using mmap in python

Question

can somebody please explain how does 0 influence mmap in python in this case:

mmap.mmap(0 , 256, "some tag")

I thought that I always need to transfer file descriptor rather than 0, so why zero?

Answer 1

From reading CPython 2.7 source code, it seems that on Windows, specifying fileno = 0 has the same effect as specifying fileno = -1, where the latter means "map anonymous memory".

Only -1 is accepted on Unix: on my 64-bit Ubuntu box with Python 2.6.5, mmap.mmap(0, 256) fails with errno=19 (No such device) and mmap.mmap(-1, 256) works fine.

Bottom line: fileno = 0 is a non-portable Windows-only variant of fileno = -1. It may get deprecated (there's even a commented-out warning in the code to that effect).

P.S. The CPython source file in question is Modules/mmapmodule.c.