diff --git a/winsup/cygwin/ChangeLog b/winsup/cygwin/ChangeLog
index 4922d6120..89a97ea1a 100644
--- a/winsup/cygwin/ChangeLog
+++ b/winsup/cygwin/ChangeLog
@@ -1,3 +1,47 @@
+2009-10-31  Corinna Vinschen  <corinna@vinschen.de>
+
+	* cygprops.h: New file.
+	* dtable.cc (handle_to_fn): Add check for correct installation_key
+	string in object name for pipes and ttys.
+	* external.cc (cygwin_internal): Add CW_GET_INSTKEY to allow fetching
+	the installation_key from cygserver.
+	* fhandler_fifo.cc (fhandler_fifo::fifo_name): Add installation_key
+	to fifo name.
+	* globals.cc: Include cygprops.h.
+	(_RDATA): Move slightly and add comment.
+	(cygwin_props): Define.
+	* mount.cc (mount_info::init): Accommodate the fact that
+	installation_root is now a global variable in DLL common shared memory,
+	rather than a member of cygwin_shared.
+	* pipe.cc (fhandler_pipe::create_selectable): Add installation_key to
+	pipe name.
+	* shared.cc (installation_root): Define here for storage in DLL
+	common shared memory.
+	(installation_key): Ditto.
+	(installation_key_buf): Ditto.
+	(init_installation_root): Convert from shared_info method to ordinary
+	function.  Add initializing installation_key.  Invalidate
+	installation_key depending of value of disable_key property.  Add
+	comment to explain.
+	(get_shared_parent_dir): Add installation_key to directory name.
+	(get_session_parent_dir): Ditto.
+	(shared_info::initialize): Move call to init_installation_root from
+	here...
+	(memory_init): ...to here.  Add debug output to print installation root
+	and installation key.  Add comment to explain why.
+	* shared_info.h (SHARED_INFO_CB): Recalculate.
+	(CURR_SHARED_MAGIC): Ditto.
+	(class shared_info): Remove definition of installation_root and
+	declaration of init_installation_root.
+	(init_installation_root): Declare.
+	(installation_root): Declare.
+	(installation_key): Declare.
+	* uinfo.cc (pwdgrp::load): Accommodate the fact that installation_root
+	is now a global variable in DLL common shared memory.
+	* include/cygwin/version.h: Bump API minor number.
+	(CYGWIN_INFO_INSTALLATIONS_NAME): Add.
+	* include/sys/cygwin.h (cygwin_getinfo_types): Add CW_GET_INSTKEY.
+
 2009-10-31  Corinna Vinschen  <corinna@vinschen.de>
 
 	* sec_helper.cc (security_descriptor::realloc): Call free first if
diff --git a/winsup/cygwin/cygprops.h b/winsup/cygwin/cygprops.h
new file mode 100644
index 000000000..fa367bc5b
--- /dev/null
+++ b/winsup/cygwin/cygprops.h
@@ -0,0 +1,20 @@
+/* cygprops.h: Cygwin DLL properties
+
+   Copyright 2009 Red Hat, Inc.
+
+This file is part of Cygwin.
+
+This software is a copyrighted work licensed under the terms of the
+Cygwin license.  Please consult the file "CYGWIN_LICENSE" for
+details. */
+
+/* DLL properties data. */
+struct cygwin_props_t
+{
+  char magic[68];
+  ULONG size;
+  ULONG disable_key;
+};
+
+#define CYGWIN_PROPS_MAGIC \
+  "Fortunately, I keep my feathers numbered for just such an emergency"
diff --git a/winsup/cygwin/dtable.cc b/winsup/cygwin/dtable.cc
index ab4d1fbfd..61be7fc97 100644
--- a/winsup/cygwin/dtable.cc
+++ b/winsup/cygwin/dtable.cc
@@ -929,6 +929,14 @@ handle_to_fn (HANDLE h, char *posix_fn)
 	  if (wcsncmp (w32, L"cygwin-", WCLEN (L"cygwin-")) != 0)
 	    return false;
 	  w32 += WCLEN (L"cygwin-");
+	  /* Check for installation key and trailing dash. */
+	  w32len = installation_key.Length / sizeof (WCHAR);
+	  if (w32len && wcsncmp (w32, installation_key.Buffer, w32len) != 0)
+	    return false;
+	  w32 += w32len;
+	  if (*w32 != L'-')
+	    return false;
+	  ++w32;
 	  bool istty = wcsncmp (w32, L"tty", WCLEN (L"tty")) == 0;
 	  if (istty)
 	    decode_tty (posix_fn, w32 + WCLEN (L"tty"));
diff --git a/winsup/cygwin/external.cc b/winsup/cygwin/external.cc
index 0b607f178..4d7f03d1b 100644
--- a/winsup/cygwin/external.cc
+++ b/winsup/cygwin/external.cc
@@ -460,6 +460,14 @@ cygwin_internal (cygwin_getinfo_types t, ...)
 	  res = 0;
 	}
 	break;
+      case CW_GET_INSTKEY:
+	{
+	  extern WCHAR installation_key_buf[18];
+	  PWCHAR dest = va_arg (arg, PWCHAR);
+	  wcscpy (dest, installation_key_buf);
+	  res = 0;
+	}
+	break;
 
