ios_base::app Standard behavior

Hello

     According C++ Standard 27.8.1.3 Table 92 when a file stream is
opened with ios_base::out | ios_base::app it should behave like fopen
with "a" open mode.But I discovered that fopen is platform dependent,
under my Linux file pointer is set at end of file after it has been
open on Windows it stays at beginning. So what should a C++ Standard
implementation do:

1. Follow platform dependent fopen behavior ?
2. Always stay at the beginning of file once the file is open?
3. Always move to the end of file once the file is open (like ate) ?

Thanks for your feedback.

Bests

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