[Python-Dev] Convert int() to size_t in Python/C

Random832 random832 at fastmail.com
Fri Apr 29 11:26:31 EDT 2016

On Fri, Apr 29, 2016, at 10:45, Marcos Dione wrote:
>     One possible solution hat was suggested to me in the #python IRC
> channel was to use that, then test if the resulting value is negative,
> and adjust accordingly, but I wonder if there is a cleaner, more general
> solution (for instance, what if the type was something else, like loff_t,
> although for that one in particular there *is* a convertion
> function/macro).

In principle, you could just use PyLong_AsUnsignedLong (or LongLong),
and raise OverflowError manually if the value happens to be out of
size_t's range. (99% sure that on every linux platform unsigned long is
the same size as size_t.

But it's not like it'd be the first function in OS to call a system call
that takes a size_t. Read just uses Py_ssize_t. Write uses the buffer
protocol, which uses Py_ssize_t. How concerned are you really about the
lost range here? What does the system call return (its return type is
ssize_t) if it writes more than SSIZE_MAX bytes? (This shouldn't be hard
to test, just try copying a >2GB file on a 32-bit system)

I'm more curious about what your calling convention is going to be for
off_in and off_out. I can't think of any other interfaces that have
optional output parameters. Python functions generally deal with output
parameters in the underlying C function (there are a few examples in
math) by returning a tuple.

Maybe return a tuple (returned value, off_in, off_out), where None
corresponds to the input parameter having been NULL (and passing None in
makes it use NULL)?

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