docs/libcurl/libcurl-thread.3 - third_party/curl - Git at Google

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 .\" * Copyright (C) 2015 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
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 .\" * KIND, either express or implied.
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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 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.

 \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.
 .SH 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:
 .IP 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." In that case the
 engine is used by libcurl in a way that is fully thread-safe.

 https://www.openssl.org/docs/man1.1.0/man3/CRYPTO_THREAD_run_once.html#DESCRIPTION

 OpenSSL <= 1.0.2 the user must set callbacks.

 https://www.openssl.org/docs/man1.0.2/man3/CRYPTO_set_locking_callback.html#DESCRIPTION

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

 .IP GnuTLS
 https://gnutls.org/manual/html_node/Thread-safety.html
 .IP NSS
 thread-safe already without anything required.
 .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.
 .IP BoringSSL
 The engine is used by libcurl in a way that is fully thread-safe.
 .SH "Other areas of caution"
 .IP 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 \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 or threaded-resolver support. c-ares is a library that provides
 asynchronous name resolves. On some platforms, libcurl simply will not
 function properly multi-threaded unless the \fICURLOPT_NOSIGNAL(3)\fP option
 is set.

 When \fICURLOPT_NOSIGNAL(3)\fP 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 \fICURLOPT_NOSIGNAL(3)\fP to 0L will not work in a
 threaded situation as there will be race where libcurl risks restoring the
 former signal handler while another thread should still ignore it.
 .IP "Name resolving"
 \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.
 .IP "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
 \fIcurl_global_init(3)\fP or \fIcurl_global_init_mem(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.
 .IP "Memory functions"
 These functions, provided either by your operating system or your own
 replacements, must be thread safe. You can use \fIcurl_global_init_mem(3)\fP
 to set your own replacement memory functions.
 .IP "Non-safe functions"
 \fICURLOPT_DNS_USE_GLOBAL_CACHE(3)\fP is not thread-safe.
	.\" **************************************************************************
	.\" * _ _ ____ _
	.\" * Project ___\| \| \| \| _ \\| \|
	.\" * / __\| \| \| \| \|_) \| \|
	.\" * \| (__\| \|_\| \| _ <\| \|___
	.\" * \___\|\___/\|_\| \_\_____\|
	.\" *
	.\" * Copyright (C) 2015 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
	.\" *
	.\" * This software is licensed as described in the file COPYING, which
	.\" * you should have received as part of this distribution. The terms
	.\" * are also available at https://curl.haxx.se/docs/copyright.html.
	.\" *
	.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
	.\" * copies of the Software, and permit persons to whom the Software is
	.\" * furnished to do so, under the terms of the COPYING file.
	.\" *
	.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
	.\" * KIND, either express or implied.
	.\" *
	.\" **************************************************************************
	.\"
	.TH libcurl-thread 3 "13 Jul 2015" "libcurl" "libcurl thread safety"
	.SH NAME
	libcurl-thread \- libcurl thread safety
	.SH "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.

	\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.
	.SH 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:
	.IP 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." In that case the
	engine is used by libcurl in a way that is fully thread-safe.

	https://www.openssl.org/docs/man1.1.0/man3/CRYPTO_THREAD_run_once.html#DESCRIPTION

	OpenSSL <= 1.0.2 the user must set callbacks.

	https://www.openssl.org/docs/man1.0.2/man3/CRYPTO_set_locking_callback.html#DESCRIPTION

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

	.IP GnuTLS
	https://gnutls.org/manual/html_node/Thread-safety.html
	.IP NSS
	thread-safe already without anything required.
	.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.
	.IP BoringSSL
	The engine is used by libcurl in a way that is fully thread-safe.
	.SH "Other areas of caution"
	.IP 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 \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 or threaded-resolver support. c-ares is a library that provides
	asynchronous name resolves. On some platforms, libcurl simply will not
	function properly multi-threaded unless the \fICURLOPT_NOSIGNAL(3)\fP option
	is set.

	When \fICURLOPT_NOSIGNAL(3)\fP 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 \fICURLOPT_NOSIGNAL(3)\fP to 0L will not work in a
	threaded situation as there will be race where libcurl risks restoring the
	former signal handler while another thread should still ignore it.
	.IP "Name resolving"
	\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.
	.IP "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
	\fIcurl_global_init(3)\fP or \fIcurl_global_init_mem(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.
	.IP "Memory functions"
	These functions, provided either by your operating system or your own
	replacements, must be thread safe. You can use \fIcurl_global_init_mem(3)\fP
	to set your own replacement memory functions.
	.IP "Non-safe functions"
	\fICURLOPT_DNS_USE_GLOBAL_CACHE(3)\fP is not thread-safe.