When are .so's reloaded?

[krotuss]

Hi, what will happen when I reemerge shared library, which is already loaded in memory, and than launch some program that is linked with this library? Will linker find out so library has changed and load its new version, or program will be linked with old version which is already loaded? Thanks.

[Naib]

Presently running apps will still be referencing the "old" so (be it in RAM or via inode location).
Newer apps will force the newer so to be loaded

[krotuss]

Thanks. How does linker know that library has changed, is there some mechanism similar to inotify used, or it simply compares image of library on disk with the one in RAM every time it links another program?

[eccerr0r]

Linker doesn't know, it just checks the current location. The linker is invoked every time a new program is run.

Existing programs (that are already loaded) have pointers to the old version, as Naib says, in RAM as code or an inode location as a file handle to the deleted version. Once all handles are gone/programs terminated, the file will then truly be "removed" from the disk and space reclaimed. Until then you'll see shared libraries still eating disk space and RAM, despite them being removed.

This is why it's good to reboot the machine after certain upgrades such as libc, so programs like init can pick up a new copy of libc.
(unlike in Windows it simply forces you to reboot... which may not always be a bad thing.)
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[Naib]