Ratiu: A tale of two toolchains and glibc

* The only formal change in semantics after calling dlclose() is that the application is no longer permitted to dereference certain pointers. Perhaps it's a tad obvious, but a conformant application must not dereference those pointers. Therefore, the application is not permitted to assume that dereferencing those pointers will, say, generate SIGSEGV, trip a guard page, or have any other desired or undesired effect, because the standard flatly forbids such dereferencing in the first place.
* The caller is expressly forbidden from interpreting the handle returned by dlopen "in any way." This presumably includes comparing it for equality with other handles returned by dlopen. Therefore, a conformant implementation may return the same handle every time you dlopen the same file, and keep an internal reference count (which the non-normative section of the dlclose standard explicitly calls out as a thing that implementations may do). If dlclose does nothing, then you just omit the reference count.
* Conformant implementations are also permitted to reuse closed handles, and a conformant implementation could even keep track of which object files were opened in the past and conspire to reuse their handle values if they are ever reopened in the future. Of course, if dlclose does nothing, then that's not really much of a "conspiracy."
* Maybe you're short on memory and trying to reclaim it? Well, that's not a very good reason at all. The pages which dlclose would free are backed by an object file on disk. If those pages are not in active use, the kernel should drop them automatically under memory pressure.
* Maybe you're trying to implement some crazy mechanism where you can replace object files without stopping and restarting the applications which are using them? Eh, that's probably a pipe dream anyway. Stopping and restarting your app is way easier than carefully shutting down an entire module of your program and then starting it up again. Also, the stop/restart dance is a general pattern, well supported by tools such as APT and systemd.