If foes not read all the characters because you are not trying to read them. You are seeking the end position. Remove that line, and the file will be opened in rewound position, at zero. Then read data in a single block or in a loop.
Also note that
SEEK_END
has no real standard portability. Better avoid using it:
http://www.cplusplus.com/reference/cstdio/fseek[
^].
—SA