FAQ: refer to thread safety guidelines

  your system has such.  Note that you must never share the same handle in

  multiple threads.

  libcurl's implementation of timeouts might use signals (depending on what it

  was built to use for name resolving), and signal handling is generally not

  thread-safe.  Multi-threaded Applicationss that call libcurl from different

  threads (on different handles) might want to use CURLOPT_NOSIGNAL, e.g.:

    curl_easy_setopt(handle, CURLOPT_NOSIGNAL, true);

  If you use a OpenSSL-powered libcurl in a multi-threaded environment, you

  need to provide one or two locking functions:

    https://www.openssl.org/docs/crypto/threads.html

  If you use a GnuTLS-powered libcurl in a multi-threaded environment, you

  need to provide locking function(s) for libgcrypt (which is used by GnuTLS

  for the crypto functions).

    https://web.archive.org/web/20111103083330/http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-applications.html

  No special locking is needed with a NSS-powered libcurl. NSS is thread-safe.

  There may be some exceptions to thread safety depending on how libcurl was

  built. Please review the guidelines for thread safety to learn more:

  https://curl.haxx.se/libcurl/c/threadsafe.html

  5.2 How can I receive all data into a large memory chunk?