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_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).
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
library. 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
circumstances:
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)
option 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 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 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 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.
SEE ALSO
libcurl-security(3)
libcurl 2024-08-05 libcurl-thread(3)
curl 8.9.1 - Generated Wed Aug 7 10:25:38 CDT 2024