> I've started a wiki page where I already added an entry about the above > question. > Do you think the entry would have answered your question? > http://starship.python.net/crew/theller/moin.cgi/CodeSnippets
As concise as I now can imagine, yes thank you. I see the main point leads: "The truth value of any NULL ctypes pointer instance is False." That angle neatly ducks my ongoing newbie confusion over a null constructed in Python via c_void_p(0) being different from a null constructed in C via return (void *) 0. > ... > > And so then the next related Faq is: > > > > Q: How should I test for ((void *) -1)? > > For me, on Windows, c_void_p(-1).value is the same > as c_void_p(0xFFFFFFFF).value, but it is -1 not 0xFFFFFFFF. Different results at my desk. Specifically, I see: >>> from ctypes import * ; c_void_p(-1).value 4294967295L >>> in Mac OS X 10.4.7 Python 2.5 r25:51918 and Win XpSp2 Python 2.5 r25:51908. > Worse, on 64-bit systems c_void_p(-1).value is 184467440737009661615, which > is the same as 0xFFFFFFFFFFFFFFFF. > So, to be portable, INVALID_HANDLE_VALUE should be defined like this: > INVALID_HANDLE_VALUE = c_void_p(-1).value Exactly what I should have written, yes, thank you. And somewhat generalisable: c_void_p(0).value is None. Maybe this is the thing that's been astonishing me: maybe C often does return the .value of the pointer, rather than a new instance of the POINTER class? > and your test would become > (ctypes.cast(pv, ctypes.c_void_p).value == INVALID_HANDLE_VALUE Yes. Mind you, typing out all that noise inspires me to ask, does (pv == INVALID_HANDLE_VALUE) give the wrong answer for any pv other than c_void_p(-1)? In other words, if by convention I get all my pointers from C and never construct a pointer in Python except to take its value, then can I safely write the more obvious translation of (pv == INVALID_HANDLE_VALUE)? > PS: It is inconsistent that the .value of a c_void_p instance is signed on > 32-bit systems, and unsigned on 64-bit systems, but it seems we have to live > with that. I'm interested, if you can make the time to say more ... > Fortunately, the c_void_p constructor accepts both signed and unsigned > Python ints and longs, so it should be possible to work around that fact. Good. > ... > might be better to use another wiki for ctypes, maybe the python.org one, for > this > stuff.) ... Could be. Offline I heard we might have more pages coming soon. Checking at random just now, I see http://wiki.python.org/moin/ is "immutable", so I guess I can't add "A Wiki page to collect the Faq's highlit by Clp" to the "Starting points" > "Asking for Help" > "Wanted..."> -- http://mail.python.org/mailman/listinfo/python-list
