ffmpeg获取h264视频数据

源资源来自教程，来自慕课网 李超，本人仅仅是添加注释 

#include <stdio.h>
#include <libavutil/log.h>
#include <libavformat/avio.h>
#include <libavformat/avformat.h>

#ifndef AV_WB32
#   define AV_WB32(p, val) do {                 \
        uint32_t d = (val);                     \
        ((uint8_t*)(p))[3] = (d);               \
        ((uint8_t*)(p))[2] = (d)>>8;            \
        ((uint8_t*)(p))[1] = (d)>>16;           \
        ((uint8_t*)(p))[0] = (d)>>24;           \
    } while(0)
#endif

#ifndef AV_RB16
#   define AV_RB16(x)                           \
    ((((const uint8_t*)(x))[0] << 8) |          \
      ((const uint8_t*)(x))[1])
#endif

static int alloc_and_copy(AVPacket *out,
                          const uint8_t *sps_pps, uint32_t sps_pps_size,
                          const uint8_t *in, uint32_t in_size)
{
    uint32_t offset         = out->size;
    uint8_t nal_header_size = offset ? 3 : 4;
    int err;

    err = av_grow_packet(out, sps_pps_size + in_size + nal_header_size);
    if (err < 0)
        return err;

    if (sps_pps)
        memcpy(out->data + offset, sps_pps, sps_pps_size);
    memcpy(out->data + sps_pps_size + nal_header_size + offset, in, in_size);
    if (!offset) {
        AV_WB32(out->data + sps_pps_size, 1);
    } else {
        (out->data + offset + sps_pps_size)[0] =
        (out->data + offset + sps_pps_size)[1] = 0;
        (out->data + offset + sps_pps_size)[2] = 1;
    }

    return 0;
}

int h264_extradata_to_annexb(const uint8_t *codec_extradata, const int codec_extradata_size, AVPacket *out_extradata, int padding)
{
    uint16_t unit_size;
    uint64_t total_size                 = 0;
    uint8_t *out                        = NULL, unit_nb, sps_done = 0,
             sps_seen                   = 0, pps_seen = 0, sps_offset = 0, pps_offset = 0;
	
    const uint8_t *extradata            = codec_extradata + 4; //扩展数据的前4个字节无效
	//nalu数据头
    static const uint8_t nalu_header[4] = { 0, 0, 0, 1 };
	//第一个数据的低2位用于表明每一个pps和sps数据的大小，一般是两个字节
    int length_size = (*extradata++ & 0x3) + 1; // retrieve length coded size, 用于指示表示编码数据长度所需字节数

    sps_offset = pps_offset = -1;

    /* retrieve sps and pps unit(s) */
	
	//第2个数据的低5位用于指示pps和sps的个数
    unit_nb = *extradata++ & 0x1f; /* number of sps unit(s) */
    if (!unit_nb) {
        goto pps;
    }else {
        sps_offset = 0;
        sps_seen = 1;
    }
	//依次处理每一个
    while (unit_nb--) {
        int err;
		
        unit_size   = AV_RB16(extradata);//读取2个字节 获取pps和sps的长度
        total_size += unit_size + 4; //sps和pps前边也有一个started code 就是4个字节
        if (total_size > INT_MAX - padding) {
            av_log(NULL, AV_LOG_ERROR,
                   "Too big extradata size, corrupted stream or invalid MP4/AVCC bitstream\n");
            av_free(out);
            return AVERROR(EINVAL);
        }
        if (extradata + 2 + unit_size > codec_extradata + codec_extradata_size) {
            av_log(NULL, AV_LOG_ERROR, "Packet header is not contained in global extradata, "
                   "corrupted stream or invalid MP4/AVCC bitstream\n");
            av_free(out);
            return AVERROR(EINVAL);
        }
		//分配空间
        if ((err = av_reallocp(&out, total_size + padding)) < 0)
            return err;
		//拷贝started code 数据
        memcpy(out + total_size - unit_size - 4, nalu_header, 4);
		//拷贝真正的sps和pps
        memcpy(out + total_size - unit_size, extradata + 2, unit_size);
        extradata += 2 + unit_size;
pps:
        if (!unit_nb && !sps_done++) {
            unit_nb = *extradata++; /* number of pps unit(s) */
            if (unit_nb) {
                pps_offset = total_size;
                pps_seen = 1;
            }
        }
    }

