=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