.\" ************************************************************************** .\" * _ _ ____ _ .\" * Project ___| | | | _ \| | .\" * / __| | | | |_) | | .\" * | (__| |_| | _ <| |___ .\" * \___|\___/|_| \_\_____| .\" * .\" * Copyright (C) 2015, Daniel Stenberg, , 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 http://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 Issues" libcurl is thread safe with the following exceptions: Do \fBNOT\fP use library functions to access or modify the same handle concurrently from multiple threads. If concurrent access to the same handle from multiple threads could be an issue then you must implement your own locking to ensure it won't happen. Shared objects. You can share certain data between multiple handles by using the share interface but you must implement your own locking and set \fIcurl_share_setopt(3)\fP CURLSHOPT_LOCKFUNC and CURLSHOPT_UNLOCKFUNC. SSL/TLS handlers. 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: .RS .IP OpenSSL http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION http://curl.haxx.se/libcurl/c/opensslthreadlock.html .IP GnuTLS http://gnutls.org/manual/html_node/Thread-safety.html .IP NSS is claimed to be thread-safe already without anything required. .IP PolarSSL Required actions unknown. .IP yassl Required actions unknown. .IP axTLS Required actions unknown. .IP Secure-Transport The engine is fully thread-safe, and no additional steps are required. .RE 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 1 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. 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 \fIcurl_global_init(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. \fICURLOPT_DNS_USE_GLOBAL_CACHE(3)\fP is not thread-safe.