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        !            23: .TH CURLOPT_SOCKOPTFUNCTION 3 "May 15, 2017" "libcurl 7.70.0" "curl_easy_setopt options"
        !            24: 
        !            25: .SH NAME
        !            26: CURLOPT_SOCKOPTFUNCTION \- set callback for setting socket options
        !            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_ACCEPT, /* socket created by accept() call */
        !            34:   CURLSOCKTYPE_LAST    /* never use */
        !            35: } curlsocktype;
        !            36: 
        !            37: #define CURL_SOCKOPT_OK 0
        !            38: #define CURL_SOCKOPT_ERROR 1 /* causes libcurl to abort and return
        !            39:                                 CURLE_ABORTED_BY_CALLBACK */
        !            40: #define CURL_SOCKOPT_ALREADY_CONNECTED 2
        !            41: 
        !            42: int sockopt_callback(void *clientp,
        !            43:                      curl_socket_t curlfd,
        !            44:                      curlsocktype purpose);
        !            45: 
        !            46: CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SOCKOPTFUNCTION, sockopt_callback);
        !            47: .SH DESCRIPTION
        !            48: Pass a pointer to your callback function, which should match the prototype
        !            49: shown above.
        !            50: 
        !            51: When set, this callback function gets called by libcurl when the socket has
        !            52: been created, but before the connect call to allow applications to change
        !            53: specific socket options. The callback's \fIpurpose\fP argument identifies the
        !            54: exact purpose for this particular socket:
        !            55: 
        !            56: \fICURLSOCKTYPE_IPCXN\fP for actively created connections or since 7.28.0
        !            57: \fICURLSOCKTYPE_ACCEPT\fP for FTP when the connection was setup with PORT/EPSV
        !            58: (in earlier versions these sockets weren't passed to this callback).
        !            59: 
        !            60: Future versions of libcurl may support more purposes. libcurl passes the newly
        !            61: created socket descriptor to the callback in the \fIcurlfd\fP parameter so
        !            62: additional setsockopt() calls can be done at the user's discretion.
        !            63: 
        !            64: The \fIclientp\fP pointer contains whatever user-defined value set using the
        !            65: \fICURLOPT_SOCKOPTDATA(3)\fP function.
        !            66: 
        !            67: Return \fICURL_SOCKOPT_OK\fP from the callback on success. Return
        !            68: \fICURL_SOCKOPT_ERROR\fP from the callback function to signal an unrecoverable
        !            69: error to the library and it will close the socket and return
        !            70: \fICURLE_COULDNT_CONNECT\fP.
        !            71: Alternatively, the callback function can return
        !            72: \fICURL_SOCKOPT_ALREADY_CONNECTED\fP, to tell libcurl that the socket is
        !            73: already connected and then libcurl will not attempt to connect it. This allows
        !            74: an application to pass in an already connected socket with
        !            75: \fICURLOPT_OPENSOCKETFUNCTION(3)\fP and then have this function make libcurl
        !            76: not attempt to connect (again).
        !            77: .SH DEFAULT
        !            78: By default, this callback is NULL and unused.
        !            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: .fi
        !           119: .SH AVAILABILITY
        !           120: Added in 7.16.0. The \fICURL_SOCKOPT_ALREADY_CONNECTED\fP return code was
        !           121: added in 7.21.5.
        !           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_SOCKOPTDATA "(3), " CURLOPT_OPENSOCKETFUNCTION "(3), "