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.
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
OpenSSL 1.1.0 "can be safely used in multi-threaded applications
provided that support for the underlying OS threading API is
built-in."
https://www.openssl.org/docs/manmaster/crypto/threads.html#DESCRIPTION
OpenSSL <= 1.0.2 the user must set callbacks.
https://www.openssl.org/docs/man1.0.2/crypto/threads.html#DESCRIPTION
https://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.
BoringSSL
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
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.
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
functions.
Non-safe functions
CURLOPT_DNS_USE_GLOBAL_CACHE(3) is not thread-safe.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
