Re: [Bug-guile-ncurses] GC bug (?) with menu's

[Mike Gran]

On Thu, Apr 12, 2018 at 04:45:18PM +0200, Thomas Danckaert wrote:
> Hi,
> 
> I'm writing a guile-ncurses program where I want to update a menu (using
> guile-ncurses 2.2).  This mostly works, but when refreshing the menu
> repeatedly, the program eventually segfaults.  I've attached a small example
> to illustrate the problem:
> 
> If you run the attached example, pressing 'r' updates the menu (here it just
> updates a counter in front of the menu items).  If you keep pressing 'r',
> the program eventually segfaults.  When exactly it happens is unpredictable,
> but shouldn't take more than a few seconds if you just hold down the 'r'
> key.  I assume the unpredictability is due to the garbage collector.
> 
> From the gdb stacktrace (and my limited understanding of the gc mechanisms),
> it seems that the list of menu items is somehow getting gc'ed prematurely:
> 
>      [New Thread 0x7ffecf65a700 (LWP 27338)]
>         11: Choice 2 Item 2
> Program received signal SIGSEGV, Segmentation fault.
> [Switching to Thread 0x7ffecf65a700 (LWP 27338)]
> gc_free_menu (x=0x83bc60) at menu_type.c:403
> 403             pitem_store[i]->left = NULL;
> 
> Is this a bug in the Guile-ncurses gc code, or am I making a wrong
> assumption in my code?

That's a bug, alright.

The work around is to not let your menu items be garbage collected
while the menu exists. Create a variable that holds onto your list
of menu items that has the same lifetime as the menu itself.


> 
> Thanks!
> 
> Thomas
> 
> ps Is there a mailing list for more general questions about guile-ncurses,
> or should I just post to the guile-user list?

Well, this e-mail gets to me, the maintainer.  I'm on guile-user, too.  And
in the IRC, I'm spk121.  So whatever works.



> (use-modules (srfi srfi-1)
>            (ice-9 format)
>            (ncurses curses)
>            (ncurses menu))
> 
> (define stdscr (initscr))
> (cbreak!)
> (noecho!)
> (keypad! stdscr #t)
 
> (define my-menu-win (newwin 10 40 4 4))
> (keypad! my-menu-win #t)
> 
> (define (make-menu i)
>   (let* ((names (map
>                (lambda (s) (string-append (format #f "~d: " i) s))
>                '("Choice 1" "Choice 2" "Choice 3" "Choice 4" "Exit")))
>        (descriptions '("Item 1" "Item 2" "Item 3" "Item 4" "")))
>     (new-menu (map (lambda (name desc) (new-item name desc))
>                  names descriptions))))
> 
> (with-throw-handler #t
>   (lambda ()
>     (let draw-menu ((i 0))
>       (let* ((my-menu (make-menu (+ 1 i))))
> 
>       ;; Set the main window and subwindow
>       (set-menu-win! my-menu my-menu-win)
>       (set-menu-sub! my-menu (derwin my-menu-win 6 38 3 1))
> 
>       ;; Set the menu mark string
>       (set-menu-mark! my-menu " * ")
> 
>       ;; Post the menu
>       (post-menu my-menu)
>       (refresh my-menu-win)
> 
>       ;; Process the up and down arrow keys.  Break the loop if F1 is
>       ;; pressed.  Ignore other keys.
>       (let loop ((c (getch my-menu-win)))
>         (cond
> 
>          ;; Move down the menu when down arrow is pressed and then loop.
>          ((eqv? c KEY_DOWN)
>           (begin
>             (menu-driver my-menu REQ_DOWN_ITEM)
>             (loop (getch my-menu-win))))
> 
>          ;; Move up the menu when the up arrow is pressed and then loop.
>          ((eqv? c KEY_UP)
>           (begin
>             (menu-driver my-menu REQ_UP_ITEM)
>             (loop (getch my-menu-win))))
> 
>          ((eqv? c #\r)
>           (begin
>             (unpost-menu my-menu)
>             (draw-menu (+ 1 i))))
> 
>          ((not (eqv? c (key-f 1)))
>           (loop (getch stdscr)))
>          ))))
>     )
>   (lambda (key . parameters)
>     (endwin)))
> 
> (endwin)
> 

Thanks,

- Mike Gran