If the same file descriptor is open in another process (e.g. one that has it via fork(2) FD sharing either without child processes exec(2)ing or with CLOEXEC not set, or some older and esoteric abuses of fdpassing over UNIX sockets), then close(2) just decrements a refcount. The actual file isn't closed until the last holder of a reference to its file descriptor calls close(2).
This is rarely relevant, but when it comes up, it sure is wild. Common sources of confusion and pain due to this behavior are: files opened for "global library reasons" (e.g. /dev/shm buffers) in preforking servers, signalfd descriptors in preforking servers, processes that fork off and daemonize but leave their spawner around for e.g. CoW memory sharing efficiencies (looking at you, Python multiprocessing backends--the docs make it sound like they all act more or less the same, but in this regard they very much do not), libraries that usually swap STDIN/OUT/ERR file descriptors around with CLOEXEC disabled for a manually fork+exec'd child (e.g. situations where posix_spawn doesn't support needed pre-exec setup) but that are then used as part of larger applications that fork/spawn for other reasons and don't realize that file descriptor allocation/forking needs care and synchronization with the manually-fork/execing library in question: mixing forks and threads is one of those things that everyone says is a fast-track ticket to nasal demons, but that everyone also does regularly, I've found--if this describes you, be careful!
If you end up in one of those situations, suddenly invariants like "I called unlink on this path and then close(2)'d the descriptor to it, so that (maybe large) chunk of allocated space isn't taking up space on the filesystem/buffers any more" and "this externally-observable lockfile/directory is now unlocked due to its absence, now processes coordinating using that file on NFS will work as expected" no longer hold.
https://www.ibm.com/docs/en/aix/7.1.0?topic=domains-unix-dom...
I know that close(2)'s weirdness isn't a superset of dlopen(3)'s and there are different reasons for both behaviors. But it's still interesting that they "rhyme" as it were.
File descriptor = reference/pointer-ish thing.
File description = referenced data.
close(2) = decrement refcount to description by closing descriptor; actual destruction of the file's open-ness doesn't happen until refcount == 0.
One of the less well designed APIs, but as an aside it is still widely used for IPC.
The only safe, consistent, reliable approach is not to close DLLs.
That doesn't address the need of some DLLs to malloc() resources in the context of the applications linking to them.
This problem _cannot_ be solved _generically_. Any solutions are extremely API-specific and impose restrictions on their users (the linking applications) which, if violated, will lead to Undefined Behavior.
Edit: as an example of cases which must bind resources in the application's context: see the Classloading in C++ paper at <https://wanderinghorse.net/computing/papers/index.html#class...> (disclosure: i wrote that article).
[0] https://wiki.musl-libc.org/functional-differences-from-glibc...
As this article details, there are so many circumstances that preclude DLLs being unloaded completely that I was surprised that their design actually worked at all. So many language constructs do not play nicely with the idea that code and static data can just disappear at runtime.
A memory leak caused by a library isn't necessarily something that should prevent it from being unloaded. Unless it is the leaked objects that are holding a reference! (Then we need a full blown GC system to detect cycles.)
The only safe, consistent, reliable approach is not to deallocate memory.
The init function could return a status that indicates "already initialized."
The calling code could observe this and passively warn of the unusual condition but otherwise proceed.
My guess they made an all-new Rust plugin which used an existing C++ library. Pretty common case when existing code base is slowly being converted to rust.
Since the library is going away, it means it's not used any more. Not being used means that no thread that is currently running will call into that library any more.
If no thread that is currently running will use the code of that library, it has no business hanging on to the data belonging to the library; that data should only be manipulable via that code. Therefore, it should be forcibly taken away.
The reason they flunk on this issue is that there isn't a nice way to enumerate through all the threads and snipe away a particular class of thread local storage from each. The architecture is oriented around the POSIX-induced brain-damaged idea that a thread must clean up all thread-specific storage after itself.
Fight me!
Of course, if the library doesn't probably attempt to close libraries it's responsible for during its shutdown procedure, that's another can of worms.
What if lib.B had been loaded explicitly somewhere else such that it did not appear in any other module's dependency list?
