open-stream-p

(open-stream-p stream)

Returns t while the stream is open and nil after it has been closed -- the question the close-if-open idiom asks so it neither double-closes nor leaks. The answer is the same on every backend, for file, string and socket streams alike; a synonym stream answers what its target answers, and the t designator is always open. The interpreter and the JVM answer from the stream table (a close removes the entry) and additionally report nil for a socket closed from the other side; the WASM backends read a mark close sets on the stream value, since a descriptor is reused by the next open.

The close case touches a file, so it is shown statically:

(let ((s (open "f.txt" :direction :input)))
  (open-stream-p s)   ; => T
  (close s)
  (open-stream-p s))  ; => NIL