libcurl-thread - libcurl thread safety
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.
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.
- 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 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.