QFile::resize takes forever for a 300MB file

I know, that QFile::resize() writes 0x00 on every position and might need that long, that's the reason, why I have put it into a QThread, but still all action halts until it is ready.
(I init the QThread in the main-window constructor, maybe that's wrong?)

Also because it is not necessary to initialize the file with 0x00s, is there a way just to allocate the file-space without initializing (that should be totally fast)?

Thanks

(using: QT4.3 mingw XP)

PS: how can I edit a thread-title to "[solved]..." if its solved?