file-length

file-length stream → length

stream — a stream associated with a file.

length — a non-negative integer or nil.

file-length returns the length of stream, or nil if the length cannot be determined.

For a binary file, the length is measured in units of the element type of the stream.

 (with-open-file (s "decimal-digits.text" 
                    :direction :output :if-exists :error) 
   (princ "0123456789" s) 
   (truename s)) 
→ #P"A:>Joe>decimal-digits.text.1" 
 (with-open-file (s "decimal-digits.text") 
   (file-length s)) 
→ 10

Should signal an error of type type-error if stream is not a stream associated with a file.

open