C11の仕様-脆弱性対応に関連する機能強化点 (2014/3/14) https://www.buildinsider.net/language/clang/02
printf_s()、OpenBSDでは実装してないみたいだけどそもそもOpenBSDのprintf()は%nを受け付けない(警告文の表示に加えてcore吐いて落ちる)。manにもこうあるし。
The %n conversion specifier has serious security implications, so it was changed to no longer store the number of bytes written so far into the variable indicated by the pointer argument. Instead a syslog(3) message will be generated, after which the program is aborted with SIGABRT.