I did this

I ran a multi-threaded app that uses libcurl under Clang 3.9 ThreadSanitizer.

I expected the following

Clean operation.

curl/libcurl version

7.44

operating system

Ubuntu 14.04, x86_64.

I encountered a data race with two threads calling Curl_ipv6works() at the same time. I think perhaps the call to Curl_ipv6works() should be moved into curl_global_init() to avoid this.