stream-external-format stream → format

stream — a file stream.

format — an external file format.

Returns an external file format designator for the stream.

 (with-open-file (stream "test" :direction :output) 
   (stream-external-format stream)) 
→ :DEFAULT 
or → :ISO8859/1-1987 
or → (:ASCII :SAIL) 
or → ACME::PROPRIETARY-FILE-FORMAT-17 
or → #<FILE-FORMAT :ISO646-1983 2343673>

the :external-format argument to the function open and the with-open-file macro.

The format returned is not necessarily meaningful to other implementations.