       default:
 	set_errno (ENOSYS);
diff --git a/winsup/cygwin/fhandler_fifo.cc b/winsup/cygwin/fhandler_fifo.cc
index d96616fc5..b612094dd 100644
--- a/winsup/cygwin/fhandler_fifo.cc
+++ b/winsup/cygwin/fhandler_fifo.cc
@@ -19,6 +19,7 @@
 #include "cygheap.h"
 #include "sigproc.h"
 #include "cygtls.h"
+#include "shared_info.h"
 
 fhandler_fifo::fhandler_fifo ():
   wait_state (fifo_unknown), dummy_client (NULL)
@@ -58,8 +59,8 @@ char *
 fhandler_fifo::fifo_name (char *buf)
 {
   /* Generate a semi-unique name to associate with this fifo. */
-  __small_sprintf (buf, "\\\\.\\pipe\\__cygfifo__%08x_%016X",
-		   get_dev (), get_ino ());
+  __small_sprintf (buf, "\\\\.\\pipe\\__cygfifo__%S_%08x_%016X",
+		   &installation_key, get_dev (), get_ino ());
   return buf;
 }
 
diff --git a/winsup/cygwin/globals.cc b/winsup/cygwin/globals.cc
index a1a4ae946..f30b6d02a 100644
--- a/winsup/cygwin/globals.cc
+++ b/winsup/cygwin/globals.cc
@@ -13,6 +13,7 @@ details. */
 #include "winsup.h"
 #include "cygtls.h"
 #include "perprocess.h"
+#include "cygprops.h"
 #include "thread.h"
 #include <malloc.h>
 #include <cygwin/version.h>
@@ -72,6 +73,9 @@ char NO_COPY almost_null[1];
 
 char *old_title;
 
+/* Define globally used, but readonly variables using the _RDATA attribute. */
+#define _RDATA __attribute__ ((section(".rdata")))
+
 /* Heavily-used const UNICODE_STRINGs are defined here once.  The idea is a
    speed improvement by not having to initialize a UNICODE_STRING every time
    we make a string comparison.  The strings are not defined as const,
@@ -83,7 +87,6 @@ char *old_title;
         { Length: sizeof (_s) - sizeof (WCHAR), \
           MaximumLength: sizeof (_s), \
           Buffer: (PWSTR) (_s) }
-#define _RDATA __attribute__ ((section(".rdata")))
 UNICODE_STRING _RDATA ro_u_empty = _ROU (L"");
 UNICODE_STRING _RDATA ro_u_lnk = _ROU (L".lnk");
 UNICODE_STRING _RDATA ro_u_exe = _ROU (L".exe");
@@ -102,9 +105,22 @@ UNICODE_STRING _RDATA ro_u_sunwnfs = _ROU (L"SUNWNFS");
 UNICODE_STRING _RDATA ro_u_udf = _ROU (L"UDF");
 UNICODE_STRING _RDATA ro_u_unixfs = _ROU (L"UNIXFS");
 UNICODE_STRING _RDATA ro_u_volume = _ROU (L"\\??\\Volume{");
-#undef _RDATA
 #undef _ROU
 
