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)