CRYPTO_THREAD_RUN_ONCE(3SSL)

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, CRYPTO_atomic_add64, CRYPTO_atomic_and, CRYPTO_atomic_or, CRYPTO_atomic_load, CRYPTO_atomic_store, CRYPTO_atomic_load_int, CRYPTO_atomic_store_int, OSSL_set_max_threads, OSSL_get_max_threads, OSSL_get_thread_support_flags, OSSL_THREAD_SUPPORT_FLAG_THREAD_POOL, OSSL_THREAD_SUPPORT_FLAG_DEFAULT_SPAWN - 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);
 int CRYPTO_atomic_add64(uint64_t *val, uint64_t op, uint64_t *ret,
                         CRYPTO_RWLOCK *lock);
 int CRYPTO_atomic_and(uint64_t *val, uint64_t op, uint64_t *ret,
                       CRYPTO_RWLOCK *lock);
 int CRYPTO_atomic_or(uint64_t *val, uint64_t op, uint64_t *ret,
                      CRYPTO_RWLOCK *lock);
 int CRYPTO_atomic_load(uint64_t *val, uint64_t *ret, CRYPTO_RWLOCK *lock);
 int CRYPTO_atomic_store(uint64_t *dst, uint64_t val, CRYPTO_RWLOCK *lock);
 int CRYPTO_atomic_load_int(int *val, int *ret, CRYPTO_RWLOCK *lock);
 int CRYPTO_atomic_store_int(int *dst, int val, CRYPTO_RWLOCK *lock);

 int OSSL_set_max_threads(OSSL_LIB_CTX *ctx, uint64_t max_threads);
 uint64_t OSSL_get_max_threads(OSSL_LIB_CTX *ctx);
 uint32_t OSSL_get_thread_support_flags(void);

 #define OSSL_THREAD_SUPPORT_FLAG_THREAD_POOL
 #define OSSL_THREAD_SUPPORT_FLAG_DEFAULT_SPAWN

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. If the argument is NULL, nothing is done.

•   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. If atomic operations are not supported and lock is NULL, then the function will fail.

•   CRYPTO_atomic_add64() atomically adds op 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_add64() then CRYPTO_atomic_add64() must be the only way that the variable is modified. If atomic operations are not supported and lock is NULL, then the function will fail.

•   CRYPTO_atomic_and() performs an atomic bitwise and of op and *val and stores the result back in *val. It also 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_and() or read by CRYPTO_atomic_load() then CRYPTO_atomic_and() must be the only way that the variable is modified. If atomic operations are not supported and lock is NULL, then the function will fail.

•   CRYPTO_atomic_or() performs an atomic bitwise or of op and *val and stores the result back in *val. It also 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_or() or read by CRYPTO_atomic_load() then CRYPTO_atomic_or() must be the only way that the variable is modified. If atomic operations are not supported and lock is NULL, then the function will fail.

•   CRYPTO_atomic_load() atomically loads the contents of *val into *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_or() or read by CRYPTO_atomic_load() then CRYPTO_atomic_load() must be the only way that the variable is read. If atomic operations are not supported and lock is NULL, then the function will fail.

•   CRYPTO_atomic_store() atomically stores the contents of val into *dst. lock will be locked, unless atomic operations are supported on the specific platform.

•   CRYPTO_atomic_load_int() works identically to CRYPTO_atomic_load() but operates on an int value instead of a uint64_t value.

•   CRYPTO_atomic_store_int() works identically to CRYPTO_atomic_store() but operates on an int value instead of a uint64_t value.

•   OSSL_set_max_threads() sets the maximum number of threads to be used by the thread pool. If the argument is 0, thread pooling is disabled. OpenSSL will not create any threads and existing threads in the thread pool will be torn down. The maximum thread count is a limit, not a target. Threads will not be spawned unless (and until) there is demand. Thread polling is disabled by default. To enable threading you must call OSSL_set_max_threads() explicitly. Under no circumstances is this done for you.

•   OSSL_get_thread_support_flags() determines what thread pool functionality OpenSSL is compiled with and is able to support in the current run time environment. OSSL_THREAD_SUPPORT_FLAG_THREAD_POOL indicates that the base thread pool functionality is available, and OSSL_THREAD_SUPPORT_FLAG_DEFAULT_SPAWN indicates that the default thread pool model is available. The default thread pool model is currently the only model available, therefore both of these flags must be set for thread pool functionality to be used.

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.

OSSL_set_max_threads() returns 1 on success and 0 on failure. Returns failure if OpenSSL-managed thread pooling is not supported (for example, if it is not supported on the current platform, or because OpenSSL is not built with the necessary support).

OSSL_get_max_threads() returns the maximum number of threads currently allowed to be used by the thread pool. If thread pooling is disabled or not available, returns 0.

OSSL_get_thread_support_flags() returns zero or more OSSL_THREAD_SUPPORT_FLAG values.

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 <openssl/crypto.h> where use of CRYPTO_THREAD_* types and functions is required.

EXAMPLES
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

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()) {
        /* Do not unlock unless the lock was successfully acquired. */
        return 0;
     }

     /* 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.

SEE ALSO
crypto(7), openssl-threads(7).

HISTORY
CRYPTO_atomic_load_int(), OSSL_set_max_threads(), OSSL_get_max_threads(), OSSL_get_thread_support_flags() were added in OpenSSL 3.2.

CRYPTO_atomic_store(), CRYPTO_atomic_add64(), CRYPTO_atomic_and() were added in OpenSSL 3.4.

CRYPTO_atomic_store_int() was added in OpenSSL 4.0.

COPYRIGHT
Copyright 2000-2026 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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>.