97 lines
4.8 KiB
Groff
97 lines
4.8 KiB
Groff
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.\" **************************************************************************
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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) 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"
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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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.SH "Handles"
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You must \fBnever\fP share the same handle in multiple threads. You can pass
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the handles around among threads, but you must never use a single handle from
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more than one thread at any given time.
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.SH "Shared objects"
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You can share certain data between multiple handles by using the share
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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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Note that some items are specifically documented as not thread-safe in the
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share API (the connection pool and HSTS cache for example).
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.SH TLS
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All current TLS libraries libcurl supports are thread-safe. OpenSSL 1.1.0+ can
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be safely used in multi-threaded applications provided that support for the
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underlying OS threading API is built-in. For older versions of OpenSSL, the
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user must set mutex callbacks.
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.SH "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. On systems that
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have a signal concept.
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When using multiple threads you should set the \fICURLOPT_NOSIGNAL(3)\fP
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option to 1L for all handles. Everything works fine except that timeouts
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cannot be honored during DNS lookups - which you can work around by building
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libcurl with c-ares or threaded-resolver support. c-ares is a library that
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provides asynchronous name resolves. On some platforms, libcurl simply cannot
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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 does not work in a
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threaded situation as there is a race condition where libcurl risks restoring
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the former signal handler while another thread should still ignore it.
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.SH "Name resolving"
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The \fBgethostbyname\fP or \fBgetaddrinfo\fP and other name resolving system
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calls used by libcurl are provided by your operating system and must be thread
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safe. It is important that libcurl can find and use thread safe versions of
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these and other system calls, as otherwise it cannot function fully thread
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safe. Some operating systems are known to have faulty thread
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implementations. We have previously received problem reports on *BSD (at least
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in the past, they may be working fine these days). Some operating systems that
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are known to have solid and working thread support are Linux, Solaris and
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Windows.
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.SH "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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.SH "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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.SH "Non-safe functions"
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\fICURLOPT_DNS_USE_GLOBAL_CACHE(3)\fP is not thread-safe.
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\fIcurl_version_info(3)\fP is not thread-safe before libcurl initialization.
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