[link|http://www.debian.org/doc/debian-policy/|Debian Policy], section 9, [link|http://www.debian.org/doc/debian-policy/ch-sharedlibs.html|Shared Libraries].
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Minor versions are updated in place via the 'link()' system call, which is atomic (rm + cp are not, which is why this creates a problem, and which is why any distro does library updates via symlinks).
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Major versions generally result in a new package, with the library major version noted.
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There's a collection of obsoleted libraries in "oldlibs", which allow, eg, libc5 programs to function properly.
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As with all other aspects of a Debian system upgrade, with the exception of the kernel and kernel modules, these updates may be made on a functioning system, in multi-user mode, without requiring a reboot or shutdown to single-user mode.
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What specific issues did you have in mind?
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