NAME

       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

SYNOPSIS

        #include <openssl/crypto.h>

        CRYPTO_ONCE CRYPTO_ONCE_STATIC_INIT;
        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);

DESCRIPTION

       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 modified.

RETURN VALUES

       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.

NOTES

       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.

EXAMPLE

       This example safely initializes and uses a lock.

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

        static CRYPTO_ONCE once = CRYPTO_ONCE_STATIC_INIT;
        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 = ... ;
            }
            myunlock();
            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.

NOTES

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

        #include <openssl/opensslconf.h>
        #if defined(OPENSSL_THREADS)
            /* thread support enabled */
        #else
            /* no thread support */
        #endif

SEE ALSO

       crypto(7)

COPYRIGHT

       Copyright 2000-2018 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 <https://www.openssl.org/source/license.html>.