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        !            23: .TH CURLOPT_OPENSOCKETFUNCTION 3 "December 03, 2019" "libcurl 7.70.0" "curl_easy_setopt options"
        !            24: 
        !            25: .SH NAME
        !            26: CURLOPT_OPENSOCKETFUNCTION \- set callback for opening sockets
        !            27: .SH SYNOPSIS
        !            28: .nf
        !            29: #include <curl/curl.h>
        !            30: 
        !            31: typedef enum  {
        !            32:   CURLSOCKTYPE_IPCXN,  /* socket created for a specific IP connection */
        !            33: } curlsocktype;
        !            34: 
        !            35: struct curl_sockaddr {
        !            36:   int family;
        !            37:   int socktype;
        !            38:   int protocol;
        !            39:   unsigned int addrlen;
        !            40:   struct sockaddr addr;
        !            41: };
        !            42: 
        !            43: curl_socket_t opensocket_callback(void *clientp,
        !            44:                                   curlsocktype purpose,
        !            45:                                   struct curl_sockaddr *address);
        !            46: 
        !            47: CURLcode curl_easy_setopt(CURL *handle, CURLOPT_OPENSOCKETFUNCTION, opensocket_callback);
        !            48: .SH DESCRIPTION
        !            49: Pass a pointer to your callback function, which should match the prototype
        !            50: shown above.
        !            51: 
        !            52: This callback function gets called by libcurl instead of the \fIsocket(2)\fP
        !            53: call. The callback's \fIpurpose\fP argument identifies the exact purpose for
        !            54: this particular socket. \fICURLSOCKTYPE_IPCXN\fP is for IP based connections
        !            55: and is the only purpose currently used in libcurl. Future versions of libcurl
        !            56: may support more purposes.
        !            57: 
        !            58: The \fIclientp\fP pointer contains whatever user-defined value set using the
        !            59: \fICURLOPT_OPENSOCKETDATA(3)\fP function.
        !            60: 
        !            61: The callback gets the resolved peer address as the \fIaddress\fP argument and
        !            62: is allowed to modify the address or refuse to connect completely. The callback
        !            63: function should return the newly created socket or \fICURL_SOCKET_BAD\fP in
        !            64: case no connection could be established or another error was detected. Any
        !            65: additional \fIsetsockopt(2)\fP calls can of course be done on the socket at
        !            66: the user's discretion.  A \fICURL_SOCKET_BAD\fP return value from the callback
        !            67: function will signal an unrecoverable error to libcurl and it will return
        !            68: \fICURLE_COULDNT_CONNECT\fP from the function that triggered this callback.
        !            69: This return code can be used for IP address blacklisting.
        !            70: 
        !            71: If you want to pass in a socket with an already established connection, pass
        !            72: the socket back with this callback and then use
        !            73: \fICURLOPT_SOCKOPTFUNCTION(3)\fP to signal that it already is connected.
        !            74: .SH DEFAULT
        !            75: The default behavior is the equivalent of this:
        !            76: .nf
        !            77:    return socket(addr->family, addr->socktype, addr->protocol);
        !            78: .fi
        !            79: .SH PROTOCOLS
        !            80: All
        !            81: .SH EXAMPLE
        !            82: .nf
        !            83: /* make libcurl use the already established socket 'sockfd' */
        !            84: 
        !            85: static curl_socket_t opensocket(void *clientp,
        !            86:                                 curlsocktype purpose,
        !            87:                                 struct curl_sockaddr *address)
        !            88: {
        !            89:   curl_socket_t sockfd;
        !            90:   sockfd = *(curl_socket_t *)clientp;
        !            91:   /* the actual externally set socket is passed in via the OPENSOCKETDATA
        !            92:      option */
        !            93:   return sockfd;
        !            94: }
        !            95: 
        !            96: static int sockopt_callback(void *clientp, curl_socket_t curlfd,
        !            97:                             curlsocktype purpose)
        !            98: {
        !            99:   /* This return code was added in libcurl 7.21.5 */
        !           100:   return CURL_SOCKOPT_ALREADY_CONNECTED;
        !           101: }
        !           102: 
        !           103: curl = curl_easy_init();
        !           104: if(curl) {
        !           105:   /* libcurl will internally think that you connect to the host
        !           106:    * and port that you specify in the URL option. */
        !           107:   curl_easy_setopt(curl, CURLOPT_URL, "http://99.99.99.99:9999");
        !           108:   /* call this function to get a socket */
        !           109:   curl_easy_setopt(curl, CURLOPT_OPENSOCKETFUNCTION, opensocket);
        !           110:   curl_easy_setopt(curl, CURLOPT_OPENSOCKETDATA, &sockfd);
        !           111: 
        !           112:   /* call this function to set options for the socket */
        !           113:   curl_easy_setopt(curl, CURLOPT_SOCKOPTFUNCTION, sockopt_callback);
        !           114: 
        !           115:   res = curl_easy_perform(curl);
        !           116: 
        !           117:   curl_easy_cleanup(curl);
        !           118: }
        !           119: .fi
        !           120: .SH AVAILABILITY
        !           121: Added in 7.17.1.
        !           122: .SH RETURN VALUE
        !           123: Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
        !           124: .SH "SEE ALSO"
        !           125: .BR CURLOPT_OPENSOCKETDATA "(3), " CURLOPT_SOCKOPTFUNCTION "(3), "
        !           126: .BR CURLOPT_CLOSESOCKETFUNCTION "(3), "