结构体AVClass
/**
* Describe the class of an AVClass context structure. That is an
* arbitrary struct of which the first field is a pointer to an
* AVClass struct (e.g. AVCodecContext, AVFormatContext etc.).
*/
typedef struct AVClass {
/**
* The name of the class; usually it is the same name as the
* context structure type to which the AVClass is associated.
*/
const char* class_name;
/**
* A pointer to a function which returns the name of a context
* instance ctx associated with the class.
*/
const char* (*item_name)(void* ctx);
/**
* a pointer to the first option specified in the class if any or NULL
*
* @see av_set_default_options()
*/
const struct AVOption *option;
/**
* LIBAVUTIL_VERSION with which this structure was created.
* This is used to allow fields to be added without requiring major
* version bumps everywhere.
*/
int version;
/**
* Offset in the structure where log_level_offset is stored.
* 0 means there is no such variable
*/
int log_level_offset_offset;
/**
* Offset in the structure where a pointer to the parent context for
* logging is stored. For example a decoder could pass its AVCodecContext
* to eval as such a parent context, which an av_log() implementation
* could then leverage to display the parent context.
* The offset can be NULL.
*/
int parent_log_context_offset;
/**
* Return next AVOptions-enabled child or NULL
*/
void* (*child_next)(void *obj, void *prev);
/**
* Return an AVClass corresponding to the next potential
* AVOptions-enabled child.
*
* The difference between child_next and this is that
* child_next iterates over _already existing_ objects, while
* child_class_next iterates over _all possible_ children.
*/
const struct AVClass* (*child_class_next)(const struct AVClass *prev);
/**
* Category used for visualization (like color)
* This is only set if the category is equal for all objects using this class.
* available since version (51 << 16 | 56 << 8 | 100)
*/
AVClassCategory category;
/**
* Callback to return the category.
* available since version (51 << 16 | 59 << 8 | 100)
*/
AVClassCategory (*get_category)(void* ctx);
/**
* Callback to return the supported/allowed ranges.
* available since version (52.12)
*/
int (*query_ranges)(struct AVOptionRanges **, void *obj, const char *key, int flags);
} AVClass;
AVOption