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前言
众所周知,FFmpeg 在解码的时候,无论输入文件是 MP4 文件还是 FLV 文件,或者其它文件格式,都能正确解封装、解码,而代码不需要针对不同的格式做出任何改变,这是面向对象中很常见的多态特性,但 FFmpeg 是用 C 语言编写的,那么它是如何使用 C 语言实现了多态特性的呢?
要解决这个问题,首先需要从函数 av_register_all 说起。
av_register_all
av_register_all 是几乎所有 FFmpeg 程序中第一个被调用的函数,用于注册在编译 FFmpeg 时设置了 --enable 选项的封装器、解封装器、编码器、解码器等。源码如下:
#define REGISTER_MUXER(X, x) \ { \ extern AVOutputFormat ff_##x##_muxer; \ if (CONFIG_##X##_MUXER) \ av_register_output_format(&ff_##x##_muxer); \ }#define REGISTER_DEMUXER(X, x) \ { \ extern AVInputFormat ff_##x##_demuxer; \ if (CONFIG_##X##_DEMUXER) \ av_register_input_format(&ff_##x##_demuxer); \ }#define REGISTER_MUXDEMUX(X, x) REGISTER_MUXER(X, x); REGISTER_DEMUXER(X, x)static void register_all(void){ // 注册编解码器 avcodec_register_all(); // 注册封装器、解封装器 /* (de)muxers */ REGISTER_MUXER (A64, a64); REGISTER_DEMUXER (AA, aa); REGISTER_DEMUXER (AAC, aac); REGISTER_MUXDEMUX(AC3, ac3); REGISTER_MUXDEMUX(FLV, flv); REGISTER_MUXDEMUX(GIF, gif); ... /* image demuxers */ REGISTER_DEMUXER (IMAGE_BMP_PIPE, image_bmp_pipe); REGISTER_DEMUXER (IMAGE_JPEG_PIPE, image_jpeg_pipe); REGISTER_DEMUXER (IMAGE_SVG_PIPE, image_svg_pipe); REGISTER_DEMUXER (IMAGE_WEBP_PIPE, image_webp_pipe); REGISTER_DEMUXER (IMAGE_PNG_PIPE, image_png_pipe); ... /* external libraries */ REGISTER_MUXER (CHROMAPRINT, chromaprint); ...}void av_register_all(void){ static AVOnce control = AV_ONCE_INIT; ff_thread_once(&control, register_all);}
define 里的 ## 用于拼接两个字符串,比如 REGISTER_DEMUXER(AAC, aac) ,它等效于:
extern AVInputFormat ff_aac_demuxer;if(CONFIG_AAC_DEMUXER) av_register_input_format(&ff_aac_demuxer);
可以看出,编译 ffmpeg 时类似于 “–enable-muxer=xxx” 这样的选项在此时发挥了作用,它决定是否注册某个格式对应的(解)封装器,以便之后处理该格式的时候找到这个(解)封装器。
av_register_input_format
av_register_input_format、av_register_output_format 源码如下:
/** head of registered input format linked list */static AVInputFormat *first_iformat = NULL;/** head of registered output format linked list */static AVOutputFormat *first_oformat = NULL;static AVInputFormat **last_iformat = &first_iformat;static AVOutputFormat **last_oformat = &first_oformat;void av_register_input_format(AVInputFormat *format){ AVInputFormat **p = last_iformat; // 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。
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