FFMPEG Tips (5) 如何利用 AVDictionary 配置参数

2024-03-01 20:45:34

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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www.imdb.com/list/ls083069869/?ref_=otl_7
www.imdb.com/list/ls083069869/?mode=desktop&ref_=m_ft_dsk

 

 

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。

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