It is everywhere even Windows. There is a little bit of weirdness on some older versions of Windows CE and some undefined behavior with edge conditions on some OS's if I recall, but you can find the capability everywhere. I added the map API to QFile back in the day and went through checking different OS's for compatibility. A quick google search here is a link to the windows method:

https://github.com/radekp/qt/blob/master/src/corelib/io/qfsf...

unix version: https://github.com/radekp/qt/blob/master/src/corelib/io/qfsf...