New submission from Erik Bray:
Since the patch to #24881 was merged the _pyio module has been non-importable
on Cygwin, due to an attempt to import from the msvcrt module.
However, the msvcrt module is not built on Cygwin (Cygwin does not use MSVCRT).
Cygwin's libc does, however, have _setmode.
The problem this is trying to solve is to call _setmode
(https://msdn.microsoft.com/en-us/library/tw4k6df8.aspx) on file descriptors to
ensure that they are O_BINARY instead of O_TEXT (a distinction that needs to
made on Windows).
Fortunately, on Cygwin, it is fairly rare that a file descriptor will have mode
O_TEXT, but it it is possible. See
This could be tricky to solve though. Removing setmode() call entirely works
for me as far as the test suite is concerned. But it leaves _pyio slightly
incongruous with the C implementation in this one small aspect, and it *is* a
I would propose for Python 3.7 adding an os.setmode() function. On Windows
this could be simply an alias for msvcrt.setmode() (or vice-versa). But
because setmode() is available also in Cygwin (and technically some other
platforms too, though none that are currently supported by Python) it would be
a good candidate for inclusion in the os module I think (for those platforms
that have it).
title: _pyio module broken on Cygwin / setmode not usable
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