CURLOPT_SEEKFUNCTION(3)    curl_easy_setopt options    CURLOPT_SEEKFUNCTION(3)

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 can't be done, so
                                           libcurl might try other means instead */

       int seek_callback(void *userp, 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.

       userp is the pointer you set with _SEEKDATA&section=3">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
       static int seek_cb(void *userp, curl_off_t offset, int origin)
       {
         struct data *d = (struct data *)userp;
         lseek(our_fd, offset, origin);
         return CURL_SEEKFUNC_OK;
       }

       {
         struct data seek_data;
         curl_easy_setopt(CURL *handle, CURLOPT_SEEKFUNCTION, seek_cb);
         curl_easy_setopt(CURL *handle, 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_SEEKDATA(3), CURLOPT_IOCTLFUNCTION(3),

libcurl 7.77.0                 November 4, 2020        CURLOPT_SEEKFUNCTION(3)