SIGWINCH
As a matter of fact, @Stefano is right: SIGWINCH means SIGNAL WINDOWS CHANGE and is sent automatically when a terminal detects a change in it’s windows size to allow for a redraw.
SIGWINCH for apache2
But unfortunately apache2 process poorly misuses this signal (in the way they divert its first meaning), but for their defense, they seems not to have a choice and had to resort to this due to a lack of signal (see bug report). One of their assumption is that apache2 process is always in background. So @mikl is also perfectly right also.
So, what to check ?
- any automatic reason to restart gracefully apache2 (cron services, apt-get installations of related services…)
- otherwise, do you run apache2 in foreground in an open tty ? If yes, this is not recommended (alas), and you’ll probably easily hit this SIGWINCH gracefull shutdown quite easily when resizing your controlling terminal.