src/code/weak.lisp

   1 ;;;; weak pointer support
   2
   3 ;;;; This software is part of the SBCL system. See the README file for
   4 ;;;; more information.
   5 ;;;;
   6 ;;;; This software is derived from the CMU CL system, which was
   7 ;;;; written at Carnegie Mellon University and released into the
   8 ;;;; public domain. The software is in the public domain and is
   9 ;;;; provided with absolutely no warranty. See the COPYING and CREDITS
  10 ;;;; files for more information.
  11
  12 (in-package "SB!IMPL")
  13
  14 (defun make-weak-pointer (object)
  15   #!+sb-doc
  16   "Allocate and return a weak pointer which points to OBJECT."
  17   (declare (values weak-pointer))
  18   (make-weak-pointer object))
  19
  20 #!-sb-fluid (declaim (inline weak-pointer-value))
  21 (defun weak-pointer-value (weak-pointer)
  22   #!+sb-doc
  23   "If WEAK-POINTER is valid, return the value of WEAK-POINTER and T.
  24    If the referent of WEAK-POINTER has been garbage collected, returns
  25    the values NIL and NIL."
  26   (declare (type weak-pointer weak-pointer)
  27            (values t (member t nil)))
  28   ;; We don't need to wrap this with a WITHOUT-GCING, because once we
  29   ;; have extracted the value, our reference to it will keep the weak
  30   ;; pointer from becoming broken. We just have to make sure the
  31   ;; compiler won't reorder these primitives.
  32   ;;
  33   ;; FIXME: Might it be a good idea to tweak the DEFKNOWNs for
  34   ;; %WEAK-POINTER-VALUE and %WEAK-POINTER-BROKEN, so that the
  35   ;; compiler will never try to reorder them even in code where we
  36   ;; neglect to frame them in a LET?
  37   (let ((value (sb!c::%weak-pointer-value weak-pointer))
  38         (broken (sb!c::%weak-pointer-broken weak-pointer)))
  39     (values value (not broken))))