ffmpeg——main()

ffmpeg的主函数入口：ffmpeg.c中的main函数

int main(int argc, char **argv)
{
    int i, ret;
    int64_t ti;

    init_dynload();

    register_exit(ffmpeg_cleanup);

    setvbuf(stderr,NULL,_IONBF,0); /* win32 runtime needs this */

    av_log_set_flags(AV_LOG_SKIP_REPEATED);
    parse_loglevel(argc, argv, options);

    if(argc>1 && !strcmp(argv[1], "-d")){
        run_as_daemon=1;
        av_log_set_callback(log_callback_null);
        argc--;
        argv++;
    }

    avcodec_register_all();//注册所有的编解码器和硬解加速器
#if CONFIG_AVDEVICE
    avdevice_register_all();
#endif
    avfilter_register_all();//注册滤波器，就是把filter加到AVFilter链表尾部
    av_register_all();//把对应的format加到AVInputFormat，AVOutputFormat链表的尾部
    avformat_network_init();

    show_banner(argc, argv, options);

    /* parse options and open all input/output files */
    ret = ffmpeg_parse_options(argc, argv);
    if (ret < 0)
        exit_program(1);

    if (nb_output_files <= 0 && nb_input_files == 0) {
        show_usage();
        av_log(NULL, AV_LOG_WARNING, "Use -h to get full help or, even better, run 'man %s'\n", program_name);
        exit_program(1);
    }

    /* file converter / grab */
    if (nb_output_files <= 0) {
        av_log(NULL, AV_LOG_FATAL, "At least one output file must be specified\n");
        exit_program(1);
    }

//     if (nb_input_files == 0) {
//         av_log(NULL, AV_LOG_FATAL, "At least one input file must be specified\n");
//         exit_program(1);
//     }

    for (i = 0; i < nb_output_files; i++) {
        if (strcmp(output_files[i]->ctx->oformat->name, "rtp"))
            want_sdp = 0;
    }

    current_time = ti = getutime();
    if (transcode() < 0)
        exit_program(1);
    ti = getutime() - ti;
    if (do_benchmark) {
        av_log(NULL, AV_LOG_INFO, "bench: utime=%0.3fs\n", ti / 1000000.0);
    }
    av_log(NULL, AV_LOG_DEBUG, "%"PRIu64" frames successfully decoded, %"PRIu64" decoding errors\n",
           decode_error_stat[0], decode_error_stat[1]);
    if ((decode_error_stat[0] + decode_error_stat[1]) * max_error_rate < decode_error_stat[1])
        exit_program(69);

    exit_program(received_nb_signals ? 255 : main_return_code);
    return main_return_code;
}

avcodec_register_all–>register_all–>REGISTER_HWACCEL REGISTER_ENCODER REGISTER_DECODER REGISTER_PARSER –>av_register_hwaccel avcodec_register avcodec_register av_register_codec_parser
(加入到AVHWAccel，AVCodec，AVCodecParser链表的尾部)

一个位置位于doc/examples/的demo，如文件decode_video.c：

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];
    outfilename = argv[2];

    avcodec_register_all();//注册所有的编解码器和硬解加速器

    pkt = av_packet_alloc();//分配pkt地址，并初始化
    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);//在解码器链表中查找对应的解码器
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }

    parser = av_parser_init(codec->id);//在解析器链表中查找对应的parser，并对AVCodecParserContext进行初始化
    if (!parser) {
        fprintf(stderr, "parser not found\n");
        exit(1);
    }

    c = avcodec_alloc_context3(codec);//对AVCodecContext进行初始化
    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
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }

    f = fopen(filename, "rb");//打开输入码流
    if (!f) {
        fprintf(stderr, "Could not open %s\n", filename);
        exit(1);
    }

    frame = av_frame_alloc();//对输出帧frame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);//从输入文件中读取数据
        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);//用于得到一帧
            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。。会调用avctx->codec->decode解码一帧
        }
    }

    /* 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;
}

了解FFMPEG API最好的方式就是参考API DEMO了：
源码位置位于tests/api/，比如文件api-h264-test.c：

int main(int argc, char **argv)
{
    if (argc < 2)
    {
        av_log(NULL, AV_LOG_ERROR, "Incorrect input\n");
        return 1;
    }

    av_register_all();//注册encoder、decoder、parser等

    if (video_decode_example(argv[1]) != 0)//用于解码
        return 1;

