ffmpeg源码分析二：main函数和transcode函数

首先从main函数看起，关键解释部分已加注释，该函数在ffmpeg.c文件中。代码如下：

int main(int argc, char **argv){    int ret;    int64_t ti;    // 注册清理回调函数    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++;    }    // 注册组件们    avcodec_register_all();#if CONFIG_AVDEVICE    avdevice_register_all();#endif    avfilter_register_all();    av_register_all();    avformat_network_init();    show_banner(argc, argv, options);    term_init();    /* parse options and open all input/output files */    // 解析参数，并且打开输入输出文件    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);//     }    current_time = ti = getutime();    // 音视频转换函数    if (transcode() < 0)        exit_program(1);    ti = getutime() - ti;    if (do_benchmark) {        printf("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;}

下面阅读transcode函数：

static int transcode(void){    int ret, i;    AVFormatContext *os;    OutputStream *ost;    InputStream *ist;    int64_t timer_start;    // 初始化，打开所有输出流的编码器,打开所有输入流的解码器,写入所有输出文件的文件头。后面详细介绍。    ret = transcode_init();    if (ret < 0)        goto fail;    if (stdin_interaction) {        av_log(NULL, AV_LOG_INFO, "Press [q] to stop, [?] for help\n");    }    timer_start = av_gettime();#if HAVE_PTHREADS    if ((ret = init_input_threads()) < 0)        goto fail;#endif       // 循环,直到收到系统信号才退出 。    while (!received_sigterm) {        int64_t cur_time= av_gettime();        /* if 'q' pressed, exits */        if (stdin_interaction)            if (check_keyboard_interaction(cur_time) < 0)                break;        /* check if there's any stream where output is still needed */        if (!need_output()) {            av_log(NULL, AV_LOG_VERBOSE, "No more output streams to write to, finishing.\n");            break;        }        // 具体的转换工作，后面将详细介绍。        ret = transcode_step();        if (ret < 0) {            if (ret == AVERROR_EOF || ret == AVERROR(EAGAIN))                continue;            av_log(NULL, AV_LOG_ERROR, "Error while filtering.\n");            break;        }        /* dump report by using the output first video and audio streams */        print_report(0, timer_start, cur_time);    }#if HAVE_PTHREADS    free_input_threads();#endif    // 文件处理完了,把缓冲中剩余的数据写到输出文件中。    /* at the end of stream, we must flush the decoder buffers */    for (i = 0; i < nb_input_streams; i++) {        ist = input_streams[i];        if (!input_files[ist->file_index]->eof_reached && ist->decoding_needed) {            output_packet(ist, NULL);        }    }    flush_encoders();    term_exit();    /* write the trailer if needed and close file */    // 为输出文件写文件尾(有的不需要)。      for (i = 0; i < nb_output_files; i++) {        os = output_files[i]->ctx;        av_write_trailer(os);    }    /* dump report by using the first video and audio streams */    print_report(1, timer_start, av_gettime());    // 关闭所有编码器。    /* close each encoder */    for (i = 0; i < nb_output_streams; i++) {        ost = output_streams[i];        if (ost->encoding_needed) {            av_freep(&ost->st->codec->stats_in);            avcodec_close(ost->st->codec);        }    }    // 关闭所有解码器。    /* close each decoder */    for (i = 0; i < nb_input_streams; i++) {        ist = input_streams[i];        if (ist->decoding_needed) {            avcodec_close(ist->st->codec);            if (ist->hwaccel_uninit)                ist->hwaccel_uninit(ist->st->codec);        }    }    /* finished ! */    ret = 0; fail:#if HAVE_PTHREADS    free_input_threads();#endif    if (output_streams) {        for (i = 0; i < nb_output_streams; i++) {            ost = output_streams[i];            if (ost) {                if (ost->stream_copy)                    av_freep(&ost->st->codec->extradata);                if (ost