libcurl-thread(3) libcurl thread safety libcurl-thread(3)NAMElibcurl-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
http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION
http://curl.haxx.se/libcurl/c/opensslthreadlock.html
GnuTLS http://gnutls.org/manual/html_node/Thread-safety.html
NSS thread-safe already without anything required.
PolarSSL
Required actions unknown.
yassl Required actions unknown.
axTLS Required actions unknown.
Secure-Transport
The engine is used by libcurl in a way that is fully thread-
safe.
WinSSL 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.
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 support. c-ares is a library that provides
asynchronous name resolves. On some platforms, libcurl simply
will not function properly multi-threaded unless this option is
set.
Name resolving
gethostby* functions and other system calls. These functions,
provided by your operating system, must be thread safe. It is
very important that libcurl can find and use thread safe ver‐
sions of these and other system calls, as otherwise it can't
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 work‐
ing 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
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 func‐
tions.
Non-safe
CURLOPT_DNS_USE_GLOBAL_CACHE(3) is not thread-safe.
libcurl 13 Jul 2015 libcurl-thread(3)
