Simple thread-safe implementation of reference count.

+/* GNU Mailutils -- a suite of utilities for electronic mail
+   Copyright (C) 1999, 2000, 2001, 2003 Free Software Foundation, Inc.
+
+   This library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2 of the License, or (at your option) any later version.
+
+   This 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with this library; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307  USA  */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include <stdlib.h>
+#include <errno.h>
+
+#include <mailutils/monitor.h>
+#include <mailutils/refcount.h>
+
+int
+mu_refcount_create (mu_refcount_t *prefcount)
+{
+  int status = 0;
+  mu_refcount_t refcount;
+  if (prefcount == NULL)
+    return EINVAL;
+  refcount = calloc (1, sizeof *refcount);
+  if (refcount != NULL)
+    {
+      refcount->ref = 1;
+      status = monitor_create (&(refcount->lock));
+      if (status == 0)
+	{
+	  *prefcount = refcount;
+	}
+      else
+	{
+	  free (refcount);
+	}
+    }
+  else
+    {
+      status = ENOMEM;
+    }
+  return status;
+}
+
+void
+mu_refcount_destroy (mu_refcount_t *prefcount)
+{
+  if (prefcount && *prefcount)
+    {
+      mu_refcount_t refcount = *prefcount;
+      monitor_destroy (&refcount->lock);
+      free (refcount);
+      *prefcount = NULL;
+    }
+}
+
+int
+mu_refcount_inc (mu_refcount_t refcount)
+{
+  int count = 0;
+  if (refcount)
+    {
+      monitor_lock (refcount->lock);
+      count = ++refcount->ref;
+      monitor_unlock (refcount->lock);
+    }
+  return count;
+}
+
+int
+mu_refcount_dec (mu_refcount_t refcount)
+{
+  int count = 0;
+  if (refcount)
+    {
+      monitor_lock (refcount->lock);
+      if (refcount->ref)
+	count = --refcount->ref;
+      monitor_unlock (refcount->lock);
+    }
+  return count;
+}