How to get buffer byte length (not number of characters)?

[Joseph Turner]

Hello!

`buffer-size' returns the number of characters in a buffer:

(with-temp-buffer
  (insert "你好")
  (buffer-size)) ;; 2

However, the buffer's byte length may be larger:

(let* ((filename (make-temp-file "buffer-size-test-"))
       (file (with-temp-file filename (insert "你好"))))
  (file-attribute-size (file-attributes filename))) ;; 6

How can I get a buffer's byte length without writing to a file?

Thank you!

Joseph