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
       object_up_ref() function that can be used to increase the reference count.  Failure to
       match object_up_ref() calls with the right number of object_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>.