file-length

(file-length stream)

The byte length of the file a file stream is open on, or nil when it cannot be determined. Every other stream answers nil: a string stream, a socket, one of the standard streams, and a handle that has already been closed. An output stream is flushed first, so the answer counts what has been written rather than what happens to have reached the disk.

Both WASM backends always answer nil, file streams included: no WASI filestat call is imported there. nil is exactly Common Lisp's answer for "the length cannot be determined", so a portable caller takes its unknown-length fallback rather than failing. The interpreter and the JVM answer for real.

(with-open-file (in "data.txt")
  (print (file-length in)))