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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	-
       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:

       o 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.

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

       o CRYPTO_THREAD_read_lock() locks the provided lock for reading.

       o CRYPTO_THREAD_write_lock() locks the provided lock for	writing.

       o CRYPTO_THREAD_unlock()	unlocks	the previously locked lock.

       o CRYPTO_THREAD_lock_free() frees the provided lock.

       o 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>.

1.1.1a				  2018-11-20	     CRYPTO_THREAD_RUN_ONCE(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | NOTES | EXAMPLE | NOTES | SEE ALSO | COPYRIGHT

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