libcurl-thread(3) Library Functions Manual 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_se-
topt(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
All current TLS libraries libcurl supports are thread-safe.
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. For older versions of OpenSSL, the user must set mutex
callbacks.
libcurl may not be able to fully clean up after multi-threaded
OpenSSL depending on how OpenSSL was built and loaded as a li-
brary. It is possible in some rare circumstances a memory leak
could occur unless you implement your own OpenSSL thread cleanup.
For example, on Windows if both libcurl and OpenSSL are linked
statically to a DLL or application then OpenSSL may leak memory
unless the DLL or application calls OPENSSL_thread_stop() before
each thread terminates. If OpenSSL is built as a DLL then it does
this cleanup automatically and there is no leak. If libcurl is
built as a DLL and OpenSSL is linked statically to it then
libcurl does this cleanup automatically and there is no leak
(added in libcurl 8.8.0).
Please review the OpenSSL documentation for a full list of cir-
cumstances:
https://docs.openssl.org/3.0/man3/OPENSSL_init_crypto/#notes
Signals
Signals are used for timing out name resolves (during DNS lookup) - when
built without using either the c-ares or threaded resolver backends. On
systems that have a signal concept.
When using multiple threads you should set the CURLOPT_NOSIGNAL(3) op-
tion to 1L for all handles. Everything works fine except that timeouts
cannot be honored during DNS lookups - 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 cannot function properly multi-threaded unless the CUR-
LOPT_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 trig-
ger). Note that setting CURLOPT_NOSIGNAL(3) to 0L does not work in a
threaded situation as there is a race condition 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 im-
plementations. 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_ver-
sion_info(3) has the CURL_VERSION_THREADSAFE feature bit set (most plat-
forms).
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.
SEE ALSO
libcurl-security(3)
libcurl 2026-07-12 libcurl-thread(3)
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