1libcurl-thread(3) libcurl libcurl-thread(3)
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6 libcurl-thread - libcurl thread safety
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9 libcurl is thread safe but has no internal thread synchronization. You
10 may have to provide your own locking should you meet any of the thread
11 safety exceptions below.
12
15 You must never share the same handle in multiple threads. You can pass
16 the handles around among threads, but you must never use a single han‐
17 dle from more than one thread at any given time.
18
20 You can share certain data between multiple handles by using the share
21 interface but you must provide your own locking and set curl_share_se‐
22 topt(3) CURLSHOPT_LOCKFUNC and CURLSHOPT_UNLOCKFUNC.
23 Note that some items are specifically documented as not thread-safe in
25 the share API (the connection pool and HSTS cache for example).
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28 If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you
29 are then of course using the underlying SSL library multi-threaded and
30 those libs might have their own requirements on this issue. You may
31 need to provide one or two functions to allow it to function properly:
32 OpenSSL
34 OpenSSL 1.1.0+ "can be safely used in multi-threaded applica‐
35 tions provided that support for the underlying OS threading API
36 is built-in." In that case the engine is used by libcurl in a
37 way that is fully thread-safe.
38 https://www.openssl.org/docs/man1.1.0/man3/CRYPTO_THREAD_run_once.html#DE‐
40 SCRIPTION
41 OpenSSL <= 1.0.2 the user must set callbacks.
43 https://www.openssl.org/docs/man1.0.2/man3/CRYPTO_set_lock‐
45 ing_callback.html#DESCRIPTION
46 https://curl.se/libcurl/c/opensslthreadlock.html
48 GnuTLS https://gnutls.org/manual/html_node/Thread-safety.html
51 NSS thread-safe already without anything required.
53 Secure-Transport
55 The engine is used by libcurl in a way that is fully thread-
56 safe.
57 Schannel
59 The engine is used by libcurl in a way that is fully thread-
60 safe.
61 wolfSSL
63 The engine is used by libcurl in a way that is fully thread-
64 safe.
65 BoringSSL
67 The engine is used by libcurl in a way that is fully thread-
68 safe.
69 AWS-LC The engine is used by libcurl in a way that is fully thread-
71 safe.
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74 Signals are used for timing out name resolves (during DNS lookup) -
75 when built without using either the c-ares or threaded resolver back‐
76 ends. When using multiple threads you should set the CURLOPT_NOSIG‐
77 NAL(3) option to 1L for all handles. Everything will or might work fine
78 except that timeouts are not honored during the DNS lookup - which you
79 can work around by building libcurl with c-ares or threaded-resolver
80 support. c-ares is a library that provides asynchronous name resolves.
81 On some platforms, libcurl simply will not function properly multi-
82 threaded unless the CURLOPT_NOSIGNAL(3) option is set.
83 When CURLOPT_NOSIGNAL(3) is set to 1L, your application needs to deal
85 with the risk of a SIGPIPE (that at least the OpenSSL backend can trig‐
86 ger). Note that setting CURLOPT_NOSIGNAL(3) to 0L will not work in a
87 threaded situation as there will be race where libcurl risks restoring
88 the former signal handler while another thread should still ignore it.
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91 The gethostbyname or getaddrinfo and other name resolving system calls
92 used by libcurl are provided by your operating system and must be
93 thread safe. It is important that libcurl can find and use thread safe
94 versions of these and other system calls, as otherwise it cannot func‐
95 tion fully thread safe. Some operating systems are known to have faulty
96 thread implementations. We have previously received problem reports on
97 *BSD (at least in the past, they may be working fine these days). Some
98 operating systems that are known to have solid and working thread sup‐
99 port are Linux, Solaris and Windows.
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102 These functions are thread-safe since libcurl 7.84.0 if curl_ver‐
103 sion_info(3) has the CURL_VERSION_THREADSAFE feature bit set (most
104 platforms).
105 If these functions are not thread-safe and you are using libcurl with
107 multiple threads it is especially important that before use you call
108 curl_global_init(3) or curl_global_init_mem(3) to explicitly initialize
109 the library and its dependents, rather than rely on the "lazy" fail-
110 safe initialization that takes place the first time curl_easy_init(3)
111 is called. For an in-depth explanation refer to libcurl(3) section
112 GLOBAL CONSTANTS.
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115 These functions, provided either by your operating system or your own
116 replacements, must be thread safe. You can use curl_global_init_mem(3)
117 to set your own replacement memory functions.
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120 CURLOPT_DNS_USE_GLOBAL_CACHE(3) is not thread-safe.
121 curl_version_info(3) is not thread-safe before libcurl initialization.
123libcurl 8.2.1 April 26, 2023 libcurl-thread(3)