本文中的程序是一個(gè)H.264碼流解析程序。該程序可以從H.264碼流中分析得到它的基本單元NALU随闺，并且可以簡(jiǎn)單解析NALU首部的字段。

原理

H.264原始碼流（又稱(chēng)為“裸流”）是由一個(gè)一個(gè)的NALU組成的。他們的結(jié)構(gòu)如下圖所示痴晦。

image.png

其中每個(gè)NALU之間通過(guò)startcode（起始碼）進(jìn)行分隔，起始碼分成兩種：0x000001（3Byte）或者0x00000001（4Byte）琳彩。如果NALU對(duì)應(yīng)的Slice為一幀的開(kāi)始就用0x00000001誊酌，否則就用0x000001。
H.264碼流解析的步驟就是首先從碼流中搜索0x000001和0x00000001露乏，分離出NALU碧浊；然后再分析NALU的各個(gè)字段。本文的程序即實(shí)現(xiàn)了上述的兩個(gè)步驟瘟仿。

『NALU』的格式如下圖3（引用H264 PDF）所示：

image.png

很明顯箱锐，『NALU』由頭和身體兩個(gè)部分組成：

• 頭：一般存儲(chǔ)標(biāo)志信息，譬如NALU的類(lèi)型猾骡。存儲(chǔ)了和編解碼信息相關(guān)的數(shù)據(jù)瑞躺；

• 身體：存儲(chǔ)了真正的數(shù)據(jù)。但實(shí)際上兴想，這塊也會(huì)相對(duì)比較復(fù)雜幢哨，過(guò)H264的一個(gè)目的是“網(wǎng)絡(luò)友好性”，說(shuō)白了就是能夠很好地適配各種傳輸格式嫂便。所以根據(jù)實(shí)際采用數(shù)據(jù)傳輸流格式捞镰，也會(huì)對(duì)這部分?jǐn)?shù)據(jù)格式再進(jìn)行處理。

代碼

#import "ViewController.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef enum {
    NALU_TYPE_SLICE    = 1,
    NALU_TYPE_DPA      = 2,
    NALU_TYPE_DPB      = 3,
    NALU_TYPE_DPC      = 4,
    NALU_TYPE_IDR      = 5,
    NALU_TYPE_SEI      = 6,
    NALU_TYPE_SPS      = 7,
    NALU_TYPE_PPS      = 8,
    NALU_TYPE_AUD      = 9,
    NALU_TYPE_EOSEQ    = 10,
    NALU_TYPE_EOSTREAM = 11,
    NALU_TYPE_FILL     = 12,
} NaluType;

typedef enum {
    NALU_PRIORITY_DISPOSABLE = 0,
    NALU_PRIRITY_LOW         = 1,
    NALU_PRIORITY_HIGH       = 2,
    NALU_PRIORITY_HIGHEST    = 3
} NaluPriority;

/**
 NALU = NALU Header + NALU Body
 1.NALU Header
 首先毙替，『NALU Header』只占1個(gè)字節(jié)岸售，即8位，其組成如下：
 
 | forbidden_zero_bit | nal_ref_idc | nal_unit_type |
 `--------------------+-------------+---------------`
 |        1 bit       |    2 bit    |    5 bit      |
 
 */
typedef struct
{
    int startcodeprefix_len;      //! 4 for parameter sets and first slice in picture, 3 for everything else (suggested)
    unsigned len;                 //! Length of the NAL unit (Excluding the start code, which does not belong to the NALU)
    unsigned max_size;            //! Nal Unit Buffer size  這個(gè)值是自己設(shè)置的厂画，項(xiàng)目里設(shè)置是100000凸丸， 和nalu的結(jié)構(gòu)沒(méi)有關(guān)系
    int forbidden_bit;            //! should be always FALSE 占用1位
    int nal_reference_idc;        //! NALU_PRIORITY_xxxx 占用2位
    int nal_unit_type;            //! NALU_TYPE_xxxx 占用5位
    char *buf;                    //! contains the first byte followed by the EBSP NALU的數(shù)據(jù)體部分?jǐn)?shù)據(jù)
} NALU_t;

FILE *h264bitstream = NULL;                //!< the bit stream file

int info2=0, info3=0;

static int FindStartCode2 (unsigned char *Buf){
    if(Buf[0]!=0 || Buf[1]!=0 || Buf[2] !=1) return 0; //查找0x000001?
    else return 1;
}

static int FindStartCode3 (unsigned char *Buf){
    if(Buf[0]!=0 || Buf[1]!=0 || Buf[2] !=0 || Buf[3] !=1) return 0;//查找0x00000001?
    else return 1;
}

