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>.

1.1.1                                              2023-05-24                       CRYPTO_THREAD_RUN_ONCE(3SSL)