Comment 42 for bug 305901

It's not defined in POSIX, but it has worked a certain way in glibc for decades. There's no _reason_ to break it for _FORTIFY_SOURCE. Pre-truncating just silently breaks programs and does weird stuff. If you want to expose it with _FORITFY_SOURCE then have vsprintf notice that the target and first format argument are the same variable, and refuse to build.

Either pretruncation should be eliminated, or the undefined behavior should be explicitly detected and dealt with. Just having programs lose data while running with no indication of the cause seems like a terrible user experience.