Re: Is there an efficient way to get file length w/o reading file?

An intelligent implementation could avoid reading the file.
But otherwise, the answer is that it's not possible portably.
Drop down to the POSIX API.

>
How does one do that? Is there an example somewhere?

 
(defun file-length-in-bytes (file)
    ;; Note the "EXT" implementation specific package.
    (with-open-stream (in (ext:run-program "/bin/ls"
                             :arguments (list "-l" (namestring file))
                             :output :stream))
        (read in)(read in)(read in)(read in)(read in)))
 
Alternatively, one could use FFI:
 
(defun file-length-in-bytes (file)
    (multiple-value-bind (res stat) (linux:stat (namestring file))
        (linux:|stat-st_size| stat)))