+/* Cygwin properties are meant to be readonly data placed in the DLL, but
+   which can be changed by external tools to make adjustments to the
+   behaviour of a DLL based on the binary of the DLL itself.  This is
+   different from $CYGWIN since it only affects that very DLL, not all
+   DLLs which have access to the $CYGWIN environment variable. */
+cygwin_props_t _RDATA cygwin_props =
+{
+  CYGWIN_PROPS_MAGIC,
+  sizeof (cygwin_props_t),
+  0
+};
+
+#undef _RDATA
+
 extern "C"
 {
   /* This is an exported copy of environ which can be used by DLLs
diff --git a/winsup/cygwin/include/cygwin/version.h b/winsup/cygwin/include/cygwin/version.h
index 89f8587ae..e88bb9d9a 100644
--- a/winsup/cygwin/include/cygwin/version.h
+++ b/winsup/cygwin/include/cygwin/version.h
@@ -370,12 +370,13 @@ details. */
       214: Export execvpe, fexecve.
       215: CW_EXIT_PROCESS added.
       216: CW_SET_EXTERNAL_TOKEN added.
+      217: CW_GET_INSTKEY added.
      */
 
      /* Note that we forgot to bump the api for ualarm, strtoll, strtoull */
 
 #define CYGWIN_VERSION_API_MAJOR 0
-#define CYGWIN_VERSION_API_MINOR 216
+#define CYGWIN_VERSION_API_MINOR 217
 
      /* There is also a compatibity version number associated with the
 	shared memory regions.  It is incremented when incompatible
@@ -410,6 +411,7 @@ details. */
 
 #define CYGWIN_INFO_CYGWIN_REGISTRY_NAME "Cygwin"
 #define CYGWIN_INFO_PROGRAM_OPTIONS_NAME "Program Options"
+#define CYGWIN_INFO_INSTALLATIONS_NAME   "Installations"
 
      /* The default cygdrive prefix. */
 
diff --git a/winsup/cygwin/include/sys/cygwin.h b/winsup/cygwin/include/sys/cygwin.h
index ce9bebfdd..60fc9f05e 100644
--- a/winsup/cygwin/include/sys/cygwin.h
+++ b/winsup/cygwin/include/sys/cygwin.h
@@ -144,7 +144,8 @@ typedef enum
     CW_SET_PRIV_KEY,
     CW_SETERRNO,
     CW_EXIT_PROCESS,
-    CW_SET_EXTERNAL_TOKEN
+    CW_SET_EXTERNAL_TOKEN,
+    CW_GET_INSTKEY
   } cygwin_getinfo_types;
 
 /* Token type for CW_SET_EXTERNAL_TOKEN */
diff --git a/winsup/cygwin/mount.cc b/winsup/cygwin/mount.cc
index a0b4b6782..8c9b72a2f 100644
--- a/winsup/cygwin/mount.cc
+++ b/winsup/cygwin/mount.cc
@@ -350,7 +350,7 @@ mount_info::init ()
   PWCHAR pathend;
   WCHAR path[PATH_MAX];
 
-  pathend = wcpcpy (path, cygwin_shared->installation_root);
+  pathend = wcpcpy (path, installation_root);
   create_root_entry (path);
   pathend = wcpcpy (pathend, L"\\etc\\fstab");
 
diff --git a/winsup/cygwin/pipe.cc b/winsup/cygwin/pipe.cc
index 5e557d8c6..aad1531d1 100644
--- a/winsup/cygwin/pipe.cc
+++ b/winsup/cygwin/pipe.cc
@@ -21,6 +21,7 @@ details. */
 #include "dtable.h"
 #include "cygheap.h"
 #include "pinfo.h"
+#include "shared_info.h"
 
 fhandler_pipe::fhandler_pipe ()
   : fhandler_base (), popen_pid (0), overlapped (NULL)
@@ -216,7 +217,10 @@ fhandler_pipe::create_selectable (LPSECURITY_ATTRIBUTES sa_ptr, HANDLE& r,
   if (psize < PIPE_BUF)
     psize = PIPE_BUF;
 
-  char pipename[MAX_PATH] = PIPE_INTRO;
+  char pipename[MAX_PATH];
+  const size_t len = __small_sprintf (pipename, PIPE_INTRO "%S-",
+				      &installation_key);
+
   /* FIXME: Eventually make ttys work with overlapped I/O. */
   DWORD overlapped = name ? 0 : FILE_FLAG_OVERLAPPED;
 
