Comment from Andrew MacIntyre (os2vacpp is the OS/2 that lacks ftruncate()):

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I actively support the OS/2 EMX port (sys.platform == "os2emx"; build
directory is PC/os2emx).  I would like to keep the VAC++ port alive, but
the reality is I don't have the resources to do so.  The VAC++ port was
the subject of discussion about removal of build support support from the
source tree for 2.6 - I don't recall there being a definitive outcome,
but if someone does delete the PC/os2vacpp directory, I'm not in a
position to argue.

AMK: (mailbox.py has a separate section of code used when file.truncate()
isn't available, and the existence of this section is bedevilling me.
It would be convenient if platforms without file.truncate() weren't a
factor; then this branch could just be removed.  In your opinion,
would it hurt OS/2 users of mailbox.py if support for platforms
without file.truncate() was removed?)

aimacintyre: No.  From what documentation I can quickly check, ftruncate() operates
on file descriptors rather than FILE pointers.  As such I am sure that,
if it became an issue, it would not be difficult to write a ftruncate()
emulation wrapper for the underlying OS/2 APIs that implement the
required functionality.