Variable: buffer-file-number
buffer-file-number is a buffer-local variable defined in files.el.gz.
Documentation
The device number and file number of the file visited in the current buffer.
The value is a list of the form (FILENUM DEVNUM). This pair of numbers uniquely identifies the file. If the buffer is visiting a new file, the value is nil.
Source Code
;; Defined in /usr/src/emacs/lisp/files.el.gz
(defvar-local buffer-file-number nil
"The device number and file number of the file visited in the current buffer.
The value is a list of the form (FILENUM DEVNUM).
This pair of numbers uniquely identifies the file.
If the buffer is visiting a new file, the value is nil.")