我们先浏览一下Dictionary的定义。
/**
* @addtogroup lavu_dict AVDictionary
* @ingroup lavu_data
*
* @brief Simple key:value store
*
* @{
* Dictionaries are used for storing key:value pairs. To create
* an AVDictionary, simply pass an address of a NULL pointer to
* av_dict_set(). NULL can be used as an empty dictionary wherever
* a pointer to an AVDictionary is required.
* Use av_dict_get() to retrieve an entry or iterate over all
* entries and finally av_dict_free() to free the dictionary
* and all its contents.
*
@code
AVDictionary *d = NULL; // "create" an empty dictionary
AVDictionaryEntry *t = NULL;
av_dict_set(&d, "foo", "bar", 0); // add an entry
char *k = av_strdup("key"); // if your strings are already allocated,
char *v = av_strdup("value"); // you can avoid copying them like this
av_dict_set(&d, k, v, AV_DICT_DONT_STRDUP_KEY | AV_DICT_DONT_STRDUP_VAL);
while (t = av_dict_get(d, "", t, AV_DICT_IGNORE_SUFFIX)) {
<....> // iterate over all entries in d
}
av_dict_free(&d);
@endcode
*/
从这里我们可以看到Dictionary实际上就是强的键值映射，我们可以看一下AVDictionary结构体的定义。
// libavutil/dict.h
typedef struct AVDictionaryEntry {
char *key;
char *value;
} AVDictionaryEntry;
// libavutil/dict.c
struct AVDictionary {
int count;
AVDictionaryEntry *elems;
};
可以看出实际上AVDictionary实际上保存的就是key:value的点对关系，还是用数组保存的，可能是为了节省空间，不过在搜索上可能还是有点差距。
然后就是这个结构体所包含的函数:
/**
* Get a dictionary entry with matching key.
*
* The returned entry key or value must not be changed, or it will
* cause undefined behavior.
*
* To iterate through all the dictionary entries, you can set the matching key
* to the null string "" and set the AV_DICT_IGNORE_SUFFIX flag.
*
* @param prev Set to the previous matching element to find the next.
* If set to NULL the first matching element is returned.
* @param key matching key
* @param flags a collection of AV_DICT_* flags controlling how the entry is retrieved
* @return found entry or NULL in case no matching entry was found in the dictionary
*/
AVDictionaryEntry *av_dict_get(const AVDictionary *m, const char *key,
const AVDictionaryEntry *prev, int flags);
/**
* Get number of entries in dictionary.
*
* @param m dictionary
* @return number of entries in dictionary
*/
int av_dict_count(const AVDictionary *m);
/**
* Set the given entry in *pm, overwriting an existing entry.
*
* Note: If AV_DICT_DONT_STRDUP_KEY or AV_DICT_DONT_STRDUP_VAL is set,
* these arguments will be freed on error.
*
* Warning: Adding a new entry to a dictionary invalidates all existing entries
* previously returned with av_dict_get.
*
* @param pm pointer to a pointer to a dictionary struct. If *pm is NULL
* a dictionary struct is allocated and put in *pm.
* @param key entry key to add to *pm (will either be av_strduped or added as a new key depending on flags)
* @param value entry value to add to *pm (will be av_strduped or added as a new key depending on flags).
* Passing a NULL value will cause an existing entry to be deleted.
* @return >= 0 on success otherwise an error code <0
*/
int av_dict_set(AVDictionary **pm, const char *key, const char *value, int flags);
/**
* Convenience wrapper for av_dict_set that converts the value to a string
* and stores it.
*
* Note: If AV_DICT_DONT_STRDUP_KEY is set, key will be freed on error.
*/
int av_dict_set_int(AVDictionary **pm, const char *key, int64_t value, int flags);
/**
* Parse the key/value pairs list and add the parsed entries to a dictionary.
*
* In case of failure, all the successfully set entries are stored in
* *pm. You may need to manually free the created dictionary.
*
* @param key_val_sep a 0-terminated list of characters used to separate
* key from value
* @param pairs_sep a 0-terminated list of characters used to separate
* two pairs from each other
* @param flags flags to use when adding to dictionary.
* AV_DICT_DONT_STRDUP_KEY and AV_DICT_DONT_STRDUP_VAL
* are ignored since the key/value tokens will always
* be duplicated.
* @return 0 on success, negative AVERROR code on failure
*/
int av_dict_parse_string(AVDictionary **pm, const char *str,
const char *key_val_sep, const char *pairs_sep,
int flags);
/**
* Copy entries from one AVDictionary struct into another.
* @param dst pointer to a pointer to a AVDictionary struct. If *dst is NULL,
* this function will allocate a struct for you and put it in *dst
* @param src pointer to source AVDictionary struct
* @param flags flags to use when setting entries in *dst
* @note metadata is read using the AV_DICT_IGNORE_SUFFIX flag
* @return 0 on success, negative AVERROR code on failure. If dst was allocated
* by this function, callers should free the associated memory.
*/
int av_dict_copy(AVDictionary **dst, const AVDictionary *src, int flags);
/**
* Free all the memory allocated for an AVDictionary struct
* and all keys and values.
*/
void av_dict_free(AVDictionary **m);
/**
* Get dictionary entries as a string.
*
* Create a string containing dictionary's entries.
* Such string may be passed back to av_dict_parse_string().
* @note String is escaped with backslashes ('\').
*
* @param[in] m dictionary
* @param[out] buffer Pointer to buffer that will be allocated with string containg entries.
* Buffer must be freed by the caller when is no longer needed.
* @param[in] key_val_sep character used to separate key from value
* @param[in] pairs_sep character used to separate two pairs from each other
* @return >= 0 on success, negative on error
* @warning Separators cannot be neither '\\' nor '\0'. They also cannot be the same.
*/
int av_dict_get_string(const AVDictionary *m, char **buffer,
const char key_val_sep, const char pairs_sep);
里面都解释的很清楚，就不做太多的解释。
由于av_dict_set用的比较多，就详细说一下。
参数介绍：
pm: 简单地说就是你要设置的dictionary的指针。
key: key:value映射的键。
value: key:value映射的值。
flag: 设置key和value是否不重新申请内存，直接使用传入的指针。比如AV_DICT_DONT_STRDUP_KEY | AV_DICT_DONT_STRDUP_VAL，就表示传入的指针不申请内存，直接使用。
之后就没什么了。
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