[Python-Dev] A cast from Py_ssize_t to long

[Thomas Wouters]

On 8/21/06, Alexander Belopolsky <alexander.belopolsky at gmail.com> wrote:
>
>
> On Aug 15, 2006, at 3:16 AM, Martin v. Löwis wrote:
>
> >
> > Where does it assume that it is safe to case ssize_t -> long?
> > That would be a bug.
>
> Is this a bug?
>
> file_readinto(PyFileObject *f, PyObject *args)
> {
> ...
>          Py_ssize_t ndone, nnow;
> ...
>          return PyInt_FromLong((long)ndone);
> }
>
> See Objects/fileobject.c (revision 51420).


Yes. Py_ssize_t can be bigger than a long (on LLP64 systems, such as Win64).
It doesn't matter on other systems, and you have to read more than 31 bits
worth of data to detect it even on Win64, but it's still a bug.
file_readinto should be using PyInt_FromSsize_t() instead. (There is the
SAFE_DOWNCAST macro for cases where we know the *value* of the (s)size_t
will always fit in a long, on any supported system, but that isn't the case
here.)

-- 
Thomas Wouters <thomas at python.org>

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