C/C uses CRYPTO_THREADID_set_callback() as

...

   CRYPTO_THREADID_set_callback(my_cb_threadid);

...

   CRYPTO_THREADID_set_callback(NULL);

...

this is in openssl.c, the first line works on load, the second — on unload.

But if you look in the openssl-1.0.2 code, you'll see

int CRYPTO_THREADID_set_callback(void (*func) (CRYPTO_THREADID *))

{

    if (threadid_callback)

        return 0;

    threadid_callback = func;

that is, CRYPTO_THREADID_set_callback(NULL) does not remove the callback, in fact there is no way to remove the callback after it's set.

This means that a shared library should never ever use CRYPTO_THREADID_set_callback(), because after the shared library is unloaded the callback will point into unallocated memory, and the callback, again, can never be removed.