[quietbritishjim]

This doesn't fully explain why a reboot is not required on Linux. If a *nix operating system updates sysfile1.so and sysfile2.so in the way you describe, then there will be some time where the filename sysfile1.so refers to the new version of that file while sysfile2.so refers to the old version. A program that is started in this brief window will get mixed versions of these libraries. It is unlikely that all combinations of versions of libraries have been tested together, so you could end up running with untested and possibly incompatible versions of libraries.

[beagle3]

> This doesn't fully explain why a reboot is not required on Linux.

Of course there is a theoretical possibility that this will happen; however, in practice, updates (especially security updates) on Linux happen with ABI compatible libraries. E.g. on debian/ubuntu

apt-get update && apt-get upgrade

Will generally only do ABI compatible updates, without installing additional packages (you need 'dist-upgrade' or 'full-upgrade' for that).

Some updates will go as far as to prevent a program restart while updating (by temporarily making the executable unavailable).

Firefox on Ubuntu is an outlier - an update will replace it with one that isn't ABI compatible. It detects this and encourages you to restart it.

All in all, it's not that a reboot is never required for linux theoretically - it is that practically, you MUST reboot only for a kernel update, and may occasionally need to restart other programs that have been updated (but are rarely forced too).

[snuxoll]

This generally should never happen, Linux distributions don’t wholesale replace shared objects with ABI incompatible versions - soname’s exist to protect against this very issue.

[CJefferson]

I had a program with a rarely reported bug that turned out to be lazy loading of .so files that was this bug. Switched to eager loading and it went away.