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.\" ************************************************************************** .\" * _ _ ____ _ .\" * Project ___| | | | _ \| | .\" * / __| | | | |_) | | .\" * | (__| |_| | _ <| |___ .\" * \___|\___/|_| \_\_____| .\" * .\" * Copyright (C) 2015 - 2020, 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.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 "November 04, 2020" "libcurl 7.77.0" "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.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 Schannel 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.
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