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=pod
=head1 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
=head1 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);
=head1 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:
=over 4
=item *
CRYPTO_THREAD_run_once() can be used to perform one-time initialization.
The B<once> argument must be a pointer to a static object of type
B<CRYPTO_ONCE> that was statically initialized to the value
B<CRYPTO_ONCE_STATIC_INIT>.
The B<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.
=item *
CRYPTO_THREAD_lock_new() allocates, initializes and returns a new read/write
lock.
=item *
CRYPTO_THREAD_read_lock() locks the provided B<lock> for reading.
=item *
CRYPTO_THREAD_write_lock() locks the provided B<lock> for writing.
=item *
CRYPTO_THREAD_unlock() unlocks the previously locked B<lock>.
=item *
CRYPTO_THREAD_lock_frees() frees the provided B<lock>.
=item *
CRYPTO_atomic_add() atomically adds B<amount> to B<val> and returns the
result of the operation in B<ret>. B<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.
=back
=head1 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_frees() returns no value.
The other functions return 1 on success or 0 on error.
=head1 EXAMPLE
This example safely initializes and uses a lock.
#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.
=head1 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
=head1 SEE ALSO
L<crypto(3)>
=head1 COPYRIGHT
Copyright 2000-2016 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
L<https://www.openssl.org/source/license.html>.
=cut
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