> By the way, at this point I think we could simply skip the test on BSDs
> and OS X. The tested functionality is cross-platform, so testing under
> a limited set of systems should be ok.

Another solution would be to rewrite the test to not use mmap() at all:

    @precisionbigmemtest(size=_4G + 4, memuse=1)
    def test_big_buffer(self, size):
        if size < _4G + 4:
            self.skipTest("not enough free memory, need at least 4 GB")
        data = bytearray(_4G + 4)
        data[-4:] = b"asdf"
        self.assertEqual(zlib.crc32(data), 3058686908)
        self.assertEqual(zlib.adler32(data), 82837919)

This is more consistent with the other bigmem tests in test_zlib, but
I'm guessing it will mean that the test gets run much less often (since a
lot of machines won't have enough memory). If that's OK, then I'd prefer
doing it this way (since it keeps things simpler). Otherwise, skipping
the test on OS X sounds fine to me.