libcurl-thread(3) libcurl thread safety 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 applica-
tions 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#DE-
SCRIPTION
OpenSSL <= 1.0.2 the user must set callbacks.
https://www.openssl.org/docs/man1.0.2/man3/CRYPTO_set_lock-
ing_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 plat-
forms, 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 back-
end 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
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 prob-
lem 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 ex-
plicitly 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 explana-
tion 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 func-
tions.
Non-safe functions
CURLOPT_DNS_USE_GLOBAL_CACHE(3) is not thread-safe.
libcurl 7.81.0 November 26, 2021 libcurl-thread(3)
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