threads

CRYPTO_set_locking_callback, CRYPTO_set_id_callback ? OpenSSL thread support

 #include <openssl/crypto.h>

 void CRYPTO_set_locking_callback(void (*locking_function)(int mode,
        int n, const char *file, int line));

 void CRYPTO_set_id_callback(unsigned long (*id_function)(void));

 int CRYPTO_num_locks(void);

OpenSSL can safely be used in multi-threaded applications provided that two callback functions are set.

locking_function(int mode, int n, const char *file, int line) is needed to perform locking on shared data stuctures. Multi-threaded applications will crash at random if it is not set.

locking_function() must be able to handle up to CRYPTO_num_locks() different mutex locks. It sets the n?th lock if mode & CRYPTO_LOCK, and releases it otherwise.

file and line are the file number of the function setting the lock. They can be useful for debugging.

id_function(void) is a function that returns a thread ID. It is not needed on Windows nor on platforms where getpid() returns a different ID for each thread (most notably Linux).

CRYPTO_num_locks() returns the required number of locks. The other functions return no values.

You can find out if OpenSSL was configured with thread support:

 #define OPENSSL_THREAD_DEFINES
 #include <openssl/opensslconf.h>
 #if defined(THREADS)
   // thread support enabled
 #else
   // no thread support
 #endif

crypto/threads/mttest.c shows examples of the callback functions on Solaris, Irix and Win32.

CRYPTO_set_locking_callback() and CRYPTO_set_id_callback() are available in all versions of SSLeay and OpenSSL. CRYPTO_num_locks() was added in OpenSSL 0.9.4.

crypto(3)