oracular (3) libcurl-thread.3.gz

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
              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.

              libcurl  may  not  be able to fully clean up after multi-threaded OpenSSL depending on how OpenSSL
              was built and loaded as a library. It is possible in some rare circumstances a memory  leak  could
              occur unless you implement your own OpenSSL thread cleanup.

              For  example, on Windows if both libcurl and OpenSSL are linked statically to a DLL or application
              then OpenSSL may leak memory unless the DLL or application calls OPENSSL_thread_stop() before each
              thread  terminates. If OpenSSL is built as a DLL then it does this cleanup automatically and there
              is no leak. If libcurl is built as a DLL and OpenSSL is linked statically to it then libcurl  does
              this cleanup automatically and there is no leak (added in libcurl 8.8.0).

              Please    review    the    OpenSSL    documentation    for   a   full   list   of   circumstances:
              https://docs.openssl.org/3.0/man3/OPENSSL_init_crypto/#notes

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.

SEE ALSO

       libcurl-security(3)