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CRYPTO_THREAD_run_once(3) OpenSSL CRYPTO_THREAD_run_once(3)
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_or, CRYPTO_atomic_load - OpenSSL thread support
LIBRARY
libcrypto, -lcrypto
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_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);
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. 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.
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 <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()) {
/* 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).
COPYRIGHT
Copyright 2000-2021 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>.
3.0.12 2023-05-07 CRYPTO_THREAD_run_once(3)