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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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