In bash(and others) I can use C-x C-d or other keybindings to edit command using external editor defined in $EDITOR or $VISUAL.
However, when I use libreadline in my program, it doesn't have similar functionality by default.
How to enable it in my program? Or maybe it is bash specific fearure, not libreadline? How to implement it then?
That readline binding is probably "edit-and-execute-command" (C-x C-e for me, according to my man bash), and turns out it's basically an alias for the fc builtin command. I just took a look at bashline.c in Bash source code and found:
1 /* bashline.c -- Bash's interface to the readline library. */
...
440 /* Called once from parse.y if we are going to use readline. */
441 void
442 initialize_readline ()
443 {
...
482 rl_add_defun ("edit-and-execute-command", emacs_edit_and_execute_command, -1);
...
592 /* Bind C-xC-e to invoke emacs and run result as commands. */
593 rl_bind_key_if_unbound_in_map (CTRL ('E'), emacs_edit_and_execute_command, emacs_ctlx_keymap);
594 #if defined (VI_MODE)
595 rl_bind_key_if_unbound_in_map ('v', vi_edit_and_execute_command, vi_movement_keymap);
...
924 #define VI_EDIT_COMMAND "fc -e \"${VISUAL:-${EDITOR:-vi}}\""
925 #define EMACS_EDIT_COMMAND "fc -e \"${VISUAL:-${EDITOR:-emacs}}\""
926 #define POSIX_VI_EDIT_COMMAND "fc -e vi"
...
1003 #if defined (VI_MODE)
1004 static int
1005 vi_edit_and_execute_command (count, c)
1006 int count, c;
1007 {
1008 if (posixly_correct)
1009 return (edit_and_execute_command (count, c, VI_EDITING_MODE, POSIX_VI_EDIT_COMMAND));
1010 else
1011 return (edit_and_execute_command (count, c, VI_EDITING_MODE, VI_EDIT_COMMAND));
1012 }
1013 #endif /* VI_MODE */
1014
1015 static int
1016 emacs_edit_and_execute_command (count, c)
1017 int count, c;
1018 {
1019 return (edit_and_execute_command (count, c, EMACS_EDITING_MODE, EMACS_EDIT_COMMAND));
1020 }
....