file-length

(file-length stream)

The length of the file a file stream is open on, in the stream's ELEMENTS (bytes, unless it was opened with a wider integer :element-type -- see open), 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.

All four backends answer for real. The interpreter and the JVM stat the path the stream was opened with; both WASM backends stat the descriptor itself (Preview 1 through fd_filestat_get, the component through wasi:filesystem's descriptor.stat), and answer nil only for what genuinely has no length -- which is the same set the other two answer nil for. Anything the host does not report as a regular file is nil as well.

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