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        !            23: .TH CURLMOPT_SOCKETFUNCTION 3 "June 24, 2019" "libcurl 7.70.0" "curl_multi_setopt options"
        !            24: 
        !            25: .SH NAME
        !            26: CURLMOPT_SOCKETFUNCTION \- callback informed about what to wait for
        !            27: .SH SYNOPSIS
        !            28: .nf
        !            29: #include <curl/curl.h>
        !            30: 
        !            31: int socket_callback(CURL *easy,      /* easy handle */
        !            32:                     curl_socket_t s, /* socket */
        !            33:                     int what,        /* describes the socket */
        !            34:                     void *userp,     /* private callback pointer */
        !            35:                     void *socketp);  /* private socket pointer */
        !            36: 
        !            37: CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_SOCKETFUNCTION, socket_callback);
        !            38: .SH DESCRIPTION
        !            39: Pass a pointer to your callback function, which should match the prototype
        !            40: shown above.
        !            41: 
        !            42: When the \fIcurl_multi_socket_action(3)\fP function is called, it informs the
        !            43: application about updates in the socket (file descriptor) status by doing
        !            44: none, one, or multiple calls to the \fBsocket_callback\fP. The callback
        !            45: function gets status updates with changes since the previous time the callback
        !            46: was called. If the given callback pointer is set to NULL, no callback will be
        !            47: called.
        !            48: .SH "CALLBACK ARGUMENTS"
        !            49: \fIeasy\fP identifies the specific transfer for which this update is related.
        !            50: 
        !            51: \fIs\fP is the specific socket this function invocation concerns. If the
        !            52: \fBwhat\fP argument is not CURL_POLL_REMOVE then it holds information about
        !            53: what activity on this socket the application is supposed to
        !            54: monitor. Subsequent calls to this callback might update the \fBwhat\fP bits
        !            55: for a socket that is already monitored.
        !            56: 
        !            57: \fBuserp\fP is set with \fICURLMOPT_SOCKETDATA(3)\fP.
        !            58: 
        !            59: \fBsocketp\fP is set with \fIcurl_multi_assign(3)\fP or will be NULL.
        !            60: 
        !            61: The \fBwhat\fP parameter informs the callback on the status of the given
        !            62: socket. It can hold one of these values:
        !            63: .IP CURL_POLL_IN
        !            64: Wait for incoming data. For the socket to become readable.
        !            65: .IP CURL_POLL_OUT
        !            66: Wait for outgoing data. For the socket to become writable.
        !            67: .IP CURL_POLL_INOUT
        !            68: Wait for incoming and outgoing data. For the socket to become readable or
        !            69: writable.
        !            70: .IP CURL_POLL_REMOVE
        !            71: The specified socket/file descriptor is no longer used by libcurl.
        !            72: .SH DEFAULT
        !            73: NULL (no callback)
        !            74: .SH PROTOCOLS
        !            75: All
        !            76: .SH EXAMPLE
        !            77: .nf
        !            78: static int sock_cb(CURL *e, curl_socket_t s, int what, void *cbp, void *sockp)
        !            79: {
        !            80:   GlobalInfo *g = (GlobalInfo*) cbp;
        !            81:   SockInfo *fdp = (SockInfo*) sockp;
        !            82: 
        !            83:   if(what == CURL_POLL_REMOVE) {
        !            84:     remsock(fdp);
        !            85:   }
        !            86:   else {
        !            87:     if(!fdp) {
        !            88:       addsock(s, e, what, g);
        !            89:     }
        !            90:     else {
        !            91:       setsock(fdp, s, e, what, g);
        !            92:     }
        !            93:   }
        !            94:   return 0;
        !            95: }
        !            96: 
        !            97: main()
        !            98: {
        !            99:   GlobalInfo setup;
        !           100:   /* ... use socket callback and custom pointer */
        !           101:   curl_multi_setopt(multi, CURLMOPT_SOCKETFUNCTION, sock_cb);
        !           102:   curl_multi_setopt(multi, CURLMOPT_SOCKETDATA, &setup);
        !           103: }
        !           104: .fi
        !           105: .SH AVAILABILITY
        !           106: Added in 7.15.4
        !           107: .SH RETURN VALUE
        !           108: Returns CURLM_OK.
        !           109: .SH "SEE ALSO"
        !           110: .BR CURLMOPT_SOCKETDATA "(3), " curl_multi_socket_action "(3), "
        !           111: .BR CURLMOPT_TIMERFUNCTION "(3) "