curl_easy_recv man page
curl_easy_recv — receives raw data on an "easy" connection
CURLcode curl_easy_recv( CURL *curl, void *buffer, size_t buflen, size_t *n);
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(3) option before calling curl_easy_perform(3) or curl_multi_perform(3). Note that curl_easy_recv(3) does not work on connections that were created without this option.
The call will return CURLE_AGAIN if there is no data to read - the socket is used in non-blocking mode internally. When CURLE_AGAIN is returned, use your operating system facilities like select(2) to wait for data. The socket may be obtained using curl_easy_getinfo(3) with CURLINFO_ACTIVESOCKET(3).
Wait on the socket only if curl_easy_recv(3) returns CURLE_AGAIN. The reason for this is libcurl or the SSL library may internally cache some data, therefore you should call curl_easy_recv(3) until all data is read which would include any cached data.
Furthermore if you wait on the socket and it tells you there is data to read, curl_easy_recv(3) may return CURLE_AGAIN if the only data that was read was for internal SSL processing, and no other data is available.
Added in 7.18.2.
On success, returns CURLE_OK, stores the received data into buffer, and the number of bytes it actually read into *n.
On failure, returns the appropriate error code.
The function may return CURLE_AGAIN. In this case, use your operating system facilities to wait until data can be read, and retry.
Reading exactly 0 bytes indicates a closed connection.
If there's no socket available to use from the previous transfer, this function returns CURLE_UNSUPPORTED_PROTOCOL.
See sendrecv.c in docs/examples directory for usage example.
curl_easy_setopt(3), curl_easy_perform(3), curl_easy_getinfo(3), curl_easy_send(3)
curl_easy_send(3), CURLOPT_CONNECT_ONLY(3), libcurl-errors(3).