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37.5 Buffer Modification

SXEmacs keeps a flag called the modified flag for each buffer, to record whether you have changed the text of the buffer. This flag is set to t whenever you alter the contents of the buffer, and cleared to nil when you save it. Thus, the flag shows whether there are unsaved changes. The flag value is normally shown in the modeline (see Modeline Variables), and controls saving (see Saving Buffers) and auto-saving (see Auto-Saving).

Some Lisp programs set the flag explicitly. For example, the function set-visited-file-name sets the flag to t, because the text does not match the newly-visited file, even if it is unchanged from the file formerly visited.

The functions that modify the contents of buffers are described in Text.

— Function: buffer-modified-p &optional buffer

This function returns t if the buffer buffer has been modified since it was last read in from a file or saved, or nil otherwise. If buffer is not supplied, the current buffer is tested.

— Function: set-buffer-modified-p flag &optional buffer

This function marks buffer as modified if flag is non-nil, or as unmodified if the flag is nil. buffer defaults to the current buffer.

Another effect of calling this function is to cause unconditional redisplay of the modeline for the current buffer. In fact, the function redraw-modeline works by doing this: 

          (set-buffer-modified-p (buffer-modified-p))
— Command: not-modified &optional arg

This command marks the current buffer as unmodified, and not needing to be saved. (If arg is non-nil, the buffer is instead marked as modified.) Don't use this function in programs, since it prints a message in the echo area; use set-buffer-modified-p (above) instead.

— Function: buffer-modified-tick &optional buffer

This function returns buffer`s modification-count. This is a counter that increments every time the buffer is modified. If buffer is nil (or omitted), the current buffer is used.