Re: Windows 9X crash

[Jussi Lahdenniemi <jussi@aprikoodi.fi>]

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On 15.1.2016 10.08, Eli Zaretskii wrote:
> Right, this can explain everything.  It probably means that a build
> configured --with-wide-int should also be tested there, as it has
> different alignment needs.

I'll test that as well, then.

>> How should we fix this?  I can write and test the fix, but I'd like to
>> hear your opinion on the preferred mechanism.
>
> What fix did you have in mind?  Over-allocating and recording the
> offset in the initial part of the block that we don't pass to the
> application?

That was my first idea, yes.  Of course, this introduces some memory 
overhead, especially if there are many small allocations.

A proposal patch is attached.  Reallocs needed some additional care, as 
there are two special cases to handle (source being in dumped memory, 
and source and destination alignments mismatching).

-- 
Jussi Lahdenniemi


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diff --git a/nt/inc/ms-w32.h b/nt/inc/ms-w32.h
index cdbfac0..a37510e 100644
--- a/nt/inc/ms-w32.h
+++ b/nt/inc/ms-w32.h
@@ -457,6 +457,10 @@ extern void *malloc_after_dump(size_t);
 extern void *realloc_after_dump(void *, size_t);
 extern void free_after_dump(void *);
 
+extern void *malloc_after_dump_9x(size_t);
+extern void *realloc_after_dump_9x(void *, size_t);
+extern void free_after_dump_9x(void *);
+
 extern malloc_fn the_malloc_fn;
 extern realloc_fn the_realloc_fn;
 extern free_fn the_free_fn;
diff --git a/src/w32heap.c b/src/w32heap.c
index 54646bf..3f6ed67 100644
--- a/src/w32heap.c
+++ b/src/w32heap.c
@@ -258,9 +258,18 @@ init_heap (void)
 	}
 #endif
 
-      the_malloc_fn = malloc_after_dump;
-      the_realloc_fn = realloc_after_dump;
-      the_free_fn = free_after_dump;
+      if (os_subtype == OS_9X)
+        {
+          the_malloc_fn = malloc_after_dump_9x;
+          the_realloc_fn = realloc_after_dump_9x;
+          the_free_fn = free_after_dump_9x;
+        }
+      else
+        {
+          the_malloc_fn = malloc_after_dump;
+          the_realloc_fn = realloc_after_dump;
+          the_free_fn = free_after_dump;
+        }
     }
   else
     {
@@ -291,9 +300,18 @@ init_heap (void)
 	  exit (-1);
 	}
       heap = s_pfn_Rtl_Create_Heap (0, data_region_base, 0, 0, NULL, &params);
-      the_malloc_fn = malloc_before_dump;
-      the_realloc_fn = realloc_before_dump;
-      the_free_fn = free_before_dump;
+
+      if (os_subtype == OS_9X)
+        {
+          fprintf (stderr, "Cannot run pre-dump Emacs on Windows 9X.\n" );
+          exit (-1);
+        }
+      else
+        {
+          the_malloc_fn = malloc_before_dump;
+          the_realloc_fn = realloc_before_dump;
+          the_free_fn = free_before_dump;
+        }
     }
 
   /* Update system version information to match current system.  */
@@ -504,6 +522,65 @@ free_before_dump (void *ptr)
     }
 }
 
+/* On Windows 98, HeapAlloc may return pointers that are not aligned
+   to 8-byte boundary, which is expected by the Lisp memory
+   management.  To circumvent this problem, manually enforce alignment
+   on Windows 9X.  */
+
+void *
+malloc_after_dump_9x (size_t size)
+{
+  void *p = malloc_after_dump (size + 8);
+  void *pa;
+  if (p == NULL)
+    return p;
+  pa = (void*)(((intptr_t)p + 8) & ~7);
+  *((void**)pa-1) = p;
+  return pa;
+}
+
+void *
+realloc_after_dump_9x (void *ptr, size_t size)
+{
+  if (FREEABLE_P (ptr))
+    {
+      void *po = *((void**)ptr-1);
+      void *p;
+      void *pa;
+      p = realloc_after_dump (po, size + 8);
+      if (p == NULL)
+        return p;
+      pa = (void*)(((intptr_t)p + 8) & ~7);
+      if (ptr != NULL &&
+          (char*)pa - (char*)p != (char*)ptr - (char*)po)
+        {
+          /* Handle the case where alignment in pre-realloc and
+             post-realloc blocks do not match.  */
+          MoveMemory (pa, (void*)((char*)p + ((char*)ptr - (char*)po)), size);
+        }
+      *((void**)pa-1) = p;
+      return pa;
+    }
+  else
+    {
+      /* Non-freeable pointers have no alignment-enforcing header
+         (since dumping is not allowed on Windows 9X).  */
+      void* p = malloc_after_dump_9x (size);
+      if (p != NULL)
+	CopyMemory (p, ptr, size);
+      return p;
+    }
+}
+
+void
+free_after_dump_9x (void *ptr)
+{
+  if (FREEABLE_P (ptr))
+    {
+      free_after_dump (*((void**)ptr-1));
+    }
+}
+
 #ifdef ENABLE_CHECKING
 void
 report_temacs_memory_usage (void)