ffmpeg——decodevideo的一个demo

doc\examples文件夹中的decode_video.c为例：
main—>avcodec_find_decoder—>av_parser_init—>avcodec_alloc_context3—>avcodec_open2—>av_parser_parse2—>decode

int main(int argc, char **argv)
{
    const char *filename, *outfilename;
    const AVCodec *codec;
    AVCodecParserContext *parser;
    AVCodecContext *c= NULL;
    FILE *f;
    AVFrame *frame;
    uint8_t inbuf[INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
    uint8_t *data;
    size_t   data_size;
    int ret;
    AVPacket *pkt;
    if (argc <= 2) {
        fprintf(stderr, "Usage: %s <input file> <output file>\n", argv[0]);
        exit(0);
    }
    filename    = argv[1];   //输入文件
    outfilename = argv[2];  //输出文件
    avcodec_register_all();   //注册register_all
    pkt = av_packet_alloc();
    if (!pkt)
        exit(1);
    /* set end of buffer to 0 (this ensures that no overreading happens for damaged MPEG streams) */
    memset(inbuf + INBUF_SIZE, 0, AV_INPUT_BUFFER_PADDING_SIZE);
    /* find the MPEG-1 video decoder */
    codec = avcodec_find_decoder(AV_CODEC_ID_MPEG1VIDEO);//在链表中找到对应的解码器，如AVCodec ff_h263_decoder
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }
    parser = av_parser_init(codec->id);//在链表中找到对应的解析器，如AVCodecParser ff_h263_parser
    if (!parser) {
        fprintf(stderr, "parser not found\n");
        exit(1);
    }
    c = avcodec_alloc_context3(codec);//初始化AVCodecContext *s
    if (!c) {
        fprintf(stderr, "Could not allocate video codec context\n");
        exit(1);
    }
    /* For some codecs, such as msmpeg4 and mpeg4, width and height
       MUST be initialized there because this information is not
       available in the bitstream. */
    /* open it */
    if (avcodec_open2(c, codec, NULL) < 0) {//打开解码器，并初始化一些信息
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }
    f = fopen(filename, "rb");
    if (!f) {
        fprintf(stderr, "Could not open %s\n", filename);
        exit(1);
    }
    frame = av_frame_alloc();//为输出帧分配地址
    if (!frame) {
        fprintf(stderr, "Could not allocate video frame\n");
        exit(1);
    }
    while (!feof(f)) {
        /* read raw data from the input file */
        data_size = fread(inbuf, 1, INBUF_SIZE, f);//从输入文件中读入数据到inbuf中
        if (!data_size)
            break;
        /* use the parser to split the data into frames */
        data = inbuf;
        while (data_size > 0) {
            ret = av_parser_parse2(parser, c, &pkt->data, &pkt->size,
                                   data, data_size, AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);//解析包，用于得到一帧数据
            if (ret < 0) {
                fprintf(stderr, "Error while parsing\n");
                exit(1);
            }
            data      += ret;
            data_size -= ret;
            if (pkt->size)
                decode(c, frame, pkt, outfilename);//解码，输出帧为frame，放入outfilename文件中
        }
    }
    /* flush the decoder */
    decode(c, frame, NULL, outfilename);
    fclose(f);
    av_parser_close(parser);
    avcodec_free_context(&c);
    av_frame_free(&frame);
    av_packet_free(&pkt);
    return 0;
}