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.

libcurl 8.5.0                                   October 30, 2023                               libcurl-thread(3)