Alexey Pesternikov documented CURLOPT_OPENSOCKETDATA and

Pass a pointer that will be untouched by libcurl and passed as the first

argument in the sockopt callback set with \fICURLOPT_SOCKOPTFUNCTION\fP.

(Option added in 7.15.6.)

.IP CURLOPT_OPENSOCKETFUNCTION

Function pointer that should match the \fIcurl_opensocket_callback\fP

prototype found in \fI<curl/curl.h>\fP. This function gets called by libcurl

instead of the \fIsocket(2)\fP call. The callback's \fIpurpose\fP argument

identifies the exact purpose for this particular socket, and currently only

one value is supported: \fICURLSOCKTYPE_IPCXN\fP for the primary connection

(meaning the control connection in the FTP case). Future versions of libcurl

may support more purposes. It passes the resolved peer address as a

\fIaddress\fP argument so the callback can modify the address or refuse to

connect at all. The callback function should return the socket or

\fICURL_SOCKET_BAD\fP in case no connection should be established or any error

detected. Any additional \fIsetsockopt(2)\fP calls can be done on the socket

at the user's discretion.  \fICURL_SOCKET_BAD\fP return value from the

callback function will signal an unrecoverable error to the library and it

will return \fICURLE_COULDNT_CONNECT\fP.  This return code can be used for IP

address blacklisting.  The default behavior is:

.Bd -literal -offset indent

   return socket(addr->family, addr->socktype, addr->protocol);

.Ed

(Option added in 7.17.1.)

.IP CURLOPT_OPENSOCKETDATA

Pass a pointer that will be untouched by libcurl and passed as the first

argument in the opensocket callback set with \fICURLOPT_OPENSOCKETFUNCTION\fP.

(Option added in 7.17.1.)

.IP CURLOPT_PROGRESSFUNCTION

Function pointer that should match the \fIcurl_progress_callback\fP prototype

found in \fI<curl/curl.h>\fP. This function gets called by libcurl instead of