@@ -228,10 +232,10 @@ fhandler_pipe::create_selectable (LPSECURITY_ATTRIBUTES sa_ptr, HANDLE& r,
     {
       static volatile ULONG pipe_unique_id;
       if (!name)
-	__small_sprintf (pipename + strlen(PIPE_INTRO), "pipe-%p-%p", myself->pid,
+	__small_sprintf (pipename + len, "pipe-%p-%p", myself->pid,
 			InterlockedIncrement ((LONG *) &pipe_unique_id));
       else
-	strcpy (pipename + strlen(PIPE_INTRO), name);
+	strcpy (pipename + len, name);
 
       debug_printf ("CreateNamedPipe: name %s, size %lu", pipename, psize);
 
diff --git a/winsup/cygwin/shared.cc b/winsup/cygwin/shared.cc
index aad6c9499..ed3021cab 100644
--- a/winsup/cygwin/shared.cc
+++ b/winsup/cygwin/shared.cc
@@ -26,12 +26,96 @@ details. */
 #include "ntdll.h"
 #include <alloca.h>
 #include <wchar.h>
+#include <wingdi.h>
+#include <winuser.h>
 
 shared_info NO_COPY *cygwin_shared;
 user_info NO_COPY *user_shared;
 HANDLE NO_COPY cygwin_shared_h;
 HANDLE NO_COPY cygwin_user_h;
 
+WCHAR installation_root[PATH_MAX] __attribute__((section (".cygwin_dll_common"), shared));
+UNICODE_STRING installation_key __attribute__((section (".cygwin_dll_common"), shared));
+WCHAR installation_key_buf[18] __attribute__((section (".cygwin_dll_common"), shared));
+
+/* Use absolute path of cygwin1.dll to derive the Win32 dir which
+   is our installation_root.  Note that we can't handle Cygwin installation
+   root dirs of more than 4K path length.  I assume that's ok...
+
+   This function also generates the installation_key value.  It's a 64 bit
+   hash value based on the path of the Cygwin DLL itself.  It's subsequently
+   used when generating shared object names.  Thus, different Cygwin
+   installations generate different object names and so are isolated from
+   each other.
+   
+   Having this information, the installation key together with the
+   installation root path is written to the registry.  The idea is that
+   cygcheck can print the paths into which the Cygwin DLL has been
+   installed for debugging purposes.
+   
+   Last but not least, the new cygwin properties datastrcuture is checked
+   for the "disabled_key" value, which is used to determine whether the
+   installation key is actually added to all object names or not.  This is
+   used as a last resort for debugging purposes, usually.  However, there
+   could be another good reason to re-enable object name collisions between
+   multiple Cygwin DLLs, which we're just not aware of right now.  Cygcheck
+   can be used to change the value in an existing Cygwin DLL binary. */
+
+void
+init_installation_root ()
+{
+  if (!GetModuleFileNameW (cygwin_hmodule, installation_root, PATH_MAX))
+    api_fatal ("Can't initialize Cygwin installation root dir.\n"
+	       "GetModuleFileNameW(%p, %p, %u), %E",
+	       cygwin_hmodule, installation_root, PATH_MAX);
+  PWCHAR p = installation_root;
+  if (wcsncmp (p, L"\\\\?\\", 4))	/* No long path prefix. */
+    {
+      if (!wcsncasecmp (p, L"\\\\", 2))	/* UNC */
+	{
+	  p = wcpcpy (p, L"\\??\\UN");
+	  GetModuleFileNameW (cygwin_hmodule, p, PATH_MAX - 6);
+	  *p = L'C';
+	}
+      else
+	{
+	  p = wcpcpy (p, L"\\??\\");
+	  GetModuleFileNameW (cygwin_hmodule, p, PATH_MAX - 4);
+	}
+    }
+  installation_root[1] = L'?';
+
+  RtlInitEmptyUnicodeString (&installation_key, installation_key_buf,
+			     sizeof installation_key_buf);
+  RtlInt64ToHexUnicodeString (hash_path_name (0, installation_root),
+			      &installation_key, FALSE);
+
+  PWCHAR w = wcsrchr (installation_root, L'\\');
+  if (w)
+    {
+      *w = L'\0';
+      w = wcsrchr (installation_root, L'\\');
+    }
+  if (!w)
+    api_fatal ("Can't initialize Cygwin installation root dir.\n"
+	       "Invalid DLL path");
+  *w = L'\0';
+
+  for (int i = 1; i >= 0; --i)
+    {
+      reg_key r (i, KEY_WRITE, CYGWIN_INFO_INSTALLATIONS_NAME, NULL);
+      if (r.set_string (installation_key_buf, installation_root)
+	  == ERROR_SUCCESS)
+      	break;
+    }
+
+  if (cygwin_props.disable_key)
+    {
+      installation_key.Length = 0;
+      installation_key.Buffer[0] = L'\0';
+    }
+}
+
 /* This function returns a handle to the top-level directory in the global
    NT namespace used to implement global objects including shared memory. */
 
