DragonFly On-Line Manual Pages

libcurl-thread(3)                   libcurl                  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.

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

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." In that case the engine is used by
              libcurl in a way that is fully thread-safe.

              https://www.openssl.org/docs/man1.1.0/man3/CRYPTO_THREAD_run_once.html#DESCRIPTION

              OpenSSL <= 1.0.2 the user must set callbacks.

              https://www.openssl.org/docs/man1.0.2/man3/CRYPTO_set_locking_callback.html#DESCRIPTION

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

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

       NSS    thread-safe already without anything required.

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

       Schannel
              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.

       AWS-LC The engine is used by libcurl in a way that is fully thread-
              safe.

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.

       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 will not work in
       a threaded situation as there will be race 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 8.1.2                   April 26, 2023               libcurl-thread(3)