// Note, format could be added after the first 2 checks but that implies that *p is no longer NULL
while(p != &format->next && !format->next && avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))
p = &(*p)->next;
if (!format->next)
last_iformat = &format->next;
}
void av_register_output_format(AVOutputFormat *format)
{
AVOutputFormat **p = last_oformat;
// Note, format could be added after the first 2 checks but that implies that *p is no longer NULL
while(p != &format->next && !format->next && avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))
p = &(*p)->next;
if (!format->next)
last_oformat = &format->next;
}
从代码中可以看到，这两个注册方法会把指定的 AVInputFormat、AVOutputFormat 加到链表的尾部。
AVInputFormat
接着看 AVInputFormat 的定义：
typedef struct AVInputFormat {
/**
* A comma separated list of short names for the format. New names
* may be appended with a minor bump.
*/
const char *name;
/**
* Descriptive name for the format, meant to be more human-readable
* than name. You should use the NULL_IF_CONFIG_SMALL() macro
* to define it.
*/
const char *long_name;
/**
* Can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER, AVFMT_SHOW_IDS,
* AVFMT_GENERIC_INDEX, AVFMT_TS_DISCONT, AVFMT_NOBINSEARCH,
* AVFMT_NOGENSEARCH, AVFMT_NO_BYTE_SEEK, AVFMT_SEEK_TO_PTS.
*/
int flags;
/**
* If extensions are defined, then no probe is done. You should
* usually not use extension format guessing because it is not
* reliable enough
*/
const char *extensions;
...
/**
* Tell if a given file has a chance of being parsed as this format.
* The buffer provided is guaranteed to be AVPROBE_PADDING_SIZE bytes
* big so you do not have to check for that unless you need more.
*/
int (*read_probe)(AVProbeData *);
/**
* Read the format header and initialize the AVFormatContext
* structure. Return 0 if OK. 'avformat_new_stream' should be
* called to create new streams.
*/
int (*read_header)(struct AVFormatContext *);
/**
* Read one packet and put it in 'pkt'. pts and flags are also
* set. 'avformat_new_stream' can be called only if the flag
* AVFMTCTX_NOHEADER is used and only in the calling thread (not in a
* background thread).
* @return 0 on success, < 0 on error.
* When returning an error, pkt must not have been allocated
* or must be freed before returning
*/
int (*read_packet)(struct AVFormatContext *, AVPacket *pkt);
...
} AVInputFormat;
可以看到，这个结构体除了 name 等变量外，还具备 read_probe、read_header 等函数指针。
以前面提到的 ff_aac_demuxer 为例，这里看一下它的实现：
AVInputFormat ff_aac_demuxer = {
// 名称
.name = "aac",
.long_name = NULL_IF_CONFIG_SMALL("raw ADTS AAC (Advanced Audio Coding)"),
// 把函数指针指向能够处理 aac 格式的函数实现
.read_probe = adts_aac_probe,
.read_header = adts_aac_read_header,
.read_packet = adts_aac_read_packet,
.flags = AVFMT_GENERIC_INDEX,
.extensions = "aac",
.mime_type = "audio/aac,audio/aacp,audio/x-aac",
.raw_codec_id = AV_CODEC_ID_AAC,
};
总结
根据以上代码的分析，此时我们就能得出问题的答案了：
FFmpeg 之所以能够作为一个平台，无论是封装、解封装，还是编码、解码，在处理对应格式的文件/数据时，都能找到对应的库来实现，而不需要修改代码，主要就是通过结构体 + 函数指针实现的。具体实现方式是：首先设计一个结构体，然后创建该结构体的多个对象，每个对象都有着自己的成员属性及函数实现。这样就使得 FFmpeg 具备了类似于面向对象编程中的多态的效果。
PS：avcodec_register_all 也是一样的，有兴趣的可以看看 AVCodec 的声明以及 ff_libx264_encoder 等编解码器的实现。