// 獲取Annexb格式的NALU，注意Annex B和 AVCC格式的區(qū)別袱院，可以自行百度
int GetAnnexbNALU (NALU_t *nalu){
    int pos = 0;
    int StartCodeFound, rewind;
    unsigned char *Buf;

    if ((Buf = (unsigned char*)calloc (nalu->max_size , sizeof(char))) == NULL)
        printf ("GetAnnexbNALU: Could not allocate Buf memory\n");

    nalu->startcodeprefix_len=3;

    //成功讀取的元素總數(shù)會(huì)以 size_t 對(duì)象返回屎慢，size_t 對(duì)象是一個(gè)整型數(shù)據(jù)類(lèi)型。如果總數(shù)與 nmemb 參數(shù)不同忽洛，則可能發(fā)生了一個(gè)錯(cuò)誤或者到達(dá)了文件末尾腻惠。
    if (3 != fread (Buf, 1, 3, h264bitstream)){// 讀取3個(gè)字節(jié)
        free(Buf);
        return 0;
    }
    info2 = FindStartCode2 (Buf);// 查找startCode 0x000001
    if(info2 != 1) {// 沒(méi)找到
        if(1 != fread(Buf+3, 1, 1, h264bitstream)){// 讀取一個(gè)字節(jié)
            free(Buf);
            return 0;
        }
        info3 = FindStartCode3 (Buf);// 查找startCode 0x00000001
        if (info3 != 1){
            free(Buf);
            return -1;
        }
        else {// 找到了startCode 0x00000001
            pos = 4;
            nalu->startcodeprefix_len = 4;
        }
    }
    else{//// 找到了startCode 0x000001
        nalu->startcodeprefix_len = 3;
        pos = 3;
    }
    StartCodeFound = 0;
    info2 = 0;
    info3 = 0;

    while (!StartCodeFound){// 循環(huán)查找startCode
        if (feof (h264bitstream)){
            nalu->len = (pos-1)-nalu->startcodeprefix_len;
            memcpy (nalu->buf, &Buf[nalu->startcodeprefix_len], nalu->len);
            nalu->forbidden_bit = nalu->buf[0] & 0x80; //1 bit 0x80的二進(jìn)制為10000000
            nalu->nal_reference_idc = nalu->buf[0] & 0x60; // 2 bit , 0x60的二進(jìn)制為1100000
            nalu->nal_unit_type = (nalu->buf[0]) & 0x1f;// 5 bit 0x1f的十進(jìn)制是31，31的二進(jìn)制為11111
            free(Buf);
            return pos-1;
        }
        /**
         int fgetc(FILE *stream)
         從指定的流 stream 獲取下一個(gè)字符（一個(gè)無(wú)符號(hào)字符）欲虚，并把位置標(biāo)識(shí)符往前移動(dòng)集灌。
         */
        Buf[pos++] = fgetc (h264bitstream);// pos位置加1
        // 查找startCode
        info3 = FindStartCode3(&Buf[pos-4]);
        if(info3 != 1)
            info2 = FindStartCode2(&Buf[pos-3]);
        StartCodeFound = (info2 == 1 || info3 == 1);
    }

    // Here, we have found another start code (and read length of startcode bytes more than we should
    // have.  Hence, go back in the file
    rewind = (info3 == 1)? -4 : -3;

    /**
     int fseek(FILE *stream, long int offset, int whence)
     設(shè)置流 stream 的文件位置為給定的偏移 offset，參數(shù) offset 意味著從給定的 whence 位置查找的字節(jié)數(shù)复哆。
     */
    if (0 != fseek (h264bitstream, rewind, SEEK_CUR)){
        free(Buf);
        printf("GetAnnexbNALU: Cannot fseek in the bit stream file");
    }

    // Here the Start code, the complete NALU, and the next start code is in the Buf.
    // The size of Buf is pos, pos+rewind are the number of bytes excluding the next
    // start code, and (pos+rewind)-startcodeprefix_len is the size of the NALU excluding the start code

    nalu->len = (pos+rewind)-nalu->startcodeprefix_len;
    memcpy (nalu->buf, &Buf[nalu->startcodeprefix_len], nalu->len);//給nalu填充數(shù)據(jù)欣喧，填充的數(shù)據(jù)是不包含startCode(本身NALU的定義就是NALU header + NALU body腌零，并不包含startCode）,但是包含NALU header 從Buf偏移startcode長(zhǎng)度的字節(jié)數(shù)位置開(kāi)始，填充長(zhǎng)度為nalu->len,
    nalu->forbidden_bit = nalu->buf[0] & 0x80; //1 bit 0x80的二進(jìn)制為10000000
    nalu->nal_reference_idc = nalu->buf[0] & 0x60; // 2 bit 0x60的二進(jìn)制為1100000
    nalu->nal_unit_type = (nalu->buf[0]) & 0x1f;// 5 bit 0x1f的十進(jìn)制是31续誉，31的二進(jìn)制為11111
    free(Buf);

