libcurl-thread.md (4750B)
1 --- 2 c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al. 3 SPDX-License-Identifier: curl 4 Title: libcurl-thread 5 Section: 3 6 Source: libcurl 7 See-also: 8 - libcurl-security (3) 9 Protocol: 10 - All 11 Added-in: n/a 12 --- 13 14 # NAME 15 16 libcurl-thread - libcurl thread safety 17 18 # Multi-threading with libcurl 19 20 libcurl is thread safe but has no internal thread synchronization. You may have 21 to provide your own locking should you meet any of the thread safety exceptions 22 below. 23 24 # Handles 25 26 You must **never** share the same handle in multiple threads. You can pass the 27 handles around among threads, but you must never use a single handle from more 28 than one thread at any given time. 29 30 # Shared objects 31 32 You can share certain data between multiple handles by using the share 33 interface but you must provide your own locking and set 34 curl_share_setopt(3) CURLSHOPT_LOCKFUNC and CURLSHOPT_UNLOCKFUNC. 35 36 Note that some items are specifically documented as not thread-safe in the 37 share API (the connection pool and HSTS cache for example). 38 39 # TLS 40 41 All current TLS libraries libcurl supports are thread-safe. 42 43 ## OpenSSL 44 45 OpenSSL 1.1.0+ can be safely used in multi-threaded applications provided that 46 support for the underlying OS threading API is built-in. For older versions of 47 OpenSSL, the user must set mutex callbacks. 48 49 libcurl may not be able to fully clean up after multi-threaded OpenSSL 50 depending on how OpenSSL was built and loaded as a library. It is possible in 51 some rare circumstances a memory leak could occur unless you implement your own 52 OpenSSL thread cleanup. 53 54 For example, on Windows if both libcurl and OpenSSL are linked statically to a 55 DLL or application then OpenSSL may leak memory unless the DLL or application 56 calls OPENSSL_thread_stop() before each thread terminates. If OpenSSL is built 57 as a DLL then it does this cleanup automatically and there is no leak. If 58 libcurl is built as a DLL and OpenSSL is linked statically to it then libcurl 59 does this cleanup automatically and there is no leak (added in libcurl 8.8.0). 60 61 Please review the OpenSSL documentation for a full list of circumstances: 62 https://docs.openssl.org/3.0/man3/OPENSSL_init_crypto/#notes 63 64 # Signals 65 66 Signals are used for timing out name resolves (during DNS lookup) - when built 67 without using either the c-ares or threaded resolver backends. On systems that 68 have a signal concept. 69 70 When using multiple threads you should set the CURLOPT_NOSIGNAL(3) 71 option to 1L for all handles. Everything works fine except that timeouts 72 cannot be honored during DNS lookups - which you can work around by building 73 libcurl with c-ares or threaded-resolver support. c-ares is a library that 74 provides asynchronous name resolves. On some platforms, libcurl simply cannot 75 function properly multi-threaded unless the CURLOPT_NOSIGNAL(3) option 76 is set. 77 78 When CURLOPT_NOSIGNAL(3) is set to 1L, your application needs to deal 79 with the risk of a SIGPIPE (that at least the OpenSSL backend can 80 trigger). Note that setting CURLOPT_NOSIGNAL(3) to 0L does not work in a 81 threaded situation as there is a race condition where libcurl risks restoring 82 the former signal handler while another thread should still ignore it. 83 84 # Name resolving 85 86 The **gethostbyname** or **getaddrinfo** and other name resolving system 87 calls used by libcurl are provided by your operating system and must be thread 88 safe. It is important that libcurl can find and use thread safe versions of 89 these and other system calls, as otherwise it cannot function fully thread 90 safe. Some operating systems are known to have faulty thread 91 implementations. We have previously received problem reports on *BSD (at least 92 in the past, they may be working fine these days). Some operating systems that 93 are known to have solid and working thread support are Linux, Solaris and 94 Windows. 95 96 # curl_global_* functions 97 98 These functions are thread-safe since libcurl 7.84.0 if 99 curl_version_info(3) has the **CURL_VERSION_THREADSAFE** feature bit 100 set (most platforms). 101 102 If these functions are not thread-safe and you are using libcurl with multiple 103 threads it is especially important that before use you call 104 curl_global_init(3) or curl_global_init_mem(3) to explicitly 105 initialize the library and its dependents, rather than rely on the "lazy" 106 fail-safe initialization that takes place the first time 107 curl_easy_init(3) is called. For an in-depth explanation refer to 108 libcurl(3) section **GLOBAL CONSTANTS**. 109 110 # Memory functions 111 112 These functions, provided either by your operating system or your own 113 replacements, must be thread safe. You can use curl_global_init_mem(3) 114 to set your own replacement memory functions. 115 116 # Non-safe functions 117 118 CURLOPT_DNS_USE_GLOBAL_CACHE(3) is not thread-safe. 119 120 curl_version_info(3) is not thread-safe before libcurl initialization.