listen

(listen &optional stream)

Returns t when a character or byte is immediately available on the designated input stream (standard input with no argument), nil otherwise. It never blocks: this is the available()/ready() question, which is what a protocol implementation asks to detect unexpected data -- cl-postgres uses it to spot an oversized SSL response.

On the interpreter and the JVM a socket answers from the kernel receive buffer, so the answer is exact. On the WASM --component backend the answer is exact for a socket too, but for a different reason: it reports whether the socket's already-read chunk still holds unconsumed bytes -- bytes still waiting host-side are not observable there without blocking. A string input stream answers whether a character remains on every backend (an unread character counts); on Preview 1 WASM anything else still rejects listen at compile time, since no non-blocking probe exists there.

(listen)          ; => NIL, with no pending standard input