FFMPEG Tips (5) 如何利用 AVDictionary 配置参数
作者：互联网
本文是我的 FFMPEG Tips 系列的第五篇文章，准备介绍下 ffmpeg 提供的一个非常好用的健值对工具：AVDictionary，特别是对于没有 map 容器的 c 代码，可以充分利用它来配置和定义播放器的参数，ffmpeg 本身也有很多 API 通过它来传递参数。
1. AVDictionary 的用法简介
AVDictionary 所在的头文件在 libavutil/dict.h，其定义如下：
struct AVDictionary {
int count;
AVDictionaryEntry *elems;
};
其中，AVDictionaryEntry 的定义如下：
typedef struct AVDictionaryEntry {
char *key;
char *value;
} AVDictionaryEntry;
下面就用示例的方式简单介绍下用法
（1）创建一个字典
AVDictionary *d = NULL;
（2） 销毁一个字典
av_dict_free(&d);
（3）添加一对 key-value
av_dict_set(&d, "name", "jhuster", 0);
av_dict_set_int(&d, "age", "29", 0);
（4） 获取 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） 遍历字典
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 参数的传递
ffmpeg 中很多 API 都是靠 AVDictionary 来传递参数的，比如常用的：
int avformat_open_input(AVFormatContext **ps, const char *url, AVInputFormat *fmt, AVDictionary **options);
最后一个参数就是 AVDictionary，我们可以在打开码流前指定各种参数，比如：探测时间、超时时间、最大延时、支持的协议的白名单等等，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;
}
那么，我们怎么知道 ffmpeg 的这个 API 支持哪些可配置的参数呢 ？
我们可以查看 ffmpeg 源码，比如 avformat_open_input 是结构体 AVFormatContext 提供的 API，在 libavformat/options_table.h 中定义了 AVFormatContext 所有支持的 options 选项，如下所示：
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同理，AVCodec 相关 API 支持的 options 选项则可以在 libavcodec/options_table.h 文件中找到，如下所示：
3. 小结
当然，我们也可以仿照 ffmpeg，给自己的核心结构体对象定义这样的 options 选项，这篇文章就不展开详述了。
标签：AVDictionary,www,otl,FFMPEG,ref,list,imdb,Tips,com 来源： https://www.cnblogs.com/notepaper/p/13621386.html