# HG changeset patch
# Parent 0505fe166dc67837469fabb459671f7040ca0656
diff --git a/dcrt0.cc b/dcrt0.cc
--- a/dcrt0.cc
+++ b/dcrt0.cc
@@ -792,6 +792,13 @@
   main_vfork = vfork_storage.create ();
 #endif
 
+  /* Windows doesn't use the lower 4MB of address space consistently,
+     and those uses arise before cygwin1.dll loads. If a dll loads
+     there we risk being unable to fork later. To avoid the problem,
+     we just reserve everything that's left in that space -- windows
+     can still do what it wants since it got there first. */
+  dlls.block_bad_address_space ();
+  
   cygbench ("pre-forkee");
   if (in_forkee)
     {
diff --git a/dll_init.cc b/dll_init.cc
--- a/dll_init.cc
+++ b/dll_init.cc
@@ -340,6 +340,44 @@
 #define A64K (64 * 1024)
 
 
+void
+dll_list::block_bad_address_space ()
+{
+  /* For some reason VirtualQuery doesn't return consistent values of
+     RegionSize for free space, so we have to compute it manually by
+     looking for MEM_FREE followed by a not-free region. We ensure not
+     to leave a danging free region by allowing the loop to examine
+     0x00400000, which is always the address of the application's
+     executable image.
+   */
+  MEMORY_BASIC_INFORMATION mb;
+  DWORD here;
+  for (DWORD i=A64K; i <= 64*A64K; i += mb.RegionSize)
+    {
+      if ( !VirtualQuery ((void*)i, &mb, sizeof(mb)))
+	api_fatal ("-> unable to examine address space at %08lx, %E", i);
+      here = (DWORD) mb.BaseAddress;
+      
+      /* this should never happen. If it does we'll need to write some
+	 code to compensate for it */
+      if (here != i)
+	api_fatal ("VirtualQuery returned info for %lx instead of %lx",
+		   here, i);
+      if (mb.State == MEM_FREE)
+	{
+	  DWORD size = mb.RegionSize, end = here + size;
+	  if (!VirtualAlloc ((void*) here, size, MEM_RESERVE, PAGE_NOACCESS))
+	    system_printf ("-> couldn't block out %08lx, %E", here);
+	}
+      else if (mb.RegionSize & (A64K-1))
+	{
+	  /* skip free space at the end of mapped slices -- they can't
+	     be used by anything else */
+	  mb.RegionSize = (mb.RegionSize + A64K - 1) & -A64K;
+	}
+    }
+}
+
 /* Reserve the chunk of free address space starting _here_ and (usually)
    covering at least _dll_size_ bytes. However, we must take care not
    to clobber the dll's target address range because it often overlaps.
diff --git a/dll_init.h b/dll_init.h
--- a/dll_init.h
+++ b/dll_init.h
@@ -82,6 +82,7 @@
   int tot;
   int loaded_dlls;
   int reload_on_fork;
+  void block_bad_address_space ();
   dll *operator [] (const PWCHAR name);
   dll *alloc (HINSTANCE, per_process *, dll_type);
   dll *find (void *);