ffmpeg结构体解析-AVClass 和 AVOption
AVClass
先来看 AVClass 的结构如下：
/** * 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;
根据 API 的意思，AVClass 就是用于描述任意包含 AVClass 的一个 Context 的信息类，并且会出现在该 Context 类的第一个字段。然后看下 AVClass 结构体的字段如下：
-
const char* class_name：表示Context 类的名称，一般和描述的 Context 类的类名/结构体名一致
-
const char* (item_name)(void ctx)：可以返回该 Context 类实例的一个函数
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const struct AVOption *option：AcOption 数组
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version：创建该结构体的版本号
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log_level_offset_offset：表示 level_offset_offset 字段在这个结构体的偏移量，如果没有这个字段则为0
-
parent_log_context_offset:同上，表示的是 Parent Context 的 level_offset_offset 字段在这个结构体的偏移量
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void* (*child_next)(void *obj, void *prev)：一个返回下一个 AVOption 的函数指针
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const struct AVClass* (*child_class_next)(const struct AVClass *prev)
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AVClassCategory category：表示 AVClass 的类型，是一组枚举类型的值，见后面备注。
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AVClassCategory (get_category)(void ctx)：
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int (*query_ranges)(struct AVOptionRanges **, void *obj, const char *key, int flags)：
在 AVFormatContext 或者 AVCodecContext 等类里面，第一个字段都是 AVClass 指针，以AVFormatContext示例，如下：
//avformat.h
typedef struct AVFormatContext {
/**
* A class for logging and @ref avoptions. Set by avformat_alloc_context().
* Exports (de)muxer private options if they exist.
*/
const AVClass *av_class;
//省略其它代码
根据API的介绍，AVClass 就是一个用于打印log 和AVOption的引用，由avformat_alloc_context() 函数创建。怎么理解这个引用呢？
就是说，AVClass 建立起了 AVOption 和 Context 之间的桥梁。
AVOption 结构体
那么这样一说，必须先来看下 AVOption 这个结构体。
先看 API 对 AVOption 结构体的说明，如下：
* AVOptions provide a generic system to declare options on arbitrary structs
* ("objects"). An option can have a help text, a type and a range of possible
* values. Options may then be enumerated, read and written to.
*
* @section avoptions_implement Implementing AVOptions
* This section describes how to add AVOptions capabilities to a struct.
*
* All AVOptions-related information is stored in an AVClass. Therefore
* the first member of the struct should be a pointer to an AVClass describing it.
* The option field of the AVClass must be set to a NULL-terminated static array
* of AVOptions. Each AVOption must have a non-empty name, a type, a default
* value and for number-type AVOptions also a range of allowed values. It must
* also declare an offset in bytes from the start of the struct, where the field
* associated with this AVOption is located. Other fields in the AVOption struct
* should also be set when applicable, but are not required.AVOption 提供了一种泛型系统用于描述任意结构体的 Option。所有 AVOption 相关的信息都是存储在 AVClass 里面的。因此所以AVOption 描述的结构体的第一个字段必定是 AVClass，其次AVClass 必须包含一个静态的 AVOption 数组。
AVOption 的定义在 libutils/opts.h 头文件中，如下：
/**
* AVOption
*/
typedef struct AVOption {
const char *name;
/**
* short English help text
* @todo What about other languages?
*/
const char *help;
/**
* The offset relative to the context structure where the option
* value is stored. It should be 0 for named constants.
*/
int offset;
enum AVOptionType type;
/**
* the default value for scalar options
*/
union {
int64_t i64;
double dbl;
const char *str;
/* TODO those are unused now */
AVRational q;
} default_val;
double min; ///< minimum valid value for the option
double max; ///< maximum valid value for the option
int flags;
#define AV_OPT_FLAG_ENCODING_PARAM 1 ///< a generic parameter which can be set by the user for muxing or encoding
#define AV_OPT_FLAG_DECODING_PARAM 2 ///< a generic parameter which can be set by the user for demuxing or decoding
#define AV_OPT_FLAG_AUDIO_PARAM 8
#define AV_OPT_FLAG_VIDEO_PARAM 16
#define AV_OPT_FLAG_SUBTITLE_PARAM 32
/**
* The option is intended for exporting values to the caller.
*/
#define AV_OPT_FLAG_EXPORT 64
/**
* The option may not be set through the AVOptions API, only read.
* This flag only makes sense when AV_OPT_FLAG_EXPORT is also set.
*/
#define AV_OPT_FLAG_READONLY 128
#define AV_OPT_FLAG_BSF_PARAM (1<<8) ///< a generic parameter which can be set by the user for bit stream filtering
#define AV_OPT_FLAG_RUNTIME_PARAM (1<<15) ///< a generic parameter which can be set by the user at runtime
#define AV_OPT_FLAG_FILTERING_PARAM (1<<16) ///< a generic parameter which can be set by the user for filtering
#define AV_OPT_FLAG_DEPRECATED (1<<17) ///< set if option is deprecated, users should refer to AVOption.help text for more information
#define AV_OPT_FLAG_CHILD_CONSTS (1<<18) ///< set if option constants can also reside in child objects
//FIXME think about enc-audio, ... style flags
/**
* The logical unit to which the option belongs. Non-constant
* options and corresponding named constants share the same
* unit. May be NULL.
*/
const char *unit;
} AVOption;来看下 AVOption 结构体的字段：
const char *name：Context Struc 结构体的字段名称
const char *help：帮助说明
int offset：表示该字段在结构体中的偏移量，一般可以使用 C标准库的 offsetof(type, member-designator) 计算该字体在结构体中的偏移量
enum AVOptionType type：枚举值，表示该字段的类型，后面讲解
default_val： 默认数值，可以是 int，double，字符串任意类型。
min max：这两个字段表示取值范围，限定了该字段的取值范围
int flags：类型标记位，取值如描述
const char *unit：该选项所属的逻辑单元，可以为空
作者：kotlon
链接：https://www.jianshu.com/p/25a087619b7c
来源：简书
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