title: Segmentation fault in libcurl, multithreaded
body: <p>So I've got a bunch of worker threads doing simple curl class, each worker thread has his own curl easy handle. They are doing only HEAD lookups on random web sites. Also locking functions are present to enable multi threaded SSL as documented <a href="http://www.openssl.org/docs/crypto/threads.html" rel="noreferrer">here</a>. Everything is working except on 2 web pages ilsole24ore.com ( seen in example down ), and ninemsn.com.au/ , they sometimes produce seg fault as shown in trace output shown here</p>
  
  <pre><code> #0  *__GI___libc_res_nquery (statp=0xb4d12df4, name=0x849e9bd "ilsole24ore.com", class=1, type=1, answer=0xb4d0ca10 "", anslen=1024, answerp=0xb4d0d234,
          answerp2=0x0, nanswerp2=0x0, resplen2=0x0) at res_query.c:182
      #1  0x00434e8b in __libc_res_nquerydomain (statp=0xb4d12df4, name=0xb4d0ca10 "", domain=0x0, class=1, type=1, answer=0xb4d0ca10 "", anslen=1024,
          answerp=0xb4d0d234, answerp2=0x0, nanswerp2=0x0, resplen2=0x0) at res_query.c:576
      #2  0x004352b5 in *__GI___libc_res_nsearch (statp=0xb4d12df4, name=0x849e9bd "ilsole24ore.com", class=1, type=1, answer=0xb4d0ca10 "", anslen=1024,
          answerp=0xb4d0d234, answerp2=0x0, nanswerp2=0x0, resplen2=0x0) at res_query.c:377
      #3  0x009c0bd6 in *__GI__nss_dns_gethostbyname3_r (name=0x849e9bd "ilsole24ore.com", af=2, result=0xb4d0d5fc, buffer=0xb4d0d300 "\177", buflen=512,
          errnop=0xb4d12b30, h_errnop=0xb4d0d614, ttlp=0x0, canonp=0x0) at nss_dns/dns-host.c:197
      #4  0x009c0f2b in _nss_dns_gethostbyname2_r (name=0x849e9bd "ilsole24ore.com", af=2, result=0xb4d0d5fc, buffer=0xb4d0d300 "\177", buflen=512,
          errnop=0xb4d12b30, h_errnop=0xb4d0d614) at nss_dns/dns-host.c:251
      #5  0x0079eacd in __gethostbyname2_r (name=0x849e9bd "ilsole24ore.com", af=2, resbuf=0xb4d0d5fc, buffer=0xb4d0d300 "\177", buflen=512, result=0xb4d0d618,
          h_errnop=0xb4d0d614) at ../nss/getXXbyYY_r.c:253
      #6  0x00760010 in gaih_inet (name=&lt;value optimized out&gt;, service=&lt;value optimized out&gt;, req=0xb4d0f83c, pai=0xb4d0d764, naddrs=0xb4d0d754)
          at ../sysdeps/posix/getaddrinfo.c:531
      #7  0x00761a65 in *__GI_getaddrinfo (name=0x849e9bd "ilsole24ore.com", service=0x0, hints=0xb4d0f83c, pai=0xb4d0f860) at ../sysdeps/posix/getaddrinfo.c:2160
      #8  0x00917f9a in ?? () from /usr/lib/libkrb5support.so.0
      #9  0x003b2f45 in krb5_sname_to_principal () from /usr/lib/libkrb5.so.3
      #10 0x0028a278 in ?? () from /usr/lib/libgssapi_krb5.so.2
      #11 0x0027eff2 in ?? () from /usr/lib/libgssapi_krb5.so.2
      #12 0x0027fb00 in gss_init_sec_context () from /usr/lib/libgssapi_krb5.so.2
      #13 0x00d8770e in ?? () from /usr/lib/libcurl.so.4
      #14 0x00d62c27 in ?? () from /usr/lib/libcurl.so.4
      #15 0x00d7e25b in ?? () from /usr/lib/libcurl.so.4
      #16 0x00d7e597 in ?? () from /usr/lib/libcurl.so.4
      #17 0x00d7f133 in curl_easy_perform () from /usr/lib/libcurl.so.4
  </code></pre>
  
  <p>My function looks something like this</p>
  
  <pre><code>int do_http_check(taskinfo *info,standardResult *data)
  {
      standardResultInit(data);
  
      char errorBuffer[CURL_ERROR_SIZE];
  
      CURL *curl;
      CURLcode result;
  
      curl = curl_easy_init();
  
      if(curl)
      {
          //required options first
          curl_easy_setopt(curl, CURLOPT_ERRORBUFFER, errorBuffer);
          curl_easy_setopt(curl, CURLOPT_URL, info-&gt;address.c_str());
          curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writer);
          curl_easy_setopt(curl, CURLOPT_WRITEDATA, &amp;data-&gt;body);
          curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, writer);
          curl_easy_setopt(curl, CURLOPT_WRITEHEADER, &amp;data-&gt;head);
          curl_easy_setopt(curl, CURLOPT_DNS_USE_GLOBAL_CACHE,0);
          curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, 30 );
          curl_easy_setopt(curl, CURLOPT_NOSIGNAL,1);
          curl_easy_setopt(curl, CURLOPT_NOBODY,1);
          curl_easy_setopt(curl, CURLOPT_TIMEOUT ,240);
  
          //optional options
          if(info-&gt;options.follow)
          {
              curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1);
              curl_easy_setopt(curl, CURLOPT_MAXREDIRS, info-&gt;options.redirects);
          }
  
          result = curl_easy_perform(curl);
  
          if (result == CURLE_OK)
          {
              data-&gt;success = true;
              curl_easy_getinfo(curl,CURLINFO_RESPONSE_CODE,&amp;data-&gt;httpMsg);
              curl_easy_getinfo(curl,CURLINFO_REDIRECT_COUNT,&amp;data-&gt;numRedirects);
              data-&gt;msg = "OK";
          }
          else
          {
              ... handle error
          }
  
  
      return 1;
  }
  </code></pre>
  
  <p>Now, when i call function without any threads, just calling it from main it never breaks, so I was thinking its connected to threads, or maybe how data return structure is being returned, but from what I saw in trace it looks like fault is generated in easy_perform() call, and its confusing me. 
  So if someone has any idea where should i look next it would be most helpful, thanks. </p>