Re: [Bug-guile-ncurses] effects in gc handlers

[John Darrington]

On Mon, May 02, 2016 at 03:08:44AM +0000, Mike Gran wrote:
     On Saturday, April 30, 2016 5:30 PM, "address@hidden" <address@hidden> 
wrote:
     
     >Hello!
     >
     >Somebody was having trouble with gc semantics in ncurses. They were 
     >asking about how to ensure X would not be garbage collected before Y. I 
     >believe that should never be an issue when writing scheme code.
     
     
     
     >So effects are being triggered by garbage collection. I suppose that 
     >this is done to try to make the library easier to use but in fact it 
     >cause an abstraction leak where programmers had to worry about GC 
     >semantics.
     
     
     Thanks for looking at this.  I'll consider it.
     

For the record, the user to whom rain1 is refering is me.

For background information, I was writing a small program like


(define stdscr (initscr))

;; Generate a window with a box around it and put another window inside it.
(let* ((h 4) (w 20) (sy 10) (sx 10)
       (win  (newwin h w sy sx))
       (inner  (derwin win (- h 2) (- w 4) 1 2))
       (panel-desc (new-panel win))
       (panel-desc2 (new-panel inner)))
  
  (box win (acs-vline) (acs-hline))
  (addstr inner "Hello world")

  (update-panels)
  (doupdate)

  (getch inner)

  (del-panel panel-desc)
  (del-panel panel-desc2)
  (endwin))


This works fine,  so I thought I could make a procedure to do it multiple times.


(define stdscr (initscr))

;; Generate a window with a box around it and return the inner box
(define (new-boxed-win h w sy sx)
  (let* ((win  (newwin h w sy sx))
         (inner  (derwin win (- h 2) (- w 4) 1 2))
         (panel-desc (new-panel win))
         (panel-desc2 (new-panel inner)))
    (box win (acs-vline) (acs-hline))
    ;; If the GC runs here, it will reap win, panel-desc and panal-desc2
    inner))


(let* ((w1 (new-boxed-win 3 20 10 10))
       (w2 (new-boxed-win 3 20 40 45)))

  (addstr w1 "Hello")
  (addstr w2 "Goodbye")

  (update-panels)
  (doupdate)
  (getch w1)
  
  (endwin))


Depending on when/if the garbage collector runs, this sometimes works, but 
sometimes displays nothing, because all panels, and outer windows have get 
garbage collected before they should be.

J'

     

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