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."

              https://www.openssl.org/docs/manmaster/crypto/threads.html#DESCRIPTION

              OpenSSL <= 1.0.2 the user must set callbacks.

              https://www.openssl.org/docs/man1.0.2/crypto/threads.html#DESCRIPTION

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

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

       NSS    thread-safe already without anything required.

       PolarSSL
              Required actions unknown.

       yassl  Required actions unknown.

       axTLS  Required actions unknown.

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

       WinSSL 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.

       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 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 7.58.0                                   August 08, 2017                               libcurl-thread(3)