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OPENSSL-THREADS(7)                  OpenSSL                 OPENSSL-THREADS(7)



NAME
       openssl-threads - Overview of thread safety in OpenSSL

DESCRIPTION
       In this man page, we use the term thread-safe to indicate that an
       object or function can be used by multiple threads at the same time.

       OpenSSL can be built with or without threads support. The most
       important use of this support is so that OpenSSL itself can use a
       single consistent API, as shown in "EXAMPLES" in
       CRYPTO_THREAD_run_once(3).  Multi-platform applications can also use
       this API.

       In particular, being configured for threads support does not imply that
       all OpenSSL objects are thread-safe.  To emphasize: most objects are
       not safe for simultaneous use.  Exceptions to this should be documented
       on the specific manual pages, and some general high-level guidance is
       given here.

       One major use of the OpenSSL thread API is to implement reference
       counting.  Many objects within OpenSSL are reference-counted, so
       resources are not released, until the last reference is removed.
       References are often increased automatically (such as when an X509
       certificate object is added into an X509_STORE trust store).  There is
       often an oobbjjeecctt_up_ref() function that can be used to increase the
       reference count.  Failure to match oobbjjeecctt_up_ref() calls with the right
       number of oobbjjeecctt_free() calls is a common source of memory leaks when a
       program exits.

       Many objects have set and get API's to set attributes in the object.  A
       "set0" passes ownership from the caller to the object and a "get0"
       returns a pointer but the attribute ownership remains with the object
       and a reference to it is returned.  A "set1" or "get1" function does
       not change the ownership, but instead updates the attribute's reference
       count so that the object is shared between the caller and the object;
       the caller must free the returned attribute when finished.  Functions
       that involve attributes that have reference counts themselves, but are
       named with just "set" or "get" are historical; and the documentation
       must state how the references are handled.  Get methods are often
       thread-safe as long as the ownership requirements are met and shared
       objects are not modified.  Set methods, or modifying shared objects,
       are generally not thread-safe as discussed below.

       Objects are thread-safe as long as the API's being invoked don't modify
       the object; in this case the parameter is usually marked in the API as
       "const".  Not all parameters are marked this way.  Note that a "const"
       declaration does not mean immutable; for example X509_cmp(3) takes
       pointers to "const" objects, but the implementation uses a C cast to
       remove that so it can lock objects, generate and cache a DER encoding,
       and so on.

       Another instance of thread-safety is when updates to an object's
       internal state, such as cached values, are done with locks.  One
       example of this is the reference counting API's described above.

       In all cases, however, it is generally not safe for one thread to
       mutate an object, such as setting elements of a private or public key,
       while another thread is using that object, such as verifying a
       signature.

       The same API's can usually be used simultaneously on different objects
       without interference.  For example, two threads can calculate a
       signature using two different EVP_PKEY_CTX objects.

       For implicit global state or singletons, thread-safety depends on the
       facility.  The CRYPTO_secure_malloc(3) and related API's have their own
       lock, while CRYPTO_malloc(3) assumes the underlying platform allocation
       will do any necessary locking.  Some API's, such as NCONF_load(3) and
       related, or OBJ_create(3) do no locking at all; this can be considered
       a bug.

       A separate, although related, issue is modifying "factory" objects when
       other objects have been created from that.  For example, an SSL_CTX
       object created by SSL_CTX_new(3) is used to create per-connection SSL
       objects by calling SSL_new(3).  In this specific case, and probably for
       factory methods in general, it is not safe to modify the factory object
       after it has been used to create other objects.

SEE ALSO
       CRYPTO_THREAD_run_once(3), local system threads documentation.

BUGS
       This page is admittedly very incomplete.

COPYRIGHT
       Copyright 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                OPENSSL-THREADS(7)