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        !            22: .\"
        !            23: .TH libcurl-thread 3 "June 30, 2019" "libcurl 7.70.0" "libcurl thread safety"
        !            24: 
        !            25: .SH NAME
        !            26: libcurl-thread \- libcurl thread safety
        !            27: .SH "Multi-threading with libcurl"
        !            28: libcurl is thread safe but has no internal thread synchronization. You may have
        !            29: to provide your own locking should you meet any of the thread safety exceptions
        !            30: below.
        !            31: 
        !            32: \fBHandles.\fP You must \fBnever\fP share the same handle in multiple threads.
        !            33: You can pass the handles around among threads, but you must never use a single
        !            34: handle from more than one thread at any given time.
        !            35: 
        !            36: \fBShared objects.\fP You can share certain data between multiple handles by
        !            37: using the share interface but you must provide your own locking and set
        !            38: \fIcurl_share_setopt(3)\fP CURLSHOPT_LOCKFUNC and CURLSHOPT_UNLOCKFUNC.
        !            39: .SH TLS
        !            40: If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you are
        !            41: then of course using the underlying SSL library multi-threaded and those libs
        !            42: might have their own requirements on this issue.  You may need to provide one
        !            43: or two functions to allow it to function properly:
        !            44: .IP OpenSSL
        !            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." In that case the
        !            47: engine is used by libcurl in a way that is fully thread-safe.
        !            48: 
        !            49: https://www.openssl.org/docs/man1.1.0/man3/CRYPTO_THREAD_run_once.html#DESCRIPTION
        !            50: 
        !            51: OpenSSL <= 1.0.2 the user must set callbacks.
        !            52: 
        !            53: https://www.openssl.org/docs/man1.0.2/man3/CRYPTO_set_locking_callback.html#DESCRIPTION
        !            54: 
        !            55: https://curl.haxx.se/libcurl/c/opensslthreadlock.html
        !            56: 
        !            57: .IP GnuTLS
        !            58: https://gnutls.org/manual/html_node/Thread-safety.html
        !            59: .IP NSS
        !            60: thread-safe already without anything required.
        !            61: .IP Secure-Transport
        !            62: The engine is used by libcurl in a way that is fully thread-safe.
        !            63: .IP WinSSL
        !            64: The engine is used by libcurl in a way that is fully thread-safe.
        !            65: .IP wolfSSL
        !            66: The engine is used by libcurl in a way that is fully thread-safe.
        !            67: .IP BoringSSL
        !            68: The engine is used by libcurl in a way that is fully thread-safe.
        !            69: .SH "Other areas of caution"
        !            70: .IP Signals
        !            71: Signals are used for timing out name resolves (during DNS lookup) - when built
        !            72: without using either the c-ares or threaded resolver backends. When using
        !            73: multiple threads you should set the \fICURLOPT_NOSIGNAL(3)\fP option to 1L for
        !            74: all handles. Everything will or might work fine except that timeouts are not
        !            75: honored during the DNS lookup - which you can work around by building libcurl
        !            76: with c-ares or threaded-resolver support. c-ares is a library that provides
        !            77: asynchronous name resolves. On some platforms, libcurl simply will not
        !            78: function properly multi-threaded unless the \fICURLOPT_NOSIGNAL(3)\fP option
        !            79: is set.
        !            80: 
        !            81: When \fICURLOPT_NOSIGNAL(3)\fP is set to 1L, your application needs to deal
        !            82: with the risk of a SIGPIPE (that at least the OpenSSL backend can
        !            83: trigger). Note that setting \fICURLOPT_NOSIGNAL(3)\fP to 0L will not work in a
        !            84: threaded situation as there will be race where libcurl risks restoring the
        !            85: former signal handler while another thread should still ignore it.
        !            86: .IP "Name resolving"
        !            87: \fBgethostby* functions and other system calls.\fP These functions, provided
        !            88: by your operating system, must be thread safe. It is very important that
        !            89: libcurl can find and use thread safe versions of these and other system calls,
        !            90: as otherwise it can't function fully thread safe. Some operating systems are
        !            91: known to have faulty thread implementations. We have previously received
        !            92: problem reports on *BSD (at least in the past, they may be working fine these
        !            93: days).  Some operating systems that are known to have solid and working thread
        !            94: support are Linux, Solaris and Windows.
        !            95: .IP "curl_global_* functions"
        !            96: These functions are not thread safe. If you are using libcurl with multiple
        !            97: threads it is especially important that before use you call
        !            98: \fIcurl_global_init(3)\fP or \fIcurl_global_init_mem(3)\fP to explicitly
        !            99: initialize the library and its dependents, rather than rely on the "lazy"
        !           100: fail-safe initialization that takes place the first time
        !           101: \fIcurl_easy_init(3)\fP is called. For an in-depth explanation refer to
        !           102: \fIlibcurl(3)\fP section \fBGLOBAL CONSTANTS\fP.
        !           103: .IP "Memory functions"
        !           104: These functions, provided either by your operating system or your own
        !           105: replacements, must be thread safe. You can use \fIcurl_global_init_mem(3)\fP
        !           106: to set your own replacement memory functions.
        !           107: .IP "Non-safe functions"
        !           108: \fICURLOPT_DNS_USE_GLOBAL_CACHE(3)\fP is not thread-safe.