1 files changed, 105 insertions, 0 deletions
diff --git a/hurd/hurd/userlink.h b/hurd/hurd/userlink.h
new file mode 100644
index 0000000000..337d46aef6
--- /dev/null
+++ b/hurd/hurd/userlink.h
@@ -0,0 +1,105 @@
+/* Support for chains recording users of a resource; `struct hurd_userlink'.
+Copyright (C) 1994 Free Software Foundation, Inc.
+This file is part of the GNU C Library.
+
+The GNU C Library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Library General Public License as
+published by the Free Software Foundation; either version 2 of the
+License, or (at your option) any later version.
+
+The GNU C Library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+Library General Public License for more details.
+
+You should have received a copy of the GNU Library General Public
+License along with the GNU C Library; see the file COPYING.LIB.  If
+not, write to the Free Software Foundation, Inc., 675 Mass Ave,
+Cambridge, MA 02139, USA.  */
+
+#ifndef	_HURD_USERLINK_H
+
+#define	_HURD_USERLINK_H	1
+#include <features.h>
+
+#define __need_NULL
+#include <stddef.h>
+
+
+/* This structure is simply a doubly-linked list.  Users of a given
+   resource are recorded by their presence in a list associated with that
+   resource.  A user attaches his own link (in local storage) to a shared
+   chain at the time he begins using some resource.  When finished with
+   that resource, the user removes his link from the chain.  If his link is
+   the last (there are no other users of the resource), and his chain has
+   been detached from the shared cell (the resource in the cell has been
+   replaced), then the user deallocates the resource that he used.  */
+
+struct hurd_userlink
+  {
+    struct hurd_userlink *next, **prevp;
+  };
+
+
+#ifndef _EXTERN_INLINE
+#define _EXTERN_INLINE extern __inline
+#endif
+
+
+/* Attach LINK to the chain of users at *CHAINP.  */
+
+_EXTERN_INLINE void
+_hurd_userlink_link (struct hurd_userlink **chainp,
+		     struct hurd_userlink *link)
+{
+  link->next = *chainp;
+  if (link->next)
+    link->next->prevp = &link->next;
+  link->prevp = chainp;
+  *chainp = link;
+}
+
+
+/* Detach LINK from its chain.  If the return value is nonzero, the caller
+   should deallocate the resource he started using after attaching LINK to
+   the chain it's on.  If the return value is zero, then someone else is
+   still using the resource.  */
+
+_EXTERN_INLINE int
+_hurd_userlink_unlink (struct hurd_userlink *link)
+{
+  /* The caller should deallocate the resource he used if his chain has
+     been detached from the cell (and thus has a nil `prevp'), and there is
+     no next link representing another user reference to the same resource. */
+  int dealloc = ! link->next && ! link->prevp;
+
+  /* Remove our link from the chain of current users.  */
+  if (link->prevp)
+    *link->prevp = link->next;
+  if (link->next)
+    link->next->prevp = link->prevp;
+
+  return dealloc;
+}
+
+
+/* Clear all users from *CHAINP.  Call this when the resource *CHAINP
+   protects is changing.  If the return value is nonzero, no users are on
+   the chain and the caller should deallocate the resource.  If the return
+   value is zero, someone is still using the resource and they will
+   deallocate it when they are finished.  */
+
+_EXTERN_INLINE int
+_hurd_userlink_clear (struct hurd_userlink **chainp)
+{
+  if (*chainp == NULL)
+    return 1;
+
+  /* Detach the chain of current users from the cell.  The last user to
+     remove his link from that chain will deallocate the old resource.  */
+  (*chainp)->prevp = NULL;
+  *chainp = NULL;
+  return 0;
+}
+
+#endif	/* hurd/userlink.h */