    return 0;
}

static int video_decode_example(const char *input_filename)
{
    AVCodec *codec = NULL;
    AVCodecContext *ctx= NULL;
    AVCodecParameters *origin_par = NULL;
    AVFrame *fr = NULL;
    uint8_t *byte_buffer = NULL;
    AVPacket pkt;
    AVFormatContext *fmt_ctx = NULL;
    int number_of_written_bytes;
    int video_stream;
    int got_frame = 0;
    int byte_buffer_size;
    int i = 0;
    int result;
    int end_of_stream = 0;

    result = avformat_open_input(&fmt_ctx, input_filename, NULL, NULL);//打开输入的文件
    if (result < 0) {
        av_log(NULL, AV_LOG_ERROR, "Can't open file\n");
        return result;
    }

    result = avformat_find_stream_info(fmt_ctx, NULL);//从文件中提取流信息
    if (result < 0) {
        av_log(NULL, AV_LOG_ERROR, "Can't get stream info\n");
        return result;
    }

    video_stream = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
    if (video_stream < 0) {
      av_log(NULL, AV_LOG_ERROR, "Can't find video stream in input file\n");
      return -1;
    }

    origin_par = fmt_ctx->streams[video_stream]->codecpar;

    codec = avcodec_find_decoder(origin_par->codec_id);//找到对应的解码器
    if (!codec) {
        av_log(NULL, AV_LOG_ERROR, "Can't find decoder\n");
        return -1;
    }

    ctx = avcodec_alloc_context3(codec);//对AVCodecContext进行初始化
    if (!ctx) {
        av_log(NULL, AV_LOG_ERROR, "Can't allocate decoder context\n");
        return AVERROR(ENOMEM);
    }

    result = avcodec_parameters_to_context(ctx, origin_par);//将参数origin_par赋值给ctx
    if (result) {
        av_log(NULL, AV_LOG_ERROR, "Can't copy decoder context\n");
        return result;
    }

    result = avcodec_open2(ctx, codec, NULL);//打开解码器
    if (result < 0) {
        av_log(ctx, AV_LOG_ERROR, "Can't open decoder\n");
        return result;
    }

    fr = av_frame_alloc();//为解码输出的帧fr分配内存
    if (!fr) {
        av_log(NULL, AV_LOG_ERROR, "Can't allocate frame\n");
        return AVERROR(ENOMEM);
    }

    byte_buffer_size = av_image_get_buffer_size(ctx->pix_fmt, ctx->width, ctx->height, 16);//根据pix_fmt、width、height得到大小
    byte_buffer = av_malloc(byte_buffer_size);
    if (!byte_buffer) {
        av_log(NULL, AV_LOG_ERROR, "Can't allocate buffer\n");
        return AVERROR(ENOMEM);
    }

    printf("#tb %d: %d/%d\n", video_stream, fmt_ctx->streams[video_stream]->time_base.num, fmt_ctx->streams[video_stream]->time_base.den);
    i = 0;
    av_init_packet(&pkt);//初始化码流包pkt
    do {
        if (!end_of_stream)
            if (av_read_frame(fmt_ctx, &pkt) < 0)//从码流中读取一帧数据，放入pkg
                end_of_stream = 1;
        if (end_of_stream) {
            pkt.data = NULL;
            pkt.size = 0;
        }
        if (pkt.stream_index == video_stream || end_of_stream) {
            got_frame = 0;
            if (pkt.pts == AV_NOPTS_VALUE)
                pkt.pts = pkt.dts = i;
            result = avcodec_decode_video2(ctx, fr, &got_frame, &pkt);//会调用avctx->codec->decode解码一帧
            if (result < 0) {
                av_log(NULL, AV_LOG_ERROR, "Error decoding frame\n");
                return result;
            }
            if (got_frame) {
                number_of_written_bytes = av_image_copy_to_buffer(byte_buffer, byte_buffer_size,
                                        (const uint8_t* const *)fr->data, (const int*) fr->linesize,
                                        ctx->pix_fmt, ctx->width, ctx->height, 1);
                if (number_of_written_bytes < 0) {
                    av_log(NULL, AV_LOG_ERROR, "Can't copy image to buffer\n");
                    return number_of_written_bytes;
                }
                printf("%d, %10"PRId64", %10"PRId64", %8"PRId64", %8d, 0x%08lx\n", video_stream,
                        fr->pts, fr->pkt_dts, fr->pkt_duration,
                        number_of_written_bytes, av_adler32_update(0, (const uint8_t*)byte_buffer, number_of_written_bytes));
            }
            av_packet_unref(&pkt);
            av_init_packet(&pkt);
        }
        i++;
    } while (!end_of_stream || got_frame);

    av_packet_unref(&pkt);
    av_frame_free(&fr);
    avcodec_close(ctx);
    avformat_close_input(&fmt_ctx);
    avcodec_free_context(&ctx);
    av_freep(&byte_buffer);
    return 0;
}