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libcurl-thread(3)                    libcurl                   libcurl-thread(3)


       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.


       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).


       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 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.


              OpenSSL <= 1.0.2 the user must set callbacks.




       NSS    thread-safe already without anything required.

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

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

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

              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 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

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 8.1.0                    April 26, 2023                libcurl-thread(3)

curl 8.1.0 - Generated Thu May 18 07:27:26 CDT 2023
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