The question of listing a buffer's markers has been raised before:

https://debbugs.gnu.org/18#47
https://lists.gnu.org/archive/html/help-gnu-emacs/2016-06/msg00050.html
https://lists.gnu.org/archive/html/emacs-devel/2007-04/msg01391.html

I attach a patch implementing this based on BUF_MARKERS, as per Martin's
suggestion.  Any reasons not to expose such a function?

Thanks,

-- 
Basil

In GNU Emacs 27.0.50 (build 4, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2019-04-30 built on thunk
Repository revision: 910d170771ac74ab76d6dcb2dda3f3167e01b705
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description: Debian GNU/Linux buster/sid