[Python-Dev] [Python 2.4] PyInt_FromLong returning NULL
tim.peters at gmail.com
Tue Dec 7 18:58:12 CET 2004
> While using Zope 2.7
Do you mean 2.7, or do you mean 2.7.3, or ...?
> with Python 2.4 we discovered some strange behaviour
> of the security machinery.
> I could track this down to some Zope code in cAccessControl.c
> where an Unauthorized exception is raised because of a call to
> PyInt_FromLong(1) which returns NULL. What could be the
> reason that such a "stupid" call return NULL in a reproducable
Any C function that returns a Python object can return NULL if
malloc() says there's not enough memory to create a new object.
PyInt_FromLong() actually allocates about 1KB at a time, and will
return NULL if malloc() can't find that much.
OTOH, 1KB isn't big, and PyInt_FromLong(1) specifically should be
returning a shared reference to a pre-existing PyIntObject (a number
of small integer objects are constructed at Python initialization
time, and PyInt_FromLong() returns references to them instead of
allocating new memory).
So PyInt_FromLong(1) should have taken this path:
v = small_ints[ival + NSMALLNEGINTS];
But if a wild store had stuffed NULL into 1's slot in the small_ints
vector, the *next* line would have blown up with a NULL-pointer
dereference before PyInt_FromLong() could have returned:
return (PyObject *) v;
So, in all, it appears impossible for PyInt_FromLong(1) to return NULL.
If it's reproducible, run it under a debugger and step into the errant
PyInt_FromLong(1) to see what's happening? Could be a compiler
optimization bug (while rare, they have happened in Python).
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