As you methioned, I move ldap_set_option() with NULL to the main thread. And, I also use libldap_r instead of libldap. A specific handler is really only used in a single thread. In every thread, it always returns NULL, because it is just a test code and it always print "connect ok" while it is running.

I think memory leak may happen when the code of openLDAP uses openssl function such as initialize connection in ldap_start_tls_s(). For every thread, there may be some special structures for SSL/TLS which need to be freed after the end of the thread. I find that Windows has a function ldap_stop_tls_s() related to ldap_start_tls_s().

For my software, I want to create processes instead of threads to avoid this problem.