FLV简介

FLV (Flash Video) 是由 Adobe 公司推出的一种封装格式，主要用于流媒体系统。

FLV 封装的媒体文件具有体积轻巧、封装播放简单等特点，很适合网络应用。

目前各浏览器普遍使用 Flash Player 作为网页播放器，使得安装有浏览器的计算机终端不需要另外安装播放器，

这也是 FLV 格式广为流行的原因之一。

FLV 文件主要由一个 Header 加上由多个 Tag 组成的 Body 构成。

一、FLV Header（UI8表示无符号8位，也就是一个字节；UB[5]表示一个字节中的5位）

 二、FLV Body（Flv Body由一个一个Tag组成，每个Tag前都有一个PerviousTagSize字段，标记着前面一个Tag的大小。）

Tag有三种类型，Audio Tag（音频Tag），Video Tag（视频Tag），script Tag（又称Metadata Tag）

每个Tag由Tag Header和Tag Data组成，对于不同类型的Tag，Tag Header的格式都是相同的（都是11byte的长度），Tag Body的格式就不一样了。

综上所述FLV整体的结构如下图：

 1、AudioTag Data

如果SoundFormat=10，那么音频数据就是AACAUDIODATA。

 2、Video Tag Data

对于H.264数据来说，CodecID = 7。
当CodecID = 7时，视频数据就是AVCVIDEOPACKET格式。

3、Script Tag Data

该类型Tag又通常被称为MetadataTag，会放一些关于FLV视频和音频的元数据信息

如：duration、width、height等。通常该类型Tag会跟在FileHeader后面作为第一个Tag出现，而且只有一个。

第一个AMF包：

第一个字节一般为0x02，表示字符串，第2-3个字节表示字符串的长度，一般为0x000A，后面跟的就是字符串，一般为"onMetaData"。

第二AMF包：

第一个字节为0x08，表示数组，第2-5个字节表示数组元素个数，后面跟着就是数组的元素，格式为：元素名长度（UI16） + 元素名（UI8[n]） + 元素的值(double)，最后以“009”结尾

原文链接：https://blog.csdn.net/weixin_42462202/article/details/88661883

此外本程序还可以分离FLV中的视频码流和音频码流。需要注意的是本程序并不能分离一些特定类型的音频（例如AAC）和视频。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

//Important!
#pragma pack(1)

#define TAG_TYPE_SCRIPT 18
#define TAG_TYPE_AUDIO  8
#define TAG_TYPE_VIDEO  9

typedef unsigned char byte;
typedef unsigned int uint;

typedef struct {
    byte Signature[3];
    byte Version;
    byte Flags;
    uint DataOffset;
} FLV_HEADER;

typedef struct {
    byte TagType;
    byte DataSize[3];
    byte Timestamp[3];
    uint Reserved;
} TAG_HEADER;

//reverse_bytes - turn a BigEndian byte array into a LittleEndian integer
uint reverse_bytes(byte *p, char c) {
    int r = 0;
    int i;
    for (i=0; i<c; i++)
        r |= ( *(p+i) << (((c-1)*8)-8*i));
    return r;
}

/**
 * Analysis FLV file
 * @param url    Location of input FLV file.
 */

int simplest_flv_parser(char *url){

    //whether output audio/video stream
    int output_a=1;
    int output_v=1;
    //-------------
    FILE *ifh=NULL,*vfh=NULL, *afh = NULL;

    //FILE *myout=fopen("output_log.txt","wb+");
    FILE *myout=stdout;

    FLV_HEADER flv;
    TAG_HEADER tagheader;
    uint previoustagsize, previoustagsize_z=0;
    uint ts=0, ts_new=0;

    ifh = fopen(url, "rb+");
    if ( ifh== NULL) {
        printf("Failed to open files!");
        return -1;
    }

    //FLV file header
    fread((char *)&flv,1,sizeof(FLV_HEADER),ifh);

    fprintf(myout,"============== FLV Header ==============\n");
    fprintf(myout,"Signature:  0x %c %c %c\n",flv.Signature[0],flv.Signature[1],flv.Signature[2]);
    fprintf(myout,"Version:    0x %X\n",flv.Version);
    fprintf(myout,"Flags  :    0x %X\n",flv.Flags);
    fprintf(myout,"HeaderSize: 0x %X\n",reverse_bytes((byte *)&flv.DataOffset, sizeof(flv.DataOffset)));
    fprintf(myout,"========================================\n");

