---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: libcurl-thread
Section: 3
Source: libcurl
See-also:
  - libcurl-security (3)
Protocol:
  - All
Added-in: n/a
---

# NAME

libcurl-thread - libcurl thread safety

# 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.

# Handles

You must **never** 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.

# Shared objects

You can share certain data between multiple handles by using the share
interface but you must provide your own locking and set
curl_share_setopt(3) CURLSHOPT_LOCKFUNC and CURLSHOPT_UNLOCKFUNC.

Note that some items are specifically documented as not thread-safe in the
share API (the connection pool and HSTS cache for example).

# TLS

All current TLS libraries libcurl supports are thread-safe.

## 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. For older versions of
OpenSSL, the user must set mutex callbacks.

libcurl may not be able to fully clean up after multi-threaded OpenSSL
depending on how OpenSSL was built and loaded as a library. It is possible in
some rare circumstances a memory leak could occur unless you implement your own
OpenSSL thread cleanup.

For example, on Windows if both libcurl and OpenSSL are linked statically to a
DLL or application then OpenSSL may leak memory unless the DLL or application
calls OPENSSL_thread_stop() before each thread terminates. If OpenSSL is built
as a DLL then it does this cleanup automatically and there is no leak. If
libcurl is built as a DLL and OpenSSL is linked statically to it then libcurl
does this cleanup automatically and there is no leak (added in libcurl 8.8.0).

Please review the OpenSSL documentation for a full list of circumstances:
https://docs.openssl.org/3.0/man3/OPENSSL_init_crypto/#notes

## mbedTLS

mbedTLS can be used safely in a multi-threaded environment provided that mbedTLS is
compiled with MBEDTLS_THREADING_C enabled.

https://mbed-tls.readthedocs.io/en/latest/kb/development/thread-safety-and-multi-threading

# Signals

Signals are used for timing out name resolves (during DNS lookup) - when built
without using either the c-ares or threaded resolver backends. On systems that
have a signal concept.

When using multiple threads you should set the CURLOPT_NOSIGNAL(3)
option to 1L for all handles. Everything works fine except that timeouts
cannot be honored during DNS lookups - 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 cannot
function properly multi-threaded unless the CURLOPT_NOSIGNAL(3) option
is set.

When CURLOPT_NOSIGNAL(3) 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 CURLOPT_NOSIGNAL(3) to 0L does not work in a
threaded situation as there is a race condition where libcurl risks restoring
the former signal handler while another thread should still ignore it.

# Name resolving

The **gethostbyname** or **getaddrinfo** and other name resolving system
calls used by libcurl are provided by your operating system and must be
thread-safe. It is important that libcurl can find and use thread-safe versions
of these and other system calls, as otherwise it cannot 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 thread-safe since libcurl 7.84.0 if
curl_version_info(3) has the **CURL_VERSION_THREADSAFE** feature bit
set (most platforms).

If these functions are not thread-safe and you are using libcurl with multiple
threads it is especially important that before use you call
curl_global_init(3) or curl_global_init_mem(3) to explicitly
initialize the library and its dependents, rather than rely on the "lazy"
fail-safe initialization that takes place the first time
curl_easy_init(3) is called. For an in-depth explanation refer to
libcurl(3) section **GLOBAL CONSTANTS**.

# Memory functions

These functions, provided either by your operating system or your own
replacements, must be thread-safe. You can use curl_global_init_mem(3)
to set your own replacement memory functions.

# Non-safe functions

CURLOPT_DNS_USE_GLOBAL_CACHE(3) is not thread-safe.

curl_version_info(3) is not thread-safe before libcurl initialization.
