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diff --git a/src/MixerPlugin.hxx b/src/MixerPlugin.hxx
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+/*
+ * Copyright (C) 2003-2013 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+/** \file
+ *
+ * This header declares the mixer_plugin class.  It should not be
+ * included directly; use MixerInternal.hxx instead in mixer
+ * implementations.
+ */
+
+#ifndef MPD_MIXER_PLUGIN_HXX
+#define MPD_MIXER_PLUGIN_HXX
+
+#include "gerror.h"
+
+#include <stdbool.h>
+
+struct config_param;
+struct mixer;
+
+struct mixer_plugin {
+	/**
+         * Alocates and configures a mixer device.
+	 *
+	 * @param ao the pointer returned by audio_output_plugin.init
+	 * @param param the configuration section, or NULL if there is
+	 * no configuration
+	 * @param error_r location to store the error occurring, or
+	 * NULL to ignore errors
+	 * @return a mixer object, or NULL on error
+	 */
+	struct mixer *(*init)(void *ao, const struct config_param *param,
+			      GError **error_r);
+
+	/**
+	 * Finish and free mixer data
+         */
+	void (*finish)(struct mixer *data);
+
+	/**
+	 * Open mixer device
+	 *
+	 * @param error_r location to store the error occurring, or
+	 * NULL to ignore errors
+	 * @return true on success, false on error
+	 */
+	bool (*open)(struct mixer *data, GError **error_r);
+
+	/**
+	 * Close mixer device
+	 */
+	void (*close)(struct mixer *data);
+
+	/**
+	 * Reads the current volume.
+	 *
+	 * @param error_r location to store the error occurring, or
+	 * NULL to ignore errors
+	 * @return the current volume (0..100 including) or -1 if
+	 * unavailable or on error (error_r set, mixer will be closed)
+	 */
+	int (*get_volume)(struct mixer *mixer, GError **error_r);
+
+	/**
+	 * Sets the volume.
+	 *
+	 * @param error_r location to store the error occurring, or
+	 * NULL to ignore errors
+	 * @param volume the new volume (0..100 including)
+	 * @return true on success, false on error
+	 */
+	bool (*set_volume)(struct mixer *mixer, unsigned volume,
+			   GError **error_r);
+
+	/**
+	 * If true, then the mixer is automatically opened, even if
+	 * its audio output is not open.  If false, then the mixer is
+	 * disabled as long as its audio output is closed.
+	 */
+	bool global;
+};
+
+#endif