The ftell function measures the byte offset from the beginning of
    the file.

    For variable-length files, VFC files, or any file with carriage-
    control attributes, if the file is opened in record mode, then
    ftell returns the starting position of the current record, not
    the current byte offset.

    When using record files, the ftell function ignores any
    characters that have been pushed back using either ungetc or
    ungetwc. This behavior does not occur if stream files are being
    used.

    For a portable way to measure the exact offset for any type of
    file, see the fgetpos function.