    //move the file pointer to the end of the header
    fseek(ifh, reverse_bytes((byte *)&flv.DataOffset, sizeof(flv.DataOffset)), SEEK_SET);

    //process each tag
    do {

        previoustagsize = _getw(ifh);

        fread((void *)&tagheader,sizeof(TAG_HEADER),1,ifh);

        //int temp_datasize1=reverse_bytes((byte *)&tagheader.DataSize, sizeof(tagheader.DataSize));
        int tagheader_datasize=tagheader.DataSize[0]*65536+tagheader.DataSize[1]*256+tagheader.DataSize[2];
        int tagheader_timestamp=tagheader.Timestamp[0]*65536+tagheader.Timestamp[1]*256+tagheader.Timestamp[2];

        char tagtype_str[10];
        switch(tagheader.TagType){
        case TAG_TYPE_AUDIO:sprintf(tagtype_str,"AUDIO");break;
        case TAG_TYPE_VIDEO:sprintf(tagtype_str,"VIDEO");break;
        case TAG_TYPE_SCRIPT:sprintf(tagtype_str,"SCRIPT");break;
        default:sprintf(tagtype_str,"UNKNOWN");break;
        }
        fprintf(myout,"[%6s] %6d %6d |",tagtype_str,tagheader_datasize,tagheader_timestamp);

        //if we are not past the end of file, process the tag
        if (feof(ifh)) {
            break;
        }

        //process tag by type
        switch (tagheader.TagType) {

        case TAG_TYPE_AUDIO:{
            char audiotag_str[100]={0};
            strcat(audiotag_str,"| ");
            char tagdata_first_byte;
            tagdata_first_byte=fgetc(ifh);
            int x=tagdata_first_byte&0xF0;
            x=x>>4;
            switch (x)
            {
            case 0:strcat(audiotag_str,"Linear PCM, platform endian");break;
            case 1:strcat(audiotag_str,"ADPCM");break;
            case 2:strcat(audiotag_str,"MP3");break;
            case 3:strcat(audiotag_str,"Linear PCM, little endian");break;
            case 4:strcat(audiotag_str,"Nellymoser 16-kHz mono");break;
            case 5:strcat(audiotag_str,"Nellymoser 8-kHz mono");break;
            case 6:strcat(audiotag_str,"Nellymoser");break;
            case 7:strcat(audiotag_str,"G.711 A-law logarithmic PCM");break;
            case 8:strcat(audiotag_str,"G.711 mu-law logarithmic PCM");break;
            case 9:strcat(audiotag_str,"reserved");break;
            case 10:strcat(audiotag_str,"AAC");break;
            case 11:strcat(audiotag_str,"Speex");break;
            case 14:strcat(audiotag_str,"MP3 8-Khz");break;
            case 15:strcat(audiotag_str,"Device-specific sound");break;
            default:strcat(audiotag_str,"UNKNOWN");break;
            }
            strcat(audiotag_str,"| ");
            x=tagdata_first_byte&0x0C;
            x=x>>2;
            switch (x)
            {
            case 0:strcat(audiotag_str,"5.5-kHz");break;
            case 1:strcat(audiotag_str,"1-kHz");break;
            case 2:strcat(audiotag_str,"22-kHz");break;
            case 3:strcat(audiotag_str,"44-kHz");break;
            default:strcat(audiotag_str,"UNKNOWN");break;
            }
            strcat(audiotag_str,"| ");
            x=tagdata_first_byte&0x02;
            x=x>>1;
            switch (x)
            {
            case 0:strcat(audiotag_str,"8Bit");break;
            case 1:strcat(audiotag_str,"16Bit");break;
            default:strcat(audiotag_str,"UNKNOWN");break;
            }
            strcat(audiotag_str,"| ");
            x=tagdata_first_byte&0x01;
            switch (x)
            {
            case 0:strcat(audiotag_str,"Mono");break;
            case 1:strcat(audiotag_str,"Stereo");break;
            default:strcat(audiotag_str,"UNKNOWN");break;
            }
            fprintf(myout,"%s",audiotag_str);