@@ -46,9 +130,10 @@ get_shared_parent_dir ()
   if (!dir)
     {
       WCHAR bnoname[MAX_PATH];
-      __small_swprintf (bnoname, L"\\BaseNamedObjects\\%s%s",
+      __small_swprintf (bnoname, L"\\BaseNamedObjects\\%s%s-%S",
 			cygwin_version.shared_id,
-			_cygwin_testing ? cygwin_version.dll_build_date : "");
+			_cygwin_testing ? cygwin_version.dll_build_date : "",
+			&installation_key);
       RtlInitUnicodeString (&uname, bnoname);
       InitializeObjectAttributes (&attr, &uname, OBJ_INHERIT | OBJ_OPENIF,
 				  NULL, everyone_sd (CYG_SHARED_DIR_ACCESS));
@@ -79,9 +164,10 @@ get_session_parent_dir ()
 	{
 	  WCHAR bnoname[MAX_PATH];
 	  __small_swprintf (bnoname,
-			    L"\\Sessions\\BNOLINKS\\%d\\%s%s",
+			    L"\\Sessions\\BNOLINKS\\%d\\%s%s-%S",
 			    psi.SessionId, cygwin_version.shared_id,
-			    _cygwin_testing ? cygwin_version.dll_build_date : "");
+			    _cygwin_testing ? cygwin_version.dll_build_date : "",
+			    &installation_key);
 	  RtlInitUnicodeString (&uname, bnoname);
 	  InitializeObjectAttributes (&attr, &uname, OBJ_INHERIT | OBJ_OPENIF,
 				      NULL, everyone_sd(CYG_SHARED_DIR_ACCESS));
@@ -270,46 +356,6 @@ shared_destroy ()
   UnmapViewOfFile (user_shared);
 }
 
-/* Use absolute path of cygwin1.dll to derive the Win32 dir which
-   is our installation root.  Note that we can't handle Cygwin installation
-   root dirs of more than 4K path length.  I assume that's ok... */
-void
-shared_info::init_installation_root ()
-{
-  if (!GetModuleFileNameW (cygwin_hmodule, installation_root, PATH_MAX))
-    api_fatal ("Can't initialize Cygwin installation root dir.\n"
-	       "GetModuleFileNameW(%p, %p, %u), %E",
-	       cygwin_hmodule, installation_root, PATH_MAX);
-  PWCHAR p = installation_root;
-  if (wcsncmp (p, L"\\\\?\\", 4))	/* No long path prefix. */
-    {
-      if (!wcsncasecmp (p, L"\\\\", 2))	/* UNC */
-	{
-	  p = wcpcpy (p, L"\\??\\UN");
-	  GetModuleFileNameW (cygwin_hmodule, p, PATH_MAX - 6);
-	  *p = L'C';
-	}
-      else
-	{
-	  p = wcpcpy (p, L"\\??\\");
-	  GetModuleFileNameW (cygwin_hmodule, p, PATH_MAX - 4);
-	}
-    }
-  installation_root[1] = L'?';
-
-  PWCHAR w = wcsrchr (installation_root, L'\\');
-  if (w)
-    {
-      *w = L'\0';
-      w = wcsrchr (installation_root, L'\\');
-    }
-  if (!w)
-    api_fatal ("Can't initialize Cygwin installation root dir.\n"
-	       "Invalid DLL path");
-
-  *w = L'\0';
-}
-
 /* Initialize obcaseinsensitive.  Default to case insensitive on pre-XP. */
 void
 shared_info::init_obcaseinsensitive ()
@@ -349,7 +395,6 @@ shared_info::initialize ()
 
   if (!sversion)
     {
-      init_installation_root ();/* Initialize installation root dir. */
       init_obcaseinsensitive ();/* Initialize obcaseinsensitive. */
       tty.init ();		/* Initialize tty table.  */
       mt.initialize ();		/* Initialize shared tape information. */
@@ -373,6 +418,10 @@ memory_init (bool init_cygheap)
       cygheap->user.init ();
     }
 
