[emacs] how to know current buffer have set-mark

Question

is there any way know there is set-mark beginning and where is the start point query in lisp

Answer 1

I'd recommend turning on transient-mark-mode

(setq transient-mark-mode t)

transient-mark-mode will highlight the region between the mark and your current point.

Alternatively, you can press C-x C-x to jump between the current point and the mark to see where the mark is set.

Answer 2

Use the variable mark-active:

mark-active is a variable defined in `C source code'.
Its value is nil
Local in buffer whole-line-or-region.el; global value is nil

  Automatically becomes buffer-local when set in any fashion.

Documentation:
Non-nil means the mark and region are currently active in this buffer.

You might also want to check if mark === point, if it's really a region you're looking for:

(if (and mark-active
    (/= (point) (mark)))

If you want to write a function that requires a region be defined, you can use interactive, like so:

(defun my-fn-that-requires-a-region (beg end)
  "Some documentation string mentioning BEG and END."
  (interactive "r")
  (message "%d / %d" beg end))

If called interactively, mark must be set or an error is generated. Called programatically, any two values must be passed in; no validation of the parameters is done.