AVOption用于在FFmpeg中描述结构体中的成员变量。一个AVOption可以包含名称，简短的帮助信息，取值等。
上篇文章中概括了AVClass，AVOption和目标结构体之间的关系。以AVFormatContext为例，可以表示为下图。
上篇文章主要概括了AVClass，AVOption和目标结构体之间的从属关系，但是并没有分析有关AVOption的源代码。本文分析有关AVOption的源代码。
AVOption有关的API
AVOption常用的API可以分成两类：用于设置参数的API和用于读取参数的API。其中最有代表性的用于设置参数的API就是av_opt_set()；而最有代表性的用于读取参数的API就是av_opt_get()。除了以上两个函数之外，本文再记录一个在FFmpeg的结构体初始化代码中最常用的用于设置默认值的函数av_opt_set_defaults()
函数调用关系图
av_opt_set()的函数调用关系图如下所示。
av_opt_set_defaults()的函数调用关系图如下所示。注：av_opt_set_defaults2是deprecated的api
av_opt_set()
通过AVOption设置参数最常用的函数就是av_opt_set()了。该函数通过字符串的方式（传入的参数是：变量名称的字符串和变量值的字符串）设置一个AVOption的值。此外，还包含了它的一系列“兄弟”函数av_opt_set_XXX()，其中“XXX”代表了int，double这些数据类型。使用这些函数的时候，可以指定int，double这些类型的变量（而不是字符串）作为输入，设定相应的AVOption的值。- /**
- * @defgroup opt_set_funcs Option setting functions
- * @{
- * Those functions set the field of obj with the given name to value.
- *
- * @param[in] obj A struct whose first element is a pointer to an AVClass.
- * @param[in] name the name of the field to set
- * @param[in] val The value to set. In case of av_opt_set() if the field is not
- * of a string type, then the given string is parsed.
- * SI postfixes and some named scalars are supported.
- * If the field is of a numeric type, it has to be a numeric or named
- * scalar. Behavior with more than one scalar and +- infix operators
- * is undefined.
- * If the field is of a flags type, it has to be a sequence of numeric
- * scalars or named flags separated by '+' or '-'. Prefixing a flag
- * with '+' causes it to be set without affecting the other flags;
- * similarly, '-' unsets a flag.
- * @param search_flags flags passed to av_opt_find2. I.e. if AV_OPT_SEARCH_CHILDREN
- * is passed here, then the option may be set on a child of obj.
- *
- * @return 0 if the value has been set, or an AVERROR code in case of
- * error:
- * AVERROR_OPTION_NOT_FOUND if no matching option exists
- * AVERROR(ERANGE) if the value is out of range
- * AVERROR(EINVAL) if the value is not valid
- */
- int av_opt_set (void *obj, const char *name, const char *val, int search_flags);
- int av_opt_set_int (void *obj, const char *name, int64_t val, int search_flags);
- int av_opt_set_double (void *obj, const char *name, double val, int search_flags);
- int av_opt_set_q (void *obj, const char *name, AVRational val, int search_flags);
- int av_opt_set_bin (void *obj, const char *name, const uint8_t *val, int size, int search_flags);
- int av_opt_set_image_size(void *obj, const char *name, int w, int h, int search_flags);
- int av_opt_set_pixel_fmt (void *obj, const char *name, enum AVPixelFormat fmt, int search_flags);
- int av_opt_set_sample_fmt(void *obj, const char *name, enum AVSampleFormat fmt, int search_flags);
- int av_opt_set_video_rate(void *obj, const char *name, AVRational val, int search_flags);
- int av_opt_set_channel_layout(void *obj, const char *name, int64_t ch_layout, int search_flags);