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+/* lv2_persist.h - C header file for the LV2 Persist extension.
+ * Copyright (C) 2010 Leonard Ritter <paniq@paniq.org>
+ *
+ * This header 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 header 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 header; if not, write to the Free Software Foundation,
+ * Inc., 59 Temple Place, Suite 330, Boston, MA 01222-1307 USA
+ */
+
+/** @file
+ * C header for the LV2 Persist extension <http://lv2plug.in/ns/ext/persist>.
+ */
+
+#ifndef LV2_PERSIST_H
+#define LV2_PERSIST_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define LV2_PERSIST_URI "http://lv2plug.in/ns/ext/persist"
+
+/** A host-provided function to store a value under a given key.
+ * @param callback_data Must be the callback_data passed to LV2_Persist.save().
+ * @param key The URI key (predicate) under which the value is to be stored.
+ * @param value Pointer to the value (object) to be stored.
+ * @param size The size of the data at @a value in bytes.
+ * @param type The type of @a value, as a URI mapped to an integer.
+ *
+ * The host passes a callback of this type to LV2_Persist.save(). This
+ * callback is called repeatedly by the plugin within LV2_Persist.save() to
+ * store all the key/value records that describe its current state.
+ *
+ * Unless @a type is 0, @a value is guaranteed to be POD (i.e. a region
+ * of memory that does not contain pointers and can safely be copied
+ * and persisted indefinitely with a simple memcpy). If @a type is 0,
+ * then @a value is a reference, as defined by the LV2 Atom extension
+ * <http://lv2plug.in/ns/ext/atom/>. Hosts are not required to support
+ * references: a plugin MUST NOT expect a host to persist references unless
+ * the host supports the feature <http://lv2plug.in/ns/ext/atom#blobSupport>.
+ * Plugins SHOULD express their state entirely with POD values.
+ *
+ * Note that @a size MUST be > 0, and @a value MUST point to a valid region of
+ * memory @a size bytes long (this is required to make restore unambiguous).
+ *
+ * The plugin MUST NOT attempt to use this function outside of the
+ * LV2_Persist.restore() context.
+ */
+typedef void (*LV2_Persist_Store_Function)(
+ void* callback_data,
+ const char* key,
+ const void* value,
+ size_t size,
+ uint32_t type);
+
+/** A host-provided function to retrieve a value under a given key.
+ * @param callback_data Must be the callback_data passed to LV2_Persist.restore().
+ * @param key The URI key (predicate) under which a value has been stored.
+ * @param size (Output) If non-NULL, set to the size of the restored value.
+ * @param type (Output) If non-NULL, set to the type of the restored value.
+ * @return A pointer to the restored value (object), or NULL if no value
+ * has been stored under @a key.
+ *
+ * A callback of this type is passed by the host to LV2_Persist.restore(). This
+ * callback is called repeatedly by the plugin within LV2_Persist.restore() to
+ * retrieve the values of any keys it requires to restore its state.
+ *
+ * The returned value MUST remain valid until LV2_Persist.restore() returns.
+ *
+ * The plugin MUST NOT attempt to use this function, or any value returned from
+ * it, outside of the LV2_Persist.restore() context. Returned values MAY be
+ * copied for later use if necessary.
+ */
+typedef const void* (*LV2_Persist_Retrieve_Function)(
+ void* callback_data,
+ const char* key,
+ size_t* size,
+ uint32_t* type);
+
+/** When the plugin's extension_data is called with argument LV2_PERSIST_URI,
+ * the plugin MUST return an LV2_Persist structure, which remains valid for
+ * the lifetime of the plugin.
+ *
+ * The host can use the contained function pointers to save and restore the
+ * state of a plugin instance at any time (provided the threading restrictions
+ * for the given function are met).
+ *
+ * The typical use case is to save the plugin's state when a project is
+ * saved, and to restore the state when a project has been loaded. Other
+ * uses are possible (e.g. cloning plugin instances or taking a snapshot
+ * of plugin state).
+ *
+ * Stored data is only guaranteed to be compatible between instances of plugins
+ * with the same URI (i.e. if a change to a plugin would cause a fatal error
+ * when restoring state saved by a previous version of that plugin, the plugin
+ * URI must change just as it must when a plugin's ports change). Plugin
+ * authors should consider this possibility, and always store sensible data
+ * with meaningful types to avoid such compatibility issues in the future.
+ */
+typedef struct _LV2_Persist {
+
+ /** Save plugin state using a host-provided @a store callback.
+ *
+ * @param instance The instance handle of the plugin.
+ * @param store The host-provided store callback.
+ * @param callback_data An opaque pointer to host data, e.g. the map or
+ * file where the values are to be stored. If @a store is called,
+ * this MUST be passed as its callback_data parameter.
+ *
+ * The plugin is expected to store everything necessary to completely
+ * restore its state later (possibly much later, in a different
+ * process, on a completely different machine, etc.)
+ *
+ * The @a callback_data pointer and @a store function MUST NOT be
+ * used beyond the scope of save().
+ *
+ * This function has its own special threading class: it may not be
+ * called concurrently with any "Instantiation" function, but it
+ * may be called concurrently with functions in any other class,
+ * unless the definition of that class prohibits it (e.g. it may
+ * not be called concurrently with a "Discovery" function, but it
+ * may be called concurrently with an "Audio" function. The plugin
+ * is responsible for any locking or lock-free techniques necessary
+ * to make this possible.
+ *
+ * Note that in the simple case where state is only modified by
+ * restore(), there are no synchronization issues since save() is
+ * never called concurrently with restore() (though run() may read
+ * it during a save).
+ *
+ * Plugins that dynamically modify state while running, however,
+ * must take care to do so in such a way that a concurrent call to
+ * save() will save a consistent representation of plugin state for a
+ * single instant in time. The simplest way to do this is to modify a
+ * copy of the state map and atomically swap a pointer to the entire
+ * map once the changes are complete (for very large state maps,
+ * a purely functional map data structure may be more appropriate
+ * since a complete copy is not necessary).
+ */
+ void (*save)(LV2_Handle instance,
+ LV2_Persist_Store_Function store,
+ void* callback_data);
+
+ /** Restore plugin state using a host-provided @a retrieve callback.
+ *
+ * @param instance The instance handle of the plugin.
+ * @param retrieve The host-provided retrieve callback.
+ * @param callback_data An opaque pointer to host data, e.g. the map or
+ * file from which the values are to be restored. If @a retrieve is
+ * called, this MUST be passed as its callback_data parameter.
+ *
+ * The plugin MAY assume a restored value was set by a previous call to
+ * LV2_Persist.save() by a plugin with the same URI.
+ *
+ * The plugin MUST gracefully fall back to a default value when a
+ * value can not be retrieved. This allows the host to reset the
+ * plugin state with an empty map.
+ *
+ * The @a callback_data pointer and @a store function MUST NOT be used
+ * beyond the scope of restore().
+ *
+ * This function is in the "Instantiation" threading class as defined
+ * by LV2. This means it MUST NOT be called concurrently with any other
+ * function on the same plugin instance.
+ */
+ void (*restore)(LV2_Handle instance,
+ LV2_Persist_Retrieve_Function retrieve,
+ void* callback_data);
+
+} LV2_Persist;
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#endif /* LV2_PERSIST_H */
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