    if (out)
        memset(out + total_size, 0, padding);

    if (!sps_seen)
        av_log(NULL, AV_LOG_WARNING,
               "Warning: SPS NALU missing or invalid. "
               "The resulting stream may not play.\n");

    if (!pps_seen)
        av_log(NULL, AV_LOG_WARNING,
               "Warning: PPS NALU missing or invalid. "
               "The resulting stream may not play.\n");

    out_extradata->data      = out;
    out_extradata->size      = total_size;

    return length_size;
}

int h264_mp4toannexb(AVFormatContext *fmt_ctx, AVPacket *in, FILE *dst_fd)
{

    AVPacket *out = NULL;
    AVPacket spspps_pkt;

    int len;
    uint8_t unit_type;
    int32_t nal_size;
    uint32_t cumul_size    = 0;
    const uint8_t *buf;
    const uint8_t *buf_end;
    int            buf_size;
    int ret = 0, i;
	//分配一个输出数据包
    out = av_packet_alloc();

    buf      = in->data;
    buf_size = in->size;
    buf_end  = in->data + in->size;

    do {
        ret= AVERROR(EINVAL);
        if (buf + 4 /*s->length_size*/ > buf_end)
            goto fail;
		
		//前4个字节是数据包的大小
        for (nal_size = 0, i = 0; i<4/*s->length_size*/; i++)
            nal_size = (nal_size << 8) | buf[i]; 

		
        buf += 4; /*s->length_size;*/
		
		//第5个字节的低5位是包的类型
        unit_type = *buf & 0x1f;
		
		//判断数据长度合法
        if (nal_size > buf_end - buf || nal_size < 0)
            goto fail;

        /*
        if (unit_type == 7)
            s->idr_sps_seen = s->new_idr = 1;
        else if (unit_type == 8) {
            s->idr_pps_seen = s->new_idr = 1;
            */
            /* if SPS has not been seen yet, prepend the AVCC one to PPS */
            /*
            if (!s->idr_sps_seen) {
                if (s->sps_offset == -1)
                    av_log(ctx, AV_LOG_WARNING, "SPS not present in the stream, nor in AVCC, stream may be unreadable\n");
                else {
                    if ((ret = alloc_and_copy(out,
                                         ctx->par_out->extradata + s->sps_offset,
                                         s->pps_offset != -1 ? s->pps_offset : ctx->par_out->extradata_size - s->sps_offset,
                                         buf, nal_size)) < 0)
                        goto fail;
                    s->idr_sps_seen = 1;
                    goto next_nal;
                }
            }
        }
        */

        /* if this is a new IDR picture following an IDR picture, reset the idr flag.
         * Just check first_mb_in_slice to be 0 as this is the simplest solution.
         * This could be checking idr_pic_id instead, but would complexify the parsing. */
        /*
        if (!s->new_idr && unit_type == 5 && (buf[1] & 0x80))
            s->new_idr = 1;

        */
        /* prepend only to the first type 5 NAL unit of an IDR picture, if no sps/pps are already present */
		//==5说明是关键帧
        if (/*s->new_idr && */unit_type == 5 /*&& !s->idr_sps_seen && !s->idr_pps_seen*/) {

			//sps 和pps在扩展数据中 下面开始处理sps和pps
            h264_extradata_to_annexb( fmt_ctx->streams[in->stream_index]->codec->extradata,
                                      fmt_ctx->streams[in->stream_index]->codec->extradata_size,
                                      &spspps_pkt,
                                      AV_INPUT_BUFFER_PADDING_SIZE);
			//在数据包前边加上sps和pps数据
            if ((ret=alloc_and_copy(out,
                               spspps_pkt.data, spspps_pkt.size,
                               buf, nal_size)) < 0)
                goto fail;
            /*s->new_idr = 0;*/
        /* if only SPS has been seen, also insert PPS */
        }
        /*else if (s->new_idr && unit_type == 5 && s->idr_sps_seen && !s->idr_pps_seen) {
            if (s->pps_offset == -1) {
                av_log(ctx, AV_LOG_WARNING, "PPS not present in the stream, nor in AVCC, stream may be unreadable\n");
                if ((ret = alloc_and_copy(out, NULL, 0, buf, nal_size)) < 0)
                    goto fail;
            } else if ((ret = alloc_and_copy(out,
                                        ctx->par_out->extradata + s->pps_offset, ctx->par_out->extradata_size - s->pps_offset,
                                        buf, nal_size)) < 0)
                goto fail;
        }*/ else {
            if ((ret=alloc_and_copy(out, NULL, 0, buf, nal_size)) < 0)
                goto fail;
            /*
            if (!s->new_idr && unit_type == 1) {
                s->new_idr = 1;
                s->idr_sps_seen = 0;
                s->idr_pps_seen = 0;
            }
            */
        }


