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        !            23: .TH CURLOPT_IOCTLFUNCTION 3 "March 23, 2020" "libcurl 7.70.0" "curl_easy_setopt options"
        !            24: 
        !            25: .SH NAME
        !            26: CURLOPT_IOCTLFUNCTION \- callback for I/O operations
        !            27: .SH SYNOPSIS
        !            28: .nf
        !            29: #include <curl/curl.h>
        !            30: 
        !            31: typedef enum {
        !            32:   CURLIOE_OK,            /* I/O operation successful */
        !            33:   CURLIOE_UNKNOWNCMD,    /* command was unknown to callback */
        !            34:   CURLIOE_FAILRESTART,   /* failed to restart the read */
        !            35:   CURLIOE_LAST           /* never use */
        !            36: } curlioerr;
        !            37: 
        !            38: typedef enum  {
        !            39:   CURLIOCMD_NOP,         /* no operation */
        !            40:   CURLIOCMD_RESTARTREAD, /* restart the read stream from start */
        !            41:   CURLIOCMD_LAST         /* never use */
        !            42: } curliocmd;
        !            43: 
        !            44: curlioerr ioctl_callback(CURL *handle, int cmd, void *clientp);
        !            45: 
        !            46: CURLcode curl_easy_setopt(CURL *handle, CURLOPT_IOCTLFUNCTION, ioctl_callback);
        !            47: .SH DESCRIPTION
        !            48: Pass a pointer to your callback function, which should match the prototype
        !            49: shown above.
        !            50: 
        !            51: This callback function gets called by libcurl when something special
        !            52: I/O-related needs to be done that the library can't do by itself. For now,
        !            53: rewinding the read data stream is the only action it can request. The
        !            54: rewinding of the read data stream may be necessary when doing an HTTP PUT or
        !            55: POST with a multi-pass authentication method.
        !            56: 
        !            57: The callback MUST return \fICURLIOE_UNKNOWNCMD\fP if the input \fIcmd\fP is
        !            58: not \fICURLIOCMD_RESTARTREAD\fP.
        !            59: 
        !            60: The \fIclientp\fP argument to the callback is set with the
        !            61: \fICURLOPT_IOCTLDATA(3)\fP option.
        !            62: 
        !            63: This option is deprecated! Do not use it. Use \fICURLOPT_SEEKFUNCTION(3)\fP
        !            64: instead to provide seeking! If \fICURLOPT_SEEKFUNCTION(3)\fP is set, this
        !            65: parameter will be ignored when seeking.
        !            66: .SH DEFAULT
        !            67: By default, this parameter is set to NULL. Not used.
        !            68: .SH PROTOCOLS
        !            69: Used with HTTP
        !            70: .SH EXAMPLE
        !            71: .nf
        !            72: static curlioerr ioctl_callback(CURL *handle, int cmd, void *clientp)
        !            73: {
        !            74:   struct data *io = (struct data *)clientp;
        !            75:   if(cmd == CURLIOCMD_RESTARTREAD) {
        !            76:     lseek(fd, 0, SEEK_SET);
        !            77:     current_offset = 0;
        !            78:     return CURLIOE_OK;
        !            79:   }
        !            80:   return CURLIOE_UNKNOWNCMD;
        !            81: }
        !            82: {
        !            83:   struct data ioctl_data;
        !            84:   curl_easy_setopt(curl, CURLOPT_IOCTLFUNCTION, ioctl_callback);
        !            85:   curl_easy_setopt(curl, CURLOPT_IOCTLDATA, &ioctl_data);
        !            86: }
        !            87: .fi
        !            88: .SH AVAILABILITY
        !            89: Added in 7.12.3
        !            90: .SH RETURN VALUE
        !            91: Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
        !            92: .SH "SEE ALSO"
        !            93: .BR CURLOPT_IOCTLDATA "(3), " CURLOPT_SEEKFUNCTION "(3), "