Emacs 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 mode
line (see Variables Used in the Mode Line), 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.
This function returns non-nil if buffer has
been modified since it was last read in from a file or saved, or
nil otherwise. If buffer has been auto-saved since the
time it was last modified, this function returns the symbol
autosaved. If buffer is nil or omitted, it
defaults to the current buffer.
This function marks the current buffer as modified if flag is
non-nil, or as unmodified if the flag is nil.
Another effect of calling this function is to cause unconditional
redisplay of the mode line for the current buffer. In fact, the
function force-mode-line-update works by doing this:
(set-buffer-modified-p (buffer-modified-p))
Like set-buffer-modified-p, but does not force redisplay of
mode lines. This function also allows flag’s value to be
the symbol autosaved, which marks the buffer as modified and
auto-saved after the last modification.
This command marks the current buffer as unmodified, and not needing
to be saved. If arg is non-nil, it marks the buffer as
modified, so that it will be saved at the next suitable occasion.
Interactively, arg is the prefix argument.
Don’t use this function in programs, since it prints a message in the
echo area; use set-buffer-modified-p (above) instead.
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.
This function returns buffer’s character-change modification-count.
Changes to text properties leave this counter unchanged; however, each
time text is inserted or removed from the buffer, the counter is reset
to the value that would be returned by buffer-modified-tick.
By comparing the values returned by two buffer-chars-modified-tick
calls, you can tell whether a character change occurred in that buffer
in between the calls. If buffer is nil (or omitted), the
current buffer is used.
Sometimes there’s a need for modifying buffer in a way that doesn’t
really change its text, like if only its text properties are changed.
If your program needs to modify a buffer without triggering any hooks
and features that react to buffer modifications, use the
with-silent-modifications macro.
Execute body pretending it does not modify the buffer. This includes checking whether the buffer’s file is locked (see File Locks), running buffer modification hooks (see Change Hooks), etc. Note that if body actually modifies the buffer text (as opposed to its text properties), its undo data may become corrupted.