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114 lines
5.5 KiB
Groff
114 lines
5.5 KiB
Groff
.\" **************************************************************************
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.\" * _ _ ____ _
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.\" * Project ___| | | | _ \| |
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.\" * / __| | | | |_) | |
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.\" * | (__| |_| | _ <| |___
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.\" * \___|\___/|_| \_\_____|
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.\" *
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.\" * Copyright (C) 2015 - 2022, Daniel Stenberg, <daniel@haxx.se>, et al.
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.\" *
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.\" * This software is licensed as described in the file COPYING, which
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.\" * you should have received as part of this distribution. The terms
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.\" * are also available at https://curl.se/docs/copyright.html.
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.\" *
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.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
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.\" * copies of the Software, and permit persons to whom the Software is
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.\" * furnished to do so, under the terms of the COPYING file.
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.\" *
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.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
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.\" * KIND, either express or implied.
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.\" *
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.\" * SPDX-License-Identifier: curl
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.\" *
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.\" **************************************************************************
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.\"
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.TH libcurl-thread 3 "13 Jul 2015" "libcurl" "libcurl thread safety"
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.SH NAME
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libcurl-thread \- libcurl thread safety
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.SH "Multi-threading with libcurl"
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libcurl is thread safe but has no internal thread synchronization. You may have
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to provide your own locking should you meet any of the thread safety exceptions
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below.
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\fBHandles.\fP You must \fBnever\fP share the same handle in multiple threads.
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You can pass the handles around among threads, but you must never use a single
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handle from more than one thread at any given time.
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\fBShared objects.\fP You can share certain data between multiple handles by
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using the share interface but you must provide your own locking and set
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\fIcurl_share_setopt(3)\fP CURLSHOPT_LOCKFUNC and CURLSHOPT_UNLOCKFUNC.
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.SH TLS
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If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you are
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then of course using the underlying SSL library multi-threaded and those libs
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might have their own requirements on this issue. You may need to provide one
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or two functions to allow it to function properly:
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.IP OpenSSL
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OpenSSL 1.1.0+ "can be safely used in multi-threaded applications provided that
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support for the underlying OS threading API is built-in." In that case the
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engine is used by libcurl in a way that is fully thread-safe.
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https://www.openssl.org/docs/man1.1.0/man3/CRYPTO_THREAD_run_once.html#DESCRIPTION
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OpenSSL <= 1.0.2 the user must set callbacks.
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https://www.openssl.org/docs/man1.0.2/man3/CRYPTO_set_locking_callback.html#DESCRIPTION
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https://curl.se/libcurl/c/opensslthreadlock.html
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.IP GnuTLS
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https://gnutls.org/manual/html_node/Thread-safety.html
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.IP NSS
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thread-safe already without anything required.
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.IP Secure-Transport
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The engine is used by libcurl in a way that is fully thread-safe.
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.IP Schannel
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The engine is used by libcurl in a way that is fully thread-safe.
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.IP wolfSSL
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The engine is used by libcurl in a way that is fully thread-safe.
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.IP BoringSSL
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The engine is used by libcurl in a way that is fully thread-safe.
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.SH "Other areas of caution"
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.IP Signals
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Signals are used for timing out name resolves (during DNS lookup) - when built
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without using either the c-ares or threaded resolver backends. When using
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multiple threads you should set the \fICURLOPT_NOSIGNAL(3)\fP option to 1L for
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all handles. Everything will or might work fine except that timeouts are not
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honored during the DNS lookup - which you can work around by building libcurl
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with c-ares or threaded-resolver support. c-ares is a library that provides
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asynchronous name resolves. On some platforms, libcurl simply will not
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function properly multi-threaded unless the \fICURLOPT_NOSIGNAL(3)\fP option
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is set.
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When \fICURLOPT_NOSIGNAL(3)\fP is set to 1L, your application needs to deal
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with the risk of a SIGPIPE (that at least the OpenSSL backend can
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trigger). Note that setting \fICURLOPT_NOSIGNAL(3)\fP to 0L will not work in a
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threaded situation as there will be race where libcurl risks restoring the
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former signal handler while another thread should still ignore it.
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.IP "Name resolving"
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\fBgethostby* functions and other system calls.\fP These functions, provided
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by your operating system, must be thread safe. It is important that libcurl
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can find and use thread safe versions of these and other system calls, as
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otherwise it cannot function fully thread safe. Some operating systems are
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known to have faulty thread implementations. We have previously received
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problem reports on *BSD (at least in the past, they may be working fine these
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days). Some operating systems that are known to have solid and working thread
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support are Linux, Solaris and Windows.
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.IP "curl_global_* functions"
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These functions are thread-safe since libcurl 7.84.0 if
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\fIcurl_version_info(3)\fP has the \fBCURL_VERSION_THREADSAFE\fP feature bit
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set (most platforms).
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If these functions are not thread-safe and you are using libcurl with multiple
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threads it is especially important that before use you call
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\fIcurl_global_init(3)\fP or \fIcurl_global_init_mem(3)\fP to explicitly
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initialize the library and its dependents, rather than rely on the "lazy"
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fail-safe initialization that takes place the first time
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\fIcurl_easy_init(3)\fP is called. For an in-depth explanation refer to
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\fIlibcurl(3)\fP section \fBGLOBAL CONSTANTS\fP.
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.IP "Memory functions"
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These functions, provided either by your operating system or your own
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replacements, must be thread safe. You can use \fIcurl_global_init_mem(3)\fP
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to set your own replacement memory functions.
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.IP "Non-safe functions"
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\fICURLOPT_DNS_USE_GLOBAL_CACHE(3)\fP is not thread-safe.
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