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  not  thread  safe.  If  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.

libcurl 7.81.0                                  November 26, 2021                              libcurl-thread(3)