FFmpeg 是如何實現多型的?
前言
眾所周知,FFmpeg 在解碼的時候,無論輸入檔案是 MP4 檔案還是 FLV 檔案,或者其它檔案格式,都能正確解封裝、解碼,而程式碼不需要針對不同的格式做出任何改變,這是面向物件中很常見的多型特性,但 FFmpeg 是用 C 語言編寫的,那麼它是如何使用 C 語言實現了多型特性的呢?
要解決這個問題,首先需要從函式 av_register_all 說起。
av_register_all
av_register_all 是幾乎所有 FFmpeg 程式中第一個被呼叫的函式,用於註冊在編譯 FFmpeg 時設定了 --enable 選項的封裝器、解封裝器、編碼HX器、解碼HX器等。原始碼如下:
#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) { // 註冊編解碼HX器 avcodec_register_all(); // 註冊封裝器、解封裝器 /* (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 裡的 ## 用於拼接兩個字串,比如 REGISTER_DEMUXER(AAC, aac) ,它等效於:
extern AVInputFormat ff_aac_demuxer;
if(CONFIG_AAC_DEMUXER) av_register_input_format(&ff_aac_demuxer);
可以看出,編譯 ffmpeg 時類似於 "--enable-muxer=xxx" 這樣的選項在此時發揮了作用,它決定是否註冊某個格式對應的(解)封裝器,以便之後處理該格式的時候找到這個(解)封裝器。
av_register_input_format
av_register_input_format、av_register_output_format 原始碼如下:
/** 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;
}
從程式碼中可以看到,這兩個註冊方法會把指定的 AVInputFormat、AVOutputFormat 加到連結串列的尾部。
AVInputFormat
接著看 AVInputFormat 的定義:
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,這個結構體除了 name 等變數外,還具備 read_probe、read_header 等函式指標。
以前面提到的 ff_aac_demuxer 為例,這裡看一下它的實現:
AVInputFormat ff_aac_demuxer = {
// 名稱
.name = "aac",
.long_name = NULL_IF_CONFIG_SMALL("raw ADTS AAC (Advanced Audio Coding)"),
// 把函式指標指向能夠處理 aac 格式的函式實現
.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,
};
總結
根據以上程式碼的分析,此時我們就能得出問題的答案了:
FFmpeg 之所以能夠作為一個平臺,無論是封裝、解封裝,還是編碼、解碼,在處理對應格式的檔案/資料時,都能找到對應的庫來實現,而不需要修改程式碼,主要就是通過結構體 + 函式指標實現的。具體實現方式是:首先設計一個結構體,然後建立該結構體的多個物件,每個物件都有著自己的成員屬性及函式實現。這樣就使得 FFmpeg 具備了類似於面向物件程式設計中的多型的效果。
PS:avcodec_register_all 也是一樣的,有興趣的可以看看 AVCodec 的宣告以及 ff_libx264_encoder 等編解碼HX器的實現。
作者:zouzhiheng
連結:https://www.jianshu.com/p/c12e6888de10
來源:簡書
簡書著作權歸作者所有,任何形式的轉載都請聯絡作者獲得授