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        !            23: .TH CURLOPT_SEEKFUNCTION 3 "March 23, 2020" "libcurl 7.70.0" "curl_easy_setopt options"
        !            24: 
        !            25: .SH NAME
        !            26: CURLOPT_SEEKFUNCTION \- user callback for seeking in input stream
        !            27: .SH SYNOPSIS
        !            28: .nf
        !            29: #include <curl/curl.h>
        !            30: 
        !            31: /* These are the return codes for the seek callbacks */
        !            32: #define CURL_SEEKFUNC_OK       0
        !            33: #define CURL_SEEKFUNC_FAIL     1 /* fail the entire transfer */
        !            34: #define CURL_SEEKFUNC_CANTSEEK 2 /* tell libcurl seeking can't be done, so
        !            35:                                     libcurl might try other means instead */
        !            36: 
        !            37: int seek_callback(void *userp, curl_off_t offset, int origin);
        !            38: 
        !            39: CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SEEKFUNCTION, seek_callback);
        !            40: .SH DESCRIPTION
        !            41: Pass a pointer to your callback function, which should match the prototype
        !            42: shown above.
        !            43: 
        !            44: This function gets called by libcurl to seek to a certain position in the
        !            45: input stream and can be used to fast forward a file in a resumed upload
        !            46: (instead of reading all uploaded bytes with the normal read
        !            47: function/callback). It is also called to rewind a stream when data has already
        !            48: been sent to the server and needs to be sent again. This may happen when doing
        !            49: an HTTP PUT or POST with a multi-pass authentication method, or when an
        !            50: existing HTTP connection is reused too late and the server closes the
        !            51: connection. The function shall work like fseek(3) or lseek(3) and it gets
        !            52: SEEK_SET, SEEK_CUR or SEEK_END as argument for \fIorigin\fP, although libcurl
        !            53: currently only passes SEEK_SET.
        !            54: 
        !            55: \fIuserp\fP is the pointer you set with \fICURLOPT_SEEKDATA(3)\fP.
        !            56: 
        !            57: The callback function must return \fICURL_SEEKFUNC_OK\fP on success,
        !            58: \fICURL_SEEKFUNC_FAIL\fP to cause the upload operation to fail or
        !            59: \fICURL_SEEKFUNC_CANTSEEK\fP to indicate that while the seek failed, libcurl
        !            60: is free to work around the problem if possible. The latter can sometimes be
        !            61: done by instead reading from the input or similar.
        !            62: 
        !            63: If you forward the input arguments directly to fseek(3) or lseek(3), note that
        !            64: the data type for \fIoffset\fP is not the same as defined for curl_off_t on
        !            65: many systems!
        !            66: .SH DEFAULT
        !            67: By default, this is NULL and unused.
        !            68: .SH PROTOCOLS
        !            69: HTTP, FTP, SFTP
        !            70: .SH EXAMPLE
        !            71: .nf
        !            72: static int seek_cb(void *userp, curl_off_t offset, int origin)
        !            73: {
        !            74:   struct data *d = (struct data *)userp;
        !            75:   lseek(our_fd, offset, origin);
        !            76:   return CURL_SEEKFUNC_OK;
        !            77: }
        !            78: 
        !            79: {
        !            80:   struct data seek_data;
        !            81:   curl_easy_setopt(CURL *handle, CURLOPT_SEEKFUNCTION, seek_cb);
        !            82:   curl_easy_setopt(CURL *handle, CURLOPT_SEEKDATA, &seek_data);
        !            83: }
        !            84: .fi
        !            85: .SH AVAILABILITY
        !            86: Added in 7.18.0
        !            87: .SH RETURN VALUE
        !            88: Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
        !            89: .SH "SEE ALSO"
        !            90: .BR CURLOPT_SEEKDATA "(3), " CURLOPT_IOCTLFUNCTION "(3), "