sigaction() returns the old action. This permits chaining of the actions. Specifically for SIGSEGV, if Library B does not want the signal, it can invoke Library A (without caring what it is). Similarily, A can invoke B. "Unloading" cannot be done, unless you are aware of the load order. This is a general issue -- dlclose() is reference counted, and you wouldn't know if it actually unloaded the library.