使用旧版本openssl时，需要设置两个回调
The documentation on OpenSSL threads states (at least for version 1.0.2):
OpenSSL can safely be used in multi-threaded applications provided that at least two
callback functions are set, locking_function and threadid_func.
但是在1.1.0以及更新的版本中，不再需要这个设置了
The ChangeLog of version 1.1.0 states:
OpenSSL now uses a new threading API. It is no longer necessary to
set locking callbacks to use OpenSSL in a multi-threaded environment. There
are two supported threading models: pthreads and windows threads. It is
also possible to configure OpenSSL at compile time for "no-threads". The
old threading API should no longer be used. The functions have been
replaced with "no-op" compatibility macros.
[Alessandro Ghedini, Matt Caswell]
# if OPENSSL_API_COMPAT < 0x10000000L
# define CRYPTO_set_id_callback(func)
# define CRYPTO_get_id_callback() (NULL)
# define CRYPTO_thread_id() (0UL)
# endif /* OPENSSL_API_COMPAT < 0x10000000L */
/*
* The old locking functions have been removed completely without compatibility
* macros. This is because the old functions either could not properly report
* errors, or the returned error values were not clearly documented.
* Replacing the locking functions with no-ops would cause race condition
* issues in the affected applications. It is far better for them to fail at
* compile time.
* On the other hand, the locking callbacks are no longer used. Consequently,
* the callback management functions can be safely replaced with no-op macros.
*/
# define CRYPTO_num_locks() (1)
# define CRYPTO_set_locking_callback(func)
# define CRYPTO_get_locking_callback() (NULL)
# define CRYPTO_set_add_lock_callback(func)
# define CRYPTO_get_add_lock_callback() (NULL)
