This function receives raw data from the established connection. You may use
it together with curl_easy_send(3) to implement custom protocols using
libcurl. This functionality can be particularly useful if you use proxies
and/or SSL encryption: libcurl will take care of proxy negotiation and
connection set-up.
buffer is a pointer to your buffer that will get the received
data. buflen is the maximum amount of data you can get in that
buffer. The variable n points to will receive the number of received
bytes.
To establish the connection, set CURLOPT_CONNECT_ONLY option before
calling curl_easy_perform(3). Note that curl_easy_recv(3) does not
work on connections that were created without this option.
You must ensure that the socket has data to read before calling
curl_easy_recv(3), otherwise the call will return CURLE_AGAIN -
the socket is used in non-blocking mode internally. Use
curl_easy_getinfo(3) with CURLINFO_LASTSOCKET to obtain the
socket; use your operating system facilities like select(2) to check if
it has any data you can read.