NAME

       CURLOPT_SEEKFUNCTION - user callback for seeking in input stream

SYNOPSIS

       #include <curl/curl.h>

       /* These are the return codes for the seek callbacks */
       #define CURL_SEEKFUNC_OK       0
       #define CURL_SEEKFUNC_FAIL     1 /* fail the entire transfer */
       #define CURL_SEEKFUNC_CANTSEEK 2 /* tell libcurl seeking cannot be done, so
                                           libcurl might try other means instead */

       int seek_callback(void *clientp, curl_off_t offset, int origin);

       CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SEEKFUNCTION, seek_callback);

DESCRIPTION

       Pass a pointer to your callback function, which should match the prototype shown above.

       This function gets called by libcurl to seek to a certain position in the input stream and can be used to
       fast forward a file in a resumed upload (instead of reading all  uploaded  bytes  with  the  normal  read
       function/callback).  It  is  also called to rewind a stream when data has already been sent to the server
       and needs to be sent again.  This  may  happen  when  doing  an  HTTP  PUT  or  POST  with  a  multi-pass
       authentication  method,  or when an existing HTTP connection is reused too late and the server closes the
       connection. The function shall work like fseek(3) or lseek(3) and it gets SEEK_SET, SEEK_CUR or  SEEK_END
       as argument for origin, although libcurl currently only passes SEEK_SET.

       clientp is the pointer you set with CURLOPT_SEEKDATA(3).

       The  callback  function  must  return CURL_SEEKFUNC_OK on success, CURL_SEEKFUNC_FAIL to cause the upload
       operation to fail or CURL_SEEKFUNC_CANTSEEK to indicate that while the seek failed, libcurl  is  free  to
       work  around  the problem if possible. The latter can sometimes be done by instead reading from the input
       or similar.

       If you forward the input arguments directly to fseek(3) or lseek(3), note that the data type  for  offset
       is not the same as defined for curl_off_t on many systems!

DEFAULT

       NULL

PROTOCOLS

       This functionality affects all supported protocols

EXAMPLE

       #include <unistd.h> /* for lseek */

       struct data {
         int our_fd;
       };
       static int seek_cb(void *clientp, curl_off_t offset, int origin)
       {
         struct data *d = (struct data *)clientp;
         lseek(d->our_fd, offset, origin);
         return CURL_SEEKFUNC_OK;
       }

       int main(void)
       {
         struct data seek_data;
         CURL *curl = curl_easy_init();
         if(curl) {
           curl_easy_setopt(curl, CURLOPT_SEEKFUNCTION, seek_cb);
           curl_easy_setopt(curl, CURLOPT_SEEKDATA, &seek_data);
         }
       }

AVAILABILITY

       Added in curl 7.18.0

RETURN VALUE

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

SEE ALSO

       CURLOPT_DEBUGFUNCTION(3), CURLOPT_IOCTLFUNCTION(3), CURLOPT_SEEKDATA(3), CURLOPT_STDERR(3)