[New-bugs-announce] [issue46858] mmap constructor resets the file pointer on Windows

[benrg]

New submission from benrg <benrudiak at gmail.com>:

On Windows, `mmap.mmap(f.fileno(), ...)` has the undocumented side effect of setting f's file pointer to 0.

The responsible code in mmapmodule is this:

    /* Win9x appears to need us seeked to zero */
    lseek(fileno, 0, SEEK_SET);

Win9x is no longer supported, and I'm quite sure that NT doesn't have whatever problem they were trying to fix. I think this code should be deleted, and a regression test added to verify that mmap leaves the file pointer alone on all platforms.

(mmap also maintains its own file pointer, the `pos` field of `mmap_object`, which is initially set to zero. This issue is about the kernel file pointer, not mmap's pointer.)

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components: IO, Library (Lib), Windows
messages: 414039
nosy: benrg, paul.moore, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: mmap constructor resets the file pointer on Windows
type: behavior
versions: Python 3.10, Python 3.11, Python 3.7, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46858>
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