+  /* Initialize installation root dir. */
+  if (!installation_root[0])
+    init_installation_root ();
+
   /* Initialize general shared memory */
   shared_locations sh_cygwin_shared;
   cygwin_shared = (shared_info *) open_shared (L"shared",
@@ -380,6 +429,11 @@ memory_init (bool init_cygheap)
 					       cygwin_shared_h,
 					       sizeof (*cygwin_shared),
 					       sh_cygwin_shared = SH_CYGWIN_SHARED);
+  /* Defer debug output printing the installation root and installation key
+     up to this point.  Debug output except for system_printf requires
+     the global shared memory to exist. */
+  debug_printf ("Installation root: <%W> key: <%S>",
+		installation_root, &installation_key);
   cygwin_shared->initialize ();
   user_shared_create (false);
 }
diff --git a/winsup/cygwin/shared_info.h b/winsup/cygwin/shared_info.h
index c92358f2c..1fcb8ff86 100644
--- a/winsup/cygwin/shared_info.h
+++ b/winsup/cygwin/shared_info.h
@@ -29,9 +29,9 @@ public:
 				  cygwin_version.api_minor)
 #define SHARED_VERSION_MAGIC CYGWIN_VERSION_MAGIC (SHARED_MAGIC, SHARED_VERSION)
 
-#define SHARED_INFO_CB 39328
+#define SHARED_INFO_CB 31136
 
-#define CURR_SHARED_MAGIC 0x22f9ff0bU
+#define CURR_SHARED_MAGIC 0x18da899eU
 
 #define USER_VERSION	1	// increment when mount table changes and
 #define USER_VERSION_MAGIC CYGWIN_VERSION_MAGIC (USER_MAGIC, USER_VERSION)
@@ -51,12 +51,10 @@ class shared_info
   DWORD sys_mount_table_counter;
   tty_list tty;
   LONG last_used_bindresvport;
-  WCHAR installation_root[PATH_MAX];
   DWORD obcaseinsensitive;
   mtinfo mt;
 
   void initialize ();
-  void init_installation_root ();
   void init_obcaseinsensitive ();
   unsigned heap_chunk_size ();
   unsigned heap_slop_size ();
@@ -105,4 +103,6 @@ void *__stdcall open_shared (const WCHAR *name, int n, HANDLE &shared_h,
 			     DWORD access = FILE_MAP_READ | FILE_MAP_WRITE);
 extern void user_shared_create (bool reinit);
 extern void user_shared_initialize ();
-
+extern void init_installation_root ();
+extern WCHAR installation_root[PATH_MAX];
+extern UNICODE_STRING installation_key;
diff --git a/winsup/cygwin/uinfo.cc b/winsup/cygwin/uinfo.cc
index 51fd8ad76..440efbb38 100644
--- a/winsup/cygwin/uinfo.cc
+++ b/winsup/cygwin/uinfo.cc
@@ -518,13 +518,13 @@ pwdgrp::load (const wchar_t *rel_path)
   curr_lines = 0;
 
   if (!path &&
-      !(path = (PWCHAR) malloc ((wcslen (cygwin_shared->installation_root)
+      !(path = (PWCHAR) malloc ((wcslen (installation_root)
 				 + wcslen (rel_path) + 1) * sizeof (WCHAR))))
     {
       paranoid_printf ("malloc (%W) failed", rel_path);
       goto out;
     }
-  wcpcpy (wcpcpy (path, cygwin_shared->installation_root), rel_path);
+  wcpcpy (wcpcpy (path, installation_root), rel_path);
   RtlInitUnicodeString (&upath, path);
 
   InitializeObjectAttributes (&attr, &upath, OBJ_CASE_INSENSITIVE, NULL, NULL);