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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
       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
              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
              CURLOPT_NOSIGNAL(3) option is set.

              When 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 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
              important that libcurl can find and use thread safe versions of
              these and other system calls, as otherwise it cannot 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 thread-safe since libcurl 7.84.0 if
              curl_version_info(3) has the CURL_VERSION_THREADSAFE feature bit
              set (most platforms).

              If these functions are not thread-safe and you are using libcurl
              with multiple threads it is especially important that before use
              you call curl_global_init(3) or 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 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
              curl_global_init_mem(3) to set your own replacement memory
              functions.

       Non-safe functions
              CURLOPT_DNS_USE_GLOBAL_CACHE(3) is not thread-safe.

              curl_version_info(3) is not thread-safe before libcurl
              initialization.



libcurl 7.87.0                 September 25, 2022              libcurl-thread(3)

curl 7.87.0 - Generated Thu Jan 12 15:23:52 CST 2023
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