Updated: 2022/Sep/29

Please read Privacy Policy. It's for your privacy.

CRYPTO_THREAD_run_once(3)           OpenSSL          CRYPTO_THREAD_run_once(3)

       CRYPTO_THREAD_run_once, CRYPTO_THREAD_lock_new,
       CRYPTO_THREAD_read_lock, CRYPTO_THREAD_write_lock,
       CRYPTO_THREAD_unlock, CRYPTO_THREAD_lock_free, CRYPTO_atomic_add -
       OpenSSL thread support

       libcrypto, -lcrypto

        #include <openssl/crypto.h>

        int CRYPTO_THREAD_run_once(CRYPTO_ONCE *once, void (*init)(void));

        CRYPTO_RWLOCK *CRYPTO_THREAD_lock_new(void);
        int CRYPTO_THREAD_read_lock(CRYPTO_RWLOCK *lock);
        int CRYPTO_THREAD_write_lock(CRYPTO_RWLOCK *lock);
        int CRYPTO_THREAD_unlock(CRYPTO_RWLOCK *lock);
        void CRYPTO_THREAD_lock_free(CRYPTO_RWLOCK *lock);

        int CRYPTO_atomic_add(int *val, int amount, int *ret, CRYPTO_RWLOCK *lock);

       OpenSSL can be safely used in multi-threaded applications provided that
       support for the underlying OS threading API is built-in. Currently,
       OpenSSL supports the pthread and Windows APIs. OpenSSL can also be
       built without any multi-threading support, for example on platforms
       that don't provide any threading support or that provide a threading
       API that is not yet supported by OpenSSL.

       The following multi-threading function are provided:

       ⊕ CRYPTO_THREAD_run_once() can be used to perform one-time
         initialization.  The once argument must be a pointer to a static
         object of type CRYPTO_ONCE that was statically initialized to the
         value CRYPTO_ONCE_STATIC_INIT.  The init argument is a pointer to a
         function that performs the desired exactly once initialization.  In
         particular, this can be used to allocate locks in a thread-safe
         manner, which can then be used with the locking functions below.

       ⊕ CRYPTO_THREAD_lock_new() allocates, initializes and returns a new
         read/write lock.

       ⊕ CRYPTO_THREAD_read_lock() locks the provided lock for reading.

       ⊕ CRYPTO_THREAD_write_lock() locks the provided lock for writing.

       ⊕ CRYPTO_THREAD_unlock() unlocks the previously locked lock.

       ⊕ CRYPTO_THREAD_lock_free() frees the provided lock.

       ⊕ CRYPTO_atomic_add() atomically adds amount to val and returns the
         result of the operation in ret. lock will be locked, unless atomic
         operations are supported on the specific platform. Because of this,
         if a variable is modified by CRYPTO_atomic_add() then
         CRYPTO_atomic_add() must be the only way that the variable is

       CRYPTO_THREAD_run_once() returns 1 on success, or 0 on error.

       CRYPTO_THREAD_lock_new() returns the allocated lock, or NULL on error.

       CRYPTO_THREAD_lock_free() returns no value.

       The other functions return 1 on success, or 0 on error.

       On Windows platforms the CRYPTO_THREAD_* types and functions in the
       openssl/crypto.h header are dependent on some of the types customarily
       made available by including windows.h. The application developer is
       likely to require control over when the latter is included, commonly as
       one of the first included headers. Therefore, it is defined as an
       application developer's responsibility to include windows.h prior to
       crypto.h where use of CRYPTO_THREAD_* types and functions is required.

       This example safely initializes and uses a lock.

        #ifdef _WIN32
        # include <windows.h>
        #include <openssl/crypto.h>

        static CRYPTO_RWLOCK *lock;

        static void myinit(void)
            lock = CRYPTO_THREAD_lock_new();

        static int mylock(void)
            if (!CRYPTO_THREAD_run_once(&once, void init) || lock == NULL)
                return 0;
            return CRYPTO_THREAD_write_lock(lock);

        static int myunlock(void)
            return CRYPTO_THREAD_unlock(lock);

        int serialized(void)
            int ret = 0;

            if (mylock()) {
                /* Your code here, do not return without releasing the lock! */
                ret = ... ;
            return ret;

       Finalization of locks is an advanced topic, not covered in this
       example.  This can only be done at process exit or when a dynamically
       loaded library is no longer in use and is unloaded.  The simplest
       solution is to just "leak" the lock in applications and not repeatedly
       load/unload shared libraries that allocate locks.

       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 */


       Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at

1.1.1i                            2020-12-10         CRYPTO_THREAD_run_once(3)