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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."

              https://www.openssl.org/docs/manmas-
              ter/crypto/threads.html#DESCRIPTION

              OpenSSL <= 1.0.2 the user must set callbacks.

              https://www.openssl.org/docs/man1.0.2/crypto/threads.html#DESCRIP-
              TION

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


       GnuTLS http://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 support. c-ares is a library  that  provides
              asynchronous  name  resolves.  On some platforms, libcurl simply
              will not function properly multi-threaded unless this 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 ver-
              sions 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 work-
              ing 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 func-
              tions.

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



libcurl                           13 Jul 2015                libcurl-thread(3)

Mac OS X 10.12.3 - Generated Wed Feb 8 05:57:01 CST 2017
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