F***ing sprintf rounding - how does it work?

[ribasushi]

Greetings everyone

I am trying to figure out which of the multiple rounding techniques is used by perl's (and I suspect C) printf. I can not find anything that matches on the interwebs. Can anyone explain in detail the output of the following oneliner:

perl -e 'printf "%s => %.1f\n", ("0.${_}5") x 2 for (0..9)'

0.05 => 0.1
0.15 => 0.1 <-- this pair precludes even/odd
0.25 => 0.2 <-- rounding as explanation
0.35 => 0.3
0.45 => 0.5 <-- looks like the threshold of down/up rounding behavior
0.55 => 0.6
0.65 => 0.7
0.75 => 0.8
0.85 => 0.8 <-- except not quite :(
0.95 => 0.9