With emacs if there is a backtrace buffer open, new backtrace is ignored. I find it is unconvenient when I am trying to find out which function/hook changes a variable. I use debug-watch as described in https://stackoverflow.com/questions/62579200/find-out-when-a-variable-is-changed-in-emacs The problem is the variable changes happens after a backtrace buffer popup is impossible to know because of the backtrace blocking described above.

(debug-watch VARIABLE)

How can I make emacs popup as many backtrace buffers as possible or log the backtraces to a file?

  • I am not sure what you mean with 'new backtrace is ignored' but you can exit the debugger and continue by pressing c (see the section debugger commands in the elisp manual). May 25, 2022 at 11:47
  • It takes time between debugger popping up and me pressing c, during that time, there is possible other backtraces buried without notice. I have come across this situation. :-(
    – godblessfq
    May 26, 2022 at 2:54

2 Answers 2


I'm not sure whether this helps answer your request, but you can use function backtrace directly, to print a backtrace at any time to standard-output (which you can bind to a buffer etc.). C-h f backtrace:

backtrace is a compiled Lisp function in subr.el.


Print a trace of Lisp function calls currently active.

Output stream used is value of standard-output.

See the Elisp manual, node Internals of Debugger, for more info about this, including and example information about variable debugger-stack-frame-as-list.

  • It surely helps in debugging one specific package. But often we don't know where is the problem. Like bookmark+ changing buffer local outline-regex and comment-start when saving bookmarks, while making outline-minor-mode crazy.
    – godblessfq
    May 26, 2022 at 2:58
  • Sorry, but I don't recognize anything you say in your comment about Bookmark+ saving bookmarks. If you think you've found bugs, please report them specifically. AFAIK, Bookmark+ does nothing wrt out-line-* or comment-start when saving bookmarks or any other time. The only occurrence of outline-* in its code is in the doc string for bookmark-after-jump-hook, where it just suggests, as an example, that you can use the hook to unhide text in outline-mode. And there are no occurrences of comment-start in the code.
    – Drew
    May 26, 2022 at 14:34
  • Bookmark+ seems to open the bookmark file in the background, and during processing of the file, it enables other mode that changes those variables. I disabled bookmark+ and everything is good now. I will look into it further later.
    – godblessfq
    May 27, 2022 at 0:36
  • I think I have got the backtrace: gist.github.com/QiangF/706e6b103c668e87b8ae79fdc76700ea
    – godblessfq
    May 27, 2022 at 0:44
  • Thanks, but I can't do anything with that really. You seem to have a lot of stuff going on, and it's not clear what - option customizations, key bindings, hydras, etc. Please (1) use M-x bmkp-send-bug-report to report the problem. But (2) do so with a minimal recipe, starting from emacs -Q to repro it.
    – Drew
    May 27, 2022 at 1:28

Enable debug-on-error and use at your own risk; I've given it the most rudimentary of testing, but wouldn't have the foggiest idea how robust this would be.

(defun my-debug-to-file (&rest _args)
  (let ((inhibit-debugger t)
        (backtrace (backtrace-get-frames 'debug))
        (print-length nil))
      (insert (backtrace-to-string backtrace))
      (insert "\n------------------------------------------------------\n\n")
      (append-to-file nil nil "/tmp/backtrace.log"))))

(advice-add 'debug :override #'my-debug-to-file)
;; (advice-remove 'debug #'my-debug-to-file)

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