新版本ffmpeg源码简单分析：Dictionary相关函数

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。