Given a security fix for a system-critical library like libc. Debian-based distros will publish after a few hours / days packages for it.
But is the library immediately used for already running applications after installing the deb-package for the fix? Or has the machine to be restarted?
You dont have to restart Linux or Unix (or BSD) unless you have a kernel upgrade.
If you upgrade an application (or libraries that the application uses) then you'll just restart the applications that depends on them.
No, and no.
The libraries are not replaced immediately for already running processes. They are replaced when the processes get restarted. For that you don't have to restart the machine. Only the processes and daemons.
If there is a libc change in particular, the apt system informs you of the services that need a restart and asks you to confirm their restart.
