curl/docs/libcurl/libcurl-thread.3

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.TH libcurl-thread 3 "13 Jul 2015" "libcurl" "libcurl thread safety"
.SH NAME
libcurl-thread \- libcurl thread safety
.SH "Multi-threading Issues"
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.

\fBHandles.\fP You must \fBnever\fP 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.

\fBShared objects.\fP You can share certain data between multiple handles by
using the share interface but you must provide your own locking and set
\fIcurl_share_setopt(3)\fP CURLSHOPT_LOCKFUNC and CURLSHOPT_UNLOCKFUNC.

\fBSSL/TLS handlers.\fP 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:

.RS
.IP OpenSSL

 http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION

 http://curl.haxx.se/libcurl/c/opensslthreadlock.html

.IP GnuTLS

 http://gnutls.org/manual/html_node/Thread-safety.html

.IP NSS

 is claimed to be thread-safe already without anything required.

.IP PolarSSL

 Required actions unknown.

.IP yassl

 Required actions unknown.

.IP axTLS

 Required actions unknown.

.IP Secure-Transport

 The engine is used by libcurl in a way that is fully thread-safe.

.IP WinSSL

 The engine is used by libcurl in a way that is fully thread-safe.

.IP wolfSSL

 The engine is used by libcurl in a way that is fully thread-safe.
.RE

\fBSignals.\fP 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
\fICURLOPT_NOSIGNAL(3)\fP 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.

\fBgethostby* functions and other system calls.\fP These functions, provided by
your operating system, must be thread safe. It is very important that libcurl
can find and use thread safe versions 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 working fine these days).
Some operating systems that are known to have solid and working thread support
are Linux, Solaris and Windows.

\fBcurl_global_* functions.\fP These functions are not thread safe. If you are
using libcurl with multiple threads it is especially important that before use
you call \fIcurl_global_init(3)\fP to explicitly initialize the library and its
dependents, rather than rely on the "lazy" fail-safe initialization that takes
place the first time \fIcurl_easy_init(3)\fP is called. For an in-depth
explanation refer to \fIlibcurl(3)\fP section \fBGLOBAL CONSTANTS\fP.

\fICURLOPT_DNS_USE_GLOBAL_CACHE(3)\fP is not thread-safe.