0.9.8.38:

[sbcl.git] / BUGS

diff --git a/BUGS b/BUGS
index 1c20aab..bc78b72 100644 (file)
--- a/BUGS
+++ b/BUGS
@@ -2122,3 +2122,31 @@ WORKAROUND:
                                              (force-output)
                                              (sb-ext:gc))))
               (sleep 1))))
+
+398: GC-unsafe SB-ALIEN string deporting
+  Translating a Lisp string to an alien string by taking a SAP to it
+  as done by the :DEPORT-GEN methods for C-STRING and UTF8-STRING
+  is not safe, since the Lisp string can move. For example the
+  following code will fail quickly on both cheneygc and pre-0.9.8.19
+  GENCGC: 
+
+  (setf (bytes-consed-between-gcs) 4096)
+  (define-alien-routine "strcmp" int (s1 c-string) (s2 c-string))
+     
+  (loop
+    (let ((string "hello, world"))
+       (assert (zerop (strcmp string string)))))
+     
+  (This will appear to work on post-0.9.8.19 GENCGC, since
+   the GC no longer zeroes memory immediately after releasing
+   it after a minor GC. Either enabling the READ_PROTECT_FREE_PAGES
+   #define in gencgc.c or modifying the example so that a major
+   GC will occasionally be triggered would unmask the bug.)
+
+  On cheneygc the only solution would seem to be allocating some alien
+  memory, copying the data over, and arranging that it's freed once we
+  return. For GENCGC we could instead try to arrange that the string
+  from which the SAP is taken is always pinned.
+
+  For some more details see comments for (define-alien-type-method
+  (c-string :deport-gen) ...)  in host-c-call.lisp.