Code below is used to get size of file.
For 6 GB file, invalid seek offset error occurs in line shown in comment.
How to get size of large file ?

* Returns size of file in bytes.
FUNCTION FILELEN( prmFileName )

local readfile, filelen
* todo: for large file next line caues invalid seek offset error
readfile = FOPEN( m.prmFileName, 10)	&& 10=r/o unbuffered
if m.readfile=-1
  return 0
  endif
filelen  = FSEEK( m.readfile, 0, 2)
fclose( m.readfile )
return m.filelen