Tribblix: manual page: libcurl-thread.3

libcurl-thread(3) Introduction to Library Functions 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.

NAME

TLS

SEE ALSO