        len = fwrite( out->data, 1, out->size, dst_fd);
        if(len != out->size){
            av_log(NULL, AV_LOG_DEBUG, "warning, length of writed data isn't equal pkt.size(%d, %d)\n",
                    len,
                    out->size);
        }
        fflush(dst_fd);

next_nal:
        buf        += nal_size;
        cumul_size += nal_size + 4;//s->length_size;
    } while (cumul_size < buf_size);

    /*
    ret = av_packet_copy_props(out, in);
    if (ret < 0)
        goto fail;

    */
fail:
    av_packet_free(&out);

    return ret;
}

int main(int argc, char *argv[])
{
    int err_code;
    char errors[1024];

    char *src_filename = NULL;
    char *dst_filename = NULL;

    FILE *dst_fd = NULL;

    int video_stream_index = -1;

    //AVFormatContext *ofmt_ctx = NULL;
    //AVOutputFormat *output_fmt = NULL;
    //AVStream *out_stream = NULL;

    AVFormatContext *fmt_ctx = NULL;
    AVPacket pkt;

    //AVFrame *frame = NULL;

    av_log_set_level(AV_LOG_DEBUG);

    if(argc < 3){
        av_log(NULL, AV_LOG_DEBUG, "the count of parameters should be more than three!\n");
        return -1;
    }

    src_filename = argv[1];
    dst_filename = argv[2];

    if(src_filename == NULL || dst_filename == NULL){
        av_log(NULL, AV_LOG_ERROR, "src or dts file is null, plz check them!\n");
        return -1;
    }

    /*register all formats and codec*/
    av_register_all();

    dst_fd = fopen(dst_filename, "wb");
    if (!dst_fd) {
        av_log(NULL, AV_LOG_DEBUG, "Could not open destination file %s\n", dst_filename);
        return -1;
    }

    /*open input media file, and allocate format context*/
    if((err_code = avformat_open_input(&fmt_ctx, src_filename, NULL, NULL)) < 0){
        av_strerror(err_code, errors, 1024);
        av_log(NULL, AV_LOG_DEBUG, "Could not open source file: %s, %d(%s)\n",
               src_filename,
               err_code,
               errors);
        return -1;
    }

    /*dump input information*/
    av_dump_format(fmt_ctx, 0, src_filename, 0);

    /*initialize packet*/
    av_init_packet(&pkt);
    pkt.data = NULL;
    pkt.size = 0;

    /*find best video stream*/
	//获取音频流通道 video_stream_index
    video_stream_index = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
    if(video_stream_index < 0){
        av_log(NULL, AV_LOG_DEBUG, "Could not find %s stream in input file %s\n",
               av_get_media_type_string(AVMEDIA_TYPE_VIDEO),
               src_filename);
        return AVERROR(EINVAL);
    }

    /*
    if (avformat_write_header(ofmt_ctx, NULL) < 0) {
        av_log(NULL, AV_LOG_DEBUG, "Error occurred when opening output file");
        exit(1);
    }
    */

    /*read frames from media file*/
    while(av_read_frame(fmt_ctx, &pkt) >=0 ){
        if(pkt.stream_index == video_stream_index){
            /*
            pkt.stream_index = 0;
            av_write_frame(ofmt_ctx, &pkt);
            av_free_packet(&pkt);
            */

            h264_mp4toannexb(fmt_ctx, &pkt, dst_fd);

        }

        //release pkt->data
        av_packet_unref(&pkt);
    }

    //av_write_trailer(ofmt_ctx);

    /*close input media file*/
    avformat_close_input(&fmt_ctx);
    if(dst_fd) {
        fclose(dst_fd);
    }

    //avio_close(ofmt_ctx->pb);

    return 0;
}