- The threads(3ssl) man page is pretty good.
- Unless you're using OpenSSL 0.9.9, the CRYPTO_set_id_callback(3ssl) is pretty much broken on pthreads implementations which don't return an integer value from pthread_self(3). This is true for the FreeBSD native implementations (the LinuxThreads emulation package should work, but is broken in other, more grave, ways). On OpenSSL 0.9.9, CRYPTO_set_idptr_callback(3ssl) can be used for implementations which return a pointer or an aggregate.
- libdank's OpenSSL layer handles this via a pthread_once_t which installs a class of thread-specific data. If the thread executing the callback has a NULL value for pthread_get_specific(), a static key is incremented and copied into the TSD. The increment, and copy of the result, forms a static mutex-protected critical section.
- "Clarification questions on OpenSSL thread-safe support", openssl-users mailing list 2008-03-10
- "When to use CRYPTO_set_locking_callback() and CRYPTO_set_id_callback()?", openssl-users mailing list 2006-08-17