buffer is a pointer to the data of length buflen that you want sent. The variable n points to will receive the number of sent bytes.
To establish the connection, set CURLOPT_CONNECT_ONLY(3) option before calling curl_easy_perform(3) or curl_multi_perform(3). Note that curl_easy_send(3) will not work on connections that were created without this option.
The call will return CURLE_AGAIN if it's not possible to send data right now - the socket is used in non-blocking mode internally. When CURLE_AGAIN is returned, use your operating system facilities like select(2) to wait until the socket is writable. The socket may be obtained using curl_easy_getinfo(3) with CURLINFO_ACTIVESOCKET(3).
Furthermore if you wait on the socket and it tells you it's writable, curl_easy_send(3) may return CURLE_AGAIN if the only data that was sent was for internal SSL processing, and no other data could be sent.
On failure, returns the appropriate error code.
This function may return CURLE_AGAIN. In this case, use your operating system facilities to wait until the socket is writable, and retry.