On Fri, Apr 29, 2016 at 12:18:46PM -0400, Random832 wrote:
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)
It's a very good point, but I don't have any 32 bits systems around with a kernel-4.5. I'll try to figure it out and/or ask in the kernel ML.
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.
These are not output parameters, even if they're pointers. they'r using the NULL pointer to signal that the current offsets should not be touched, to differentiate from a offset of 0. Something that in Python we would use None.