Variable: buffer-file-name
buffer-file-name is a buffer-local variable defined in buffer.c.
Documentation
Name of file visited in current buffer, or nil if not visiting a file.
This should be an absolute file name.
Probably introduced at or before Emacs version 17.
Source Code
// Defined in /usr/src/emacs/src/buffer.c
DEFVAR_PER_BUFFER ("buffer-file-name", &BVAR (current_buffer, filename),
Qstringp,
doc: /* Name of file visited in current buffer, or nil if not visiting a file.
This should be an absolute file name. */);