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.

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.

Other areas of caution

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
    gethostby* functions and other system calls. These functions, provided by your operating system, must be thread safe. It is very important that libcurl can find and use thread safe versions of these and other system calls, as otherwise it can't 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 not thread safe. If 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.