本文是我的 FFMPEG Tips系列的第五篇文章，準備介紹下 ffmpeg 提供的一個非常好用的健值對工具：AVDictionary，特別是對於沒有 map 容器的 c 程式碼，可以充分利用它來配置和定義播放器的引數，ffmpeg 本身也有很多 API 通過它來傳遞引數。

1. AVDictionary 的用法簡介

AVDictionary 所在的標頭檔案在 libavutil/dict.h，其定義如下：

struct AVDictionary {  
    int count;  
    AVDictionaryEntry *elems;  
};

其中，AVDictionaryEntry 的定義如下：

typedef struct AVDictionaryEntry {  
    char *key;  
    char *value;  
} AVDictionaryEntry;

下面就用示例的方式簡單介紹下用法

（1）建立一個字典

AVDictionary *d = NULL;

（2） 銷燬一個字典

av_dict_free(&d);

（3）新增一對 key-value

av_dict_set(&d, "name", "jhuster", 0);
av_dict_set_int(&d, "age", "29", 0);

（4） 獲取 key 的值

AVDictionaryEntry *t = NULL;

t = av_dict_get(d, "name", NULL, AV_DICT_IGNORE_SUFFIX);
av_log(NULL, AV_LOG_DEBUG, "name: %s", t->value);

t = av_dict_get(d, "age", NULL, AV_DICT_IGNORE_SUFFIX);
av_log(NULL, AV_LOG_DEBUG, "age: %d", (int) (*t->value));

（5） 遍歷字典

AVDictionaryEntry *t = NULL;
while ((t = av_dict_get(d, "", t, AV_DICT_IGNORE_SUFFIX))) {
    av_log(NULL, AV_LOG_DEBUG, "%s: %s", t->key, t->value);
}
 

2. ffmpeg 引數的傳遞

ffmpeg 中很多 API 都是靠 AVDictionary 來傳遞引數的，比如常用的：

int avformat_open_input(AVFormatContext **ps, const char *url, AVInputFormat *fmt, AVDictionary **options);

最後一個引數就是 AVDictionary，我們可以在開啟碼流前指定各種引數，比如：探測時間、超時時間、最大延時、支援的協議的白名單等等，例如：

AVDictionary *options = NULL;
av_dict_set(&options, “probesize”, “4096", 0);
av_dict_set(&options, “max_delay”, “5000000”, 0);

AVFormatContext *ic = avformat_alloc_context();
if (avformat_open_input(&ic, url, NULL, &options) < 0) {
    LOGE("could not open source %s", url);
    return -1;
}

那麼，我們怎麼知道 ffmpeg 的這個 API 支援哪些可配置的引數呢 ？

我們可以檢視 ffmpeg 原始碼，比如 avformat_open_input 是結構體 AVFormatContext 提供的 API，在 libavformat/options_table.h 中定義了 AVFormatContext 所有支援的 options 選項，如下所示：

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同理，AVCodec 相關 API 支援的 options 選項則可以在 libavcodec/options_table.h 檔案中找到，如下所示：

3. 小結

當然，我們也可以仿照 ffmpeg，給自己的核心結構體物件定義這樣的 options 選項，這篇文章就不展開詳述了。