CRYPTO_set_locking_callback, CRYPTO_set_id_callback, CRYPTO_num_locks, CRYPTO_set_dynlock_create_callback, CRYPTO_set_dynlock_lock_callback,
CRYPTO_set_dynlock_destroy_callback, CRYPTO_get_new_dynlockid,
CRYPTO_destroy_dynlockid, CRYPTO_lock - 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);

/* struct CRYPTO_dynlock_value needs to be defined by the user */
struct CRYPTO_dynlock_value;

void CRYPTO_set_dynlock_create_callback(struct CRYPTO_dynlock_value *
       (*dyn_create_function)(char *file, int line));
void CRYPTO_set_dynlock_lock_callback(void (*dyn_lock_function)
       (int mode, struct CRYPTO_dynlock_value *l,
       const char *file, int line));
void CRYPTO_set_dynlock_destroy_callback(void (*dyn_destroy_function)
       (struct CRYPTO_dynlock_value *l, const char *file, int line));

int CRYPTO_get_new_dynlockid(void);

void CRYPTO_destroy_dynlockid(int i);

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

#define CRYPTO_w_lock(type)    \
#define CRYPTO_w_unlock(type)  \
#define CRYPTO_r_lock(type)    \
#define CRYPTO_r_unlock(type)  \
#define CRYPTO_add(addr,amount,type)   \


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

locking_function(int mode, int n, const char *file, int line) is needed to perform locking on shared data structures. (Note that OpenSSL uses a number of global data structures that will be implicitly shared
whenever multiple threads use OpenSSL.) 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, for example
pthread_self() if it returns an integer (see NOTES below). It isn't needed on Windows nor on platforms where getpid() returns a different ID for each thread (see NOTES below).

Additionally, OpenSSL supports dynamic locks, and sometimes, some parts of OpenSSL need it for better performance. To enable this, the
following is required:

o Three additional callback function, dyn_create_function,
dyn_lock_function and dyn_destroy_function.
o A structure defined with the data that each lock needs to handle.
struct CRYPTO_dynlock_value has to be defined to contain whatever
structure is needed to handle locks.
dyn_create_function(const char *file, int line) is needed to create a
lock. Multi-threaded applications might crash at random if it is not
dyn_lock_function(int mode, CRYPTO_dynlock *l, const char *file, int
line) is needed to perform locking off dynamic lock numbered n. Multithreaded applications might crash at random if it is not set.
dyn_destroy_function(CRYPTO_dynlock *l, const char *file, int line) is needed to destroy the lock l. Multi-threaded applications might crash
at random if it is not set.
CRYPTO_get_new_dynlockid() is used to create locks. It will call dyn_create_function for the actual creation.
CRYPTO_destroy_dynlockid() is used to destroy locks. It will call dyn_destroy_function for the actual destruction.
CRYPTO_lock() is used to lock and unlock the locks. mode is a bitfield describing what should be done with the lock. n is the number of the
lock as returned from CRYPTO_get_new_dynlockid(). mode can be combined from the following values. These values are pairwise exclusive, with
undefined behaviour if misused (for example, CRYPTO_READ and
CRYPTO_WRITE should not be used together):



CRYPTO_num_locks() returns the required number of locks.

CRYPTO_get_new_dynlockid() returns the index to the newly created lock.

The other functions return no values.


You can find out if OpenSSL was configured with thread support:
#include <openssl/opensslconf.h>
#if defined(OPENSSL_THREADS)
// thread support enabled
// no thread support
Also, dynamic locks are currently not used internally by OpenSSL, but
may do so in the future.
Defining id_function(void) has it's own issues. Generally speaking,
pthread_self() should be used, even on platforms where getpid() gives different answers in each thread, since that may depend on the machine the program is run on, not the machine where the program is being
compiled. For instance, Red Hat 8 Linux and earlier used LinuxThreads, whose getpid() returns a different value for each thread. Red Hat 9 Linux and later use NPTL, which is Posix-conformant, and has a getpid() that returns the same value for all threads in a process. A program
compiled on Red Hat 8 and run on Red Hat 9 will therefore see getpid() returning the same value for all threads.
There is still the issue of platforms where pthread_self() returns something other than an integer. This is a bit unusual, and this
manual has no cookbook solution for that case.


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. All functions dealing with dynamic locks
were added in OpenSSL 0.9.5b-dev.


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