    return (pos+rewind);
}

//H.264碼流解析的步驟就是首先從碼流中搜索0x000001和0x00000001莱没，分離出NALU；然后再分析NALU的各個(gè)字段酷鸦。本文的程序即實(shí)現(xiàn)了上述的兩個(gè)步驟。
/**
 * Analysis H.264 Bitstream
 * @param url    Location of input H.264 bitstream file.
 */
int simplest_h264_parser(char *url){

    NALU_t *n;
    int buffersize=100000;

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

    h264bitstream=fopen(url, "rb+");
    if (h264bitstream==NULL){
        printf("Open file error\n");
        return 0;
    }

    n = (NALU_t*)calloc (1, sizeof (NALU_t));
    if (n == NULL){
        printf("Alloc NALU Error\n");
        return 0;
    }

    n->max_size=buffersize;
    n->buf = (char*)calloc (buffersize, sizeof (char));
    if (n->buf == NULL){
        free (n);
        printf ("AllocNALU: n->buf");
        return 0;
    }

    int data_offset=0;
    int nal_num=0;
    printf("-----+-------- NALU Table ------+---------+\n");
    printf(" NUM |    POS  |    IDC |  TYPE |   LEN   |\n");
    printf("-----+---------+--------+-------+---------+\n");

    while(!feof(h264bitstream))
    {
        int data_lenth;
        data_lenth=GetAnnexbNALU(n);// 讀取NALU

        char type_str[20]={0};
        switch(n->nal_unit_type){
            case NALU_TYPE_SLICE:sprintf(type_str,"SLICE");break;
            case NALU_TYPE_DPA:sprintf(type_str,"DPA");break;
            case NALU_TYPE_DPB:sprintf(type_str,"DPB");break;
            case NALU_TYPE_DPC:sprintf(type_str,"DPC");break;
            case NALU_TYPE_IDR:sprintf(type_str,"IDR");break;
            case NALU_TYPE_SEI:sprintf(type_str,"SEI");break;
            case NALU_TYPE_SPS:sprintf(type_str,"SPS");break;
            case NALU_TYPE_PPS:sprintf(type_str,"PPS");break;
            case NALU_TYPE_AUD:sprintf(type_str,"AUD");break;
            case NALU_TYPE_EOSEQ:sprintf(type_str,"EOSEQ");break;
            case NALU_TYPE_EOSTREAM:sprintf(type_str,"EOSTREAM");break;
            case NALU_TYPE_FILL:sprintf(type_str,"FILL");break;
        }
        char idc_str[20]={0};
        switch(n->nal_reference_idc>>5){ //nal_reference_idc 占2位牙咏，指示當(dāng)前NALU的優(yōu)先級(jí)臼隔，或者說(shuō)重要性，數(shù)值越大表明越重要妄壶。所以取值范圍為0～3
            case NALU_PRIORITY_DISPOSABLE:sprintf(idc_str,"DISPOS");break;
            case NALU_PRIRITY_LOW:sprintf(idc_str,"LOW");break;
            case NALU_PRIORITY_HIGH:sprintf(idc_str,"HIGH");break;
            case NALU_PRIORITY_HIGHEST:sprintf(idc_str,"HIGHEST");break;
        }

        fprintf(myout,"%5d| %8d| %7s| %6s| %8d|\n",nal_num,data_offset,idc_str,type_str,n->len);

        data_offset=data_offset+data_lenth;

        nal_num++;
    }

    //Free
    if (n){
        if (n->buf){
            free(n->buf);
            n->buf=NULL;
        }
        free (n);
    }
    return 0;
}

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    NSString *fileName = [[NSBundle mainBundle] pathForResource:@"sintel" ofType:@"h264"];
    
    const char *filePath = [fileName cStringUsingEncoding:NSUTF8StringEncoding];
    
    simplest_h264_parser(filePath);
}


@end

結(jié)果

本程序的輸入為一個(gè)H.264原始碼流（裸流）的文件路徑摔握，輸出為該碼流的NALU統(tǒng)計(jì)數(shù)據(jù)，如下圖所示丁寄。

image.png

參考

視音頻數(shù)據(jù)處理入門(mén)：H.264視頻碼流解析
https://blog.csdn.net/leixiaohua1020/article/details/50534369