file-position

(file-position stream [position])

With one argument, the current position of a file stream. A binary one — opened with an integer :element-type — counts in its elements (bytes for '(unsigned-byte 8)). A character one counts bytes, as SBCL does: a character advances it by its UTF-8 length, a line read by read-line ends after its terminator (both bytes of a CRLF), a character peek-char looked at or unread-char pushed back is not consumed yet, and a stream opened with :if-exists :append starts at the end of the file. With two arguments, it repositions the stream to position and answers t; the next read or write starts there. position may also be :start or :end.

A string stream has a position too, counted in characters. An input stream counts the characters read from its own start -- a stream over part of a string starts at 0 -- and repositions to an index, :start or :end; an index past the end answers nil and leaves the stream where it was. A character given back with unread-char counts as not yet read. An output stream counts the characters written since get-output-stream-string last emptied it, and cannot be repositioned anywhere but where it already is.

Anything whose position cannot be determined answers nil, which is what Common Lisp prescribes for exactly that: a socket, one of the standard streams, and a handle that has already been closed. Portable callers guard the call with ignore-errors and take their non-seeking fallback path on nil.

All four backends answer for real. The interpreter and the JVM count what the byte primitives move for a binary stream, read the channel offset of a character or bidirectional stream, and reopen or reposition the file for the set. Preview 1 WASM queries and moves the descriptor's own cursor through fd_seek; the component backend has no cursor — WASI 0.3 reads are offset-based — so it goes through a per-descriptor byte offset the adapter tracks.

(with-open-file (in "data.bin" :element-type '(unsigned-byte 8))
  (print (file-position in 5))
  (print (read-byte in))
  (print (file-position in)))