OPENSSL-THREADS(7ossl)

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version 15.1 · 3 versions
, in freebsd 15.1

okay bestie, welcome to openssl-threads from the 7ossl manual — this is the thread-safety tea for OpenSSL, and we're gonna make it make sense 💅🦋

✨ NAME ✨

openssl-threads — overview of thread safety in OpenSSL

💖 DESCRIPTION 💖

In this man page, “thread-safe” means an object or function can be used by multiple threads at the same time. Periodt. ✨

OpenSSL can be built with or without threads support. The most important use of this support is so that OpenSSL itself can use one single consistent API, exactly like the EXAMPLES section in CRYPTO_THREAD_run_once(3) shows. Multi-platform applications can also use this API. 🌈

In particular, being configured for threads support does not mean every OpenSSL object is 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 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. Here's the ownership tea:

  • set0 passes ownership from the caller to the object.
  • get0 returns a pointer, but the attribute ownership remains with the object, and a reference to it is returned.
  • set1 or get1 does not change the ownership; instead it 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; 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. 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, 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 👀

🐞 BUGS 🐞

This page is admittedly very incomplete. She said what she said. 😭

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>.

okay that's the whole vibe! This page is from OpenSSL 3.5.6, dated 2026-04-07, and it lives under OPENSSL-THREADS(7ossl) — bye bestie, stay thread-safe-ish 💖✨

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