1--- 2c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al. 3SPDX-License-Identifier: curl 4Title: libcurl-thread 5Section: 3 6Source: libcurl 7See-also: 8 - libcurl-security (3) 9Protocol: 10 - All 11--- 12 13# NAME 14 15libcurl-thread - libcurl thread safety 16 17# Multi-threading with libcurl 18 19libcurl is thread safe but has no internal thread synchronization. You may have 20to provide your own locking should you meet any of the thread safety exceptions 21below. 22 23# Handles 24 25You must **never** share the same handle in multiple threads. You can pass the 26handles around among threads, but you must never use a single handle from more 27than one thread at any given time. 28 29# Shared objects 30 31You can share certain data between multiple handles by using the share 32interface but you must provide your own locking and set 33curl_share_setopt(3) CURLSHOPT_LOCKFUNC and CURLSHOPT_UNLOCKFUNC. 34 35Note that some items are specifically documented as not thread-safe in the 36share API (the connection pool and HSTS cache for example). 37 38# TLS 39 40All current TLS libraries libcurl supports are thread-safe. 41 42## OpenSSL 43 44OpenSSL 1.1.0+ can be safely used in multi-threaded applications provided that 45support for the underlying OS threading API is built-in. For older versions of 46OpenSSL, the user must set mutex callbacks. 47 48libcurl may not be able to fully clean up after multi-threaded OpenSSL 49depending on how OpenSSL was built and loaded as a library. It is possible in 50some rare circumstances a memory leak could occur unless you implement your own 51OpenSSL thread cleanup. 52 53For example, on Windows if both libcurl and OpenSSL are linked statically to a 54DLL or application then OpenSSL may leak memory unless the DLL or application 55calls OPENSSL_thread_stop() before each thread terminates. If OpenSSL is built 56as a DLL then it does this cleanup automatically and there is no leak. If 57libcurl is built as a DLL and OpenSSL is linked statically to it then libcurl 58does this cleanup automatically and there is no leak (added in libcurl 8.8.0). 59 60Please review the OpenSSL documentation for a full list of circumstances: 61https://www.openssl.org/docs/man3.0/man3/OPENSSL_thread_stop.html#NOTES 62 63# Signals 64 65Signals are used for timing out name resolves (during DNS lookup) - when built 66without using either the c-ares or threaded resolver backends. On systems that 67have a signal concept. 68 69When using multiple threads you should set the CURLOPT_NOSIGNAL(3) 70option to 1L for all handles. Everything works fine except that timeouts 71cannot be honored during DNS lookups - which you can work around by building 72libcurl with c-ares or threaded-resolver support. c-ares is a library that 73provides asynchronous name resolves. On some platforms, libcurl simply cannot 74function properly multi-threaded unless the CURLOPT_NOSIGNAL(3) option 75is set. 76 77When CURLOPT_NOSIGNAL(3) is set to 1L, your application needs to deal 78with the risk of a SIGPIPE (that at least the OpenSSL backend can 79trigger). Note that setting CURLOPT_NOSIGNAL(3) to 0L does not work in a 80threaded situation as there is a race condition where libcurl risks restoring 81the former signal handler while another thread should still ignore it. 82 83# Name resolving 84 85The **gethostbyname** or **getaddrinfo** and other name resolving system 86calls used by libcurl are provided by your operating system and must be thread 87safe. It is important that libcurl can find and use thread safe versions of 88these and other system calls, as otherwise it cannot function fully thread 89safe. Some operating systems are known to have faulty thread 90implementations. We have previously received problem reports on *BSD (at least 91in the past, they may be working fine these days). Some operating systems that 92are known to have solid and working thread support are Linux, Solaris and 93Windows. 94 95# curl_global_* functions 96 97These functions are thread-safe since libcurl 7.84.0 if 98curl_version_info(3) has the **CURL_VERSION_THREADSAFE** feature bit 99set (most platforms). 100 101If these functions are not thread-safe and you are using libcurl with multiple 102threads it is especially important that before use you call 103curl_global_init(3) or curl_global_init_mem(3) to explicitly 104initialize the library and its dependents, rather than rely on the "lazy" 105fail-safe initialization that takes place the first time 106curl_easy_init(3) is called. For an in-depth explanation refer to 107libcurl(3) section **GLOBAL CONSTANTS**. 108 109# Memory functions 110 111These functions, provided either by your operating system or your own 112replacements, must be thread safe. You can use curl_global_init_mem(3) 113to set your own replacement memory functions. 114 115# Non-safe functions 116 117CURLOPT_DNS_USE_GLOBAL_CACHE(3) is not thread-safe. 118 119curl_version_info(3) is not thread-safe before libcurl initialization. 120