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.

       Note that some items are specifically documented as not thread-safe in the share API  (the
       connection pool and HSTS cache for example).

TLS

       All  current  TLS libraries libcurl supports are thread-safe. OpenSSL 1.1.0+ can be safely
       used in multi-threaded applications provided that support for the underlying OS  threading
       API is built-in. For older versions of OpenSSL, the user must set mutex callbacks.

Signals

       Signals  are  used  for  timing out name resolves (during DNS lookup) - when built without
       using either the c-ares or threaded resolver backends.  On  systems  that  have  a  signal
       concept.

       When  using  multiple  threads you should set the CURLOPT_NOSIGNAL(3) option to 1L for all
       handles. Everything works fine except that timeouts cannot be honored during DNS lookups -
       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  cannot  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 does not work in  a  threaded  situation  as  there  is  a  race
       condition  where  libcurl  risks  restoring the former signal handler while another thread
       should still ignore it.

Name resolving

       The gethostbyname or getaddrinfo and other name resolving system calls used by libcurl are
       provided  by  your  operating system and 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.