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diff --git a/winsup/cygserver/ipc.h b/winsup/cygserver/ipc.h
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+++ b/winsup/cygserver/ipc.h
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+/* cygserver_ipc.h
+
+   Copyright 2002 Red Hat, Inc.
+
+   Originally written by Conrad Scott <conrad.scott@dsl.pipex.com>
+
+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. */
+
+#ifndef __CYGSERVER_IPC_H__
+#define __CYGSERVER_IPC_H__
+
+#include <assert.h>
+#include <limits.h>		/* For OPEN_MAX. */
+
+/*
+ * The sysv ipc id's (msgid, semid, shmid) are integers arranged such
+ * that they no subsystem will generate the same id as some other
+ * subsystem; nor do these ids overlap file descriptors (the other
+ * common integer ids).  Since Cygwin can allocate more than OPEN_MAX
+ * file descriptors, it can't be guaranteed not to overlap, but it
+ * should help catch some errors.
+ *
+ * msgid's: OPEN_MAX,     OPEN_MAX + 3, OPEN_MAX + 6, . . .
+ * semid's: OPEN_MAX + 1, OPEN_MAX + 4, OPEN_MAX + 7, . . .
+ * shmid's: OPEN_MAX + 2, OPEN_MAX + 5, OPEN_MAX + 8, . . .
+ *
+ * To further ensure that ids are unique, if ipc objects are created
+ * and destroyed and then re-created, they are given new ids by
+ * munging the basic id (as above) with a sequence number.
+ *
+ * Internal ipc id's, which are 0, 1, ... within each subsystem (and
+ * not munged with a sequence number), are used solely by the ipcs(8)
+ * interface.
+ */
+
+enum ipc_subsys_t
+  {
+    IPC_MSGOP = 0,
+    IPC_SEMOP = 1,
+    IPC_SHMOP = 2,
+    IPC_SUBSYS_COUNT
+  };
+
+/*
+ * IPCMNI - The absolute maximum number of simultaneous ipc ids for
+ * any one subsystem.
+ */
+
+enum
+  {
+    IPCMNI = 0x10000		// Must be a power of two.
+  };
+
+inline int
+ipc_int2ext (const int intid, const ipc_subsys_t subsys, long & sequence)
+{
+  assert (0 <= intid && intid < IPCMNI);
+
+  const long tmp = InterlockedIncrement (&sequence);
+
+  return (((tmp & 0x7fff) << 16)
+	  | (OPEN_MAX + (intid * IPC_SUBSYS_COUNT) + subsys));
+}
+
+inline int
+ipc_ext2int_subsys (const int extid)
+{
+  return ((extid & (IPCMNI - 1)) - OPEN_MAX) % IPC_SUBSYS_COUNT;
+}
+
+inline int
+ipc_ext2int (const int extid, const ipc_subsys_t subsys)
+{
+  if (ipc_ext2int_subsys (extid) != subsys)
+    return -1;
+  else
+    return ((extid & (IPCMNI - 1)) - OPEN_MAX) / IPC_SUBSYS_COUNT;
+}
+
+#endif /* __CYGSERVER_IPC_H__ */