CURLOPT_SEEKFUNCTION(3)

1CURLOPT_SEEKFUNCTION(3)    curl_easy_setopt options    CURLOPT_SEEKFUNCTION(3)
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NAME

6       CURLOPT_SEEKFUNCTION - user callback for seeking in input stream
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SYNOPSIS

9       #include <curl/curl.h>
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11       /* These are the return codes for the seek callbacks */
12       #define CURL_SEEKFUNC_OK       0
13       #define CURL_SEEKFUNC_FAIL     1 /* fail the entire transfer */
14       #define CURL_SEEKFUNC_CANTSEEK 2 /* tell libcurl seeking cannot be done, so
15                                           libcurl might try other means instead */
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17       int seek_callback(void *clientp, curl_off_t offset, int origin);
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19       CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SEEKFUNCTION, seek_callback);
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DESCRIPTION

22       Pass a pointer to your callback function, which should match the proto‐
23       type shown above.
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25       This function gets called by libcurl to seek to a certain  position  in
26       the  input  stream  and can be used to fast forward a file in a resumed
27       upload (instead of reading all uploaded  bytes  with  the  normal  read
28       function/callback).  It is also called to rewind a stream when data has
29       already been sent to the server and needs to be sent  again.  This  may
30       happen  when doing an HTTP PUT or POST with a multi-pass authentication
31       method, or when an existing HTTP connection is reused too late and  the
32       server  closes the connection. The function shall work like fseek(3) or
33       lseek(3) and it gets SEEK_SET, SEEK_CUR or  SEEK_END  as  argument  for
34       origin, although libcurl currently only passes SEEK_SET.
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36       clientp is the pointer you set with CURLOPT_SEEKDATA(3).
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38       The   callback   function  must  return  CURL_SEEKFUNC_OK  on  success,
39       CURL_SEEKFUNC_FAIL to cause the upload operation to fail or  CURL_SEEK‐
40       FUNC_CANTSEEK  to  indicate that while the seek failed, libcurl is free
41       to work around the problem if possible. The  latter  can  sometimes  be
42       done by instead reading from the input or similar.
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44       If  you  forward  the input arguments directly to fseek(3) or lseek(3),
45       note that the data type for offset is  not  the  same  as  defined  for
46       curl_off_t on many systems!
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DEFAULT

49       By default, this is NULL and unused.
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PROTOCOLS

52       HTTP, FTP, SFTP
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EXAMPLE

55       static int seek_cb(void *clientp, curl_off_t offset, int origin)
56       {
57         struct data *d = (struct data *)clientp;
58         lseek(our_fd, offset, origin);
59         return CURL_SEEKFUNC_OK;
60       }
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62       {
63         struct data seek_data;
64         curl_easy_setopt(CURL *handle, CURLOPT_SEEKFUNCTION, seek_cb);
65         curl_easy_setopt(CURL *handle, CURLOPT_SEEKDATA, &seek_data);
66       }
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AVAILABILITY

69       Added in 7.18.0
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RETURN VALUE

72       Returns  CURLE_OK  if the option is supported, and CURLE_UNKNOWN_OPTION
73       if not.
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