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