The specific quote is:
When the "r+", "w+", or "a+" access type is specified, both reading and writing are allowed (the file is said to be open for "update"). However, when you switch between reading and writing, there must be an intervening fflush, fsetpos, fseek, or rewind operation. The current position can be specified for the fsetpos or fseek operation, if desired.As an example, here is what you might do in python:
>>> f = open('test', 'wb+')
>>> f.write('initial text\n')
>>> f = open('test', 'rb+')
>>> f.write('this should go at the end\n')
On most platforms, that succeeds. But on Windows, if you don't do
>>> f.seek(0, 2) # Seek to the end of the file
before you call f.write(), you will get an IOError, with e.errno = 0. (Yeah, having an error of SUCCESS is a little hard to figure out).
Anyway, it took a while for me to figure out, so I figured I'd let other people know.