -- > Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-hackers >
