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

       By default, this is NULL and unused.

PROTOCOLS

       HTTP, FTP, SFTP

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 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)

ibcurl 8.5.0                                    December 04, 2023                        CURLOPT_SEEKFUNCTION(3)