make-string-output-stream &key element-type → string-stream

element-type — a type specifier. The default is character.

string-stream — an output string stream.

Returns an output string stream that accepts characters and makes available (via get-output-stream-string) a string that contains the characters that were actually output.

The element-type names the type of the elements of the string; a string is constructed of the most specialized type that can accommodate elements of that element-type.

 (let ((s (make-string-output-stream))) 
   (write-string "testing... " s) 
   (prin1 1234 s) 
   (get-output-stream-string s)) 
→ "testing... 1234"

None..

get-output-stream-string, with-output-to-string