libcurl-thread(3)            libcurl thread safety           libcurl-thread(3)

NAME
       libcurl-thread - libcurl thread safety

Multi-threading with libcurl
       libcurl is thread safe but has no internal thread synchronization. You
       may have to provide your own locking should you meet any of the thread
       safety exceptions below.

       Handles. You must never share the same handle in multiple threads.  You
       can pass the handles around among threads, but you must never use a
       single handle from more than one thread at any given time.

       Shared objects. You can share certain data between multiple handles by
       using the share interface but you must provide your own locking and set
       _share_setopt&section=3">curl_share_setopt(3) CURLSHOPT_LOCKFUNC and CURLSHOPT_UNLOCKFUNC.

TLS
       If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you
       are then of course using the underlying SSL library multi-threaded and
       those libs might have their own requirements on this issue.  You may
       need to provide one or two functions to allow it to function properly:

       OpenSSL
              OpenSSL 1.1.0+ "can be safely used in multi-threaded
              applications provided that support for the underlying OS
              threading API is built-in." In that case the engine is used by
              libcurl in a way that is fully thread-safe.

              https://www.openssl.org/docs/man1.1.0/man3/CRYPTO_THREAD_run_once.html#DESCRIPTION

              OpenSSL <= 1.0.2 the user must set callbacks.

              https://www.openssl.org/docs/man1.0.2/man3/CRYPTO_set_locking_callback.html#DESCRIPTION

              https://curl.se/libcurl/c/opensslthreadlock.html

       GnuTLS https://gnutls.org/manual/html_node/Thread-safety.html

       NSS    thread-safe already without anything required.

       Secure-Transport
              The engine is used by libcurl in a way that is fully thread-
              safe.

       Schannel
              The engine is used by libcurl in a way that is fully thread-
              safe.

       wolfSSL
              The engine is used by libcurl in a way that is fully thread-
              safe.

       BoringSSL
              The engine is used by libcurl in a way that is fully thread-
              safe.

Other areas of caution
       Signals
              Signals are used for timing out name resolves (during DNS
              lookup) - when built without using either the c-ares or threaded
              resolver backends. When using multiple threads you should set
              the _NOSIGNAL&section=3">CURLOPT_NOSIGNAL(3) option to 1L for all handles. Everything
              will or might work fine except that timeouts are not honored
              during the DNS lookup - which you can work around by building
              libcurl with c-ares or threaded-resolver support. c-ares is a
              library that provides asynchronous name resolves. On some
              platforms, libcurl simply will not function properly multi-
              threaded unless the _NOSIGNAL&section=3">CURLOPT_NOSIGNAL(3) option is set.

              When _NOSIGNAL&section=3">CURLOPT_NOSIGNAL(3) is set to 1L, your application needs to
              deal with the risk of a SIGPIPE (that at least the OpenSSL
              backend can trigger). Note that setting _NOSIGNAL&section=3">CURLOPT_NOSIGNAL(3) to
              0L will not work in a threaded situation as there will be race
              where libcurl risks restoring the former signal handler while
              another thread should still ignore it.

       Name resolving
              gethostby* functions and other system calls. These functions,
              provided by your operating system, must be thread safe. It is
              very important that libcurl can find and use thread safe
              versions of these and other system calls, as otherwise it can't
              function fully thread safe. Some operating systems are known to
              have faulty thread implementations. We have previously received
              problem reports on *BSD (at least in the past, they may be
              working fine these days).  Some operating systems that are known
              to have solid and working thread support are Linux, Solaris and
              Windows.

       curl_global_* functions
              These functions are not thread safe. If you are using libcurl
              with multiple threads it is especially important that before use
              you call _global_init&section=3">curl_global_init(3) or _global_init_mem&section=3">curl_global_init_mem(3) to
              explicitly initialize the library and its dependents, rather
              than rely on the "lazy" fail-safe initialization that takes
              place the first time _easy_init&section=3">curl_easy_init(3) is called. For an in-
              depth explanation refer to libcurl(3) section GLOBAL CONSTANTS.

       Memory functions
              These functions, provided either by your operating system or
              your own replacements, must be thread safe. You can use
              _global_init_mem&section=3">curl_global_init_mem(3) to set your own replacement memory
              functions.

       Non-safe functions
              _DNS_USE_GLOBAL_CACHE&section=3">CURLOPT_DNS_USE_GLOBAL_CACHE(3) is not thread-safe.

libcurl 7.77.0                 November 4, 2020              libcurl-thread(3)