            //if the output file hasn't been opened, open it.
            if(output_a!=0&&afh == NULL){
                afh = fopen("output.mp3", "wb");
            }

            //TagData - First Byte Data
            int data_size=reverse_bytes((byte *)&tagheader.DataSize, sizeof(tagheader.DataSize))-1;
            if(output_a!=0){
                //TagData+1
                for (int i=0; i<data_size; i++)
                    fputc(fgetc(ifh),afh);

            }else{
                for (int i=0; i<data_size; i++)
                    fgetc(ifh);
            }
            break;
        }
        case TAG_TYPE_VIDEO:{
            char videotag_str[100]={0};
            strcat(videotag_str,"| ");
            char tagdata_first_byte;
            tagdata_first_byte=fgetc(ifh);
            int x=tagdata_first_byte&0xF0;
            x=x>>4;
            switch (x)
            {
            case 1:strcat(videotag_str,"key frame  ");break;
            case 2:strcat(videotag_str,"inter frame");break;
            case 3:strcat(videotag_str,"disposable inter frame");break;
            case 4:strcat(videotag_str,"generated keyframe");break;
            case 5:strcat(videotag_str,"video info/command frame");break;
            default:strcat(videotag_str,"UNKNOWN");break;
            }
            strcat(videotag_str,"| ");
            x=tagdata_first_byte&0x0F;
            switch (x)
            {
            case 1:strcat(videotag_str,"JPEG (currently unused)");break;
            case 2:strcat(videotag_str,"Sorenson H.263");break;
            case 3:strcat(videotag_str,"Screen video");break;
            case 4:strcat(videotag_str,"On2 VP6");break;
            case 5:strcat(videotag_str,"On2 VP6 with alpha channel");break;
            case 6:strcat(videotag_str,"Screen video version 2");break;
            case 7:strcat(videotag_str,"AVC");break;
            default:strcat(videotag_str,"UNKNOWN");break;
            }
            fprintf(myout,"%s",videotag_str);

            fseek(ifh, -1, SEEK_CUR);
            //if the output file hasn't been opened, open it.
            if (vfh == NULL&&output_v!=0) {
                //write the flv header (reuse the original file's hdr) and first previoustagsize
                    vfh = fopen("output.flv", "wb");
                    fwrite((char *)&flv,1, sizeof(flv),vfh);
                    fwrite((char *)&previoustagsize_z,1,sizeof(previoustagsize_z),vfh);
            }
#if 0
            //Change Timestamp
            //Get Timestamp
            ts = reverse_bytes((byte *)&tagheader.Timestamp, sizeof(tagheader.Timestamp));
            ts=ts*2;
            //Writeback Timestamp
            ts_new = reverse_bytes((byte *)&ts, sizeof(ts));
            memcpy(&tagheader.Timestamp, ((char *)&ts_new) + 1, sizeof(tagheader.Timestamp));
#endif

            //TagData + Previous Tag Size
            int data_size=reverse_bytes((byte *)&tagheader.DataSize, sizeof(tagheader.DataSize))+4;
            if(output_v!=0){
                //TagHeader
                fwrite((char *)&tagheader,1, sizeof(tagheader),vfh);
                //TagData
                for (int i=0; i<data_size; i++)
                    fputc(fgetc(ifh),vfh);
            }else{
                for (int i=0; i<data_size; i++)
                    fgetc(ifh);
            }
            //rewind 4 bytes, because we need to read the previoustagsize again for the loop's sake
            fseek(ifh, -4, SEEK_CUR);

            break;
            }
        default:

            //skip the data of this tag
            fseek(ifh, reverse_bytes((byte *)&tagheader.DataSize, sizeof(tagheader.DataSize)), SEEK_CUR);
        }

        fprintf(myout,"\n");

    } while (!feof(ifh));

    _fcloseall();

    return 0;
}

原文链接：https://blog.csdn.net/leixiaohua1020/article/details/50535082