Is libCurl really thread-safe?

Question

After being unable to find out what was happening in my app in the previous question, I have tried to reproduce it in my pc using valgrind to find memory / multithread problems. I have seen in many places this:

==769== Possible data race during write of size 4 at 0xACD3ADC by thread #13
==769== Locks held: none
==769==    at 0x4C36067: memset (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==769==    by 0x4E4570D: pthread_create@@GLIBC_2.2.5 (allocatestack.c:249)
==769==    by 0x4C30C90: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==769==    by 0x509F957: Curl_thread_create (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x50A261B: Curl_resolver_getaddrinfo (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x5066163: Curl_resolv (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x507D74D: Curl_connect (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x508D62F: multi_runsingle (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x508E180: curl_multi_perform (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x50857B2: curl_easy_perform (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x489D63: ConnectionMgr::checkInternetConnection() (ConnectionMgr.cpp:287)
==769==    by 0x490E9E: void std::_Mem_fn<void (ConnectionMgr::*)()>::operator()<, void>(ConnectionMgr*) const (in /home/user/app)
==769== 
==769== Address 0xACD3ADC is 44 bytes inside a block of size 304 alloc'd
==769==    at 0x4C2DFF0: calloc (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==769==    by 0x4012E54: _dl_allocate_tls (dl-tls.c:296)
==769==    by 0x4E45DA0: pthread_create@@GLIBC_2.2.5 (allocatestack.c:589)
==769==    by 0x4C30C90: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==769==    by 0x509F957: Curl_thread_create (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x50A261B: Curl_resolver_getaddrinfo (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x5066163: Curl_resolv (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x507D74D: Curl_connect (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x508D62F: multi_runsingle (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x508E180: curl_multi_perform (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x50857B2: curl_easy_perform (in /usr/lib/x86_64-linux-gnu/libcurl.so.4.3.0)
==769==    by 0x489D63: ConnectionMgr::checkInternetConnection() (ConnectionMgr.cpp:287)

The function I'm running in a separated thread is this one.

void ConnectionMgr::checkInternetConnection()
{
    CURL* curl = curl_easy_init();

    curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, 15);
    curl_easy_setopt(curl, CURLOPT_URL,"http://www.google.com");
    curl_easy_setopt (curl, CURLOPT_FAILONERROR, 1L);
    curl_easy_setopt (curl, CURLOPT_CONNECT_ONLY, 1L);

    long responseCode = 404;
    curl_easy_perform(curl);
    curl_easy_getinfo (curl, CURLINFO_HTTP_CONNECTCODE, &responseCode);

    curl_easy_cleanup(curl);

    mHasInternet = (responseCode == 200);
}

Thanks a lot.

Answer 1

Maybe this question is a bit outdated but I found the answer to it. The problem was that Curl was using signals to notify timeouts on requests and seems not to work fine with multi-threading.

To fix it I used curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1); and after that it worked properly.

Answer 2

Yes!

libcurl is thread-safe under the documented restrictions. Hundreds of applications are using it every day successfully in a threaded way.

Are there any bugs? Sure, and we work hard at fixing all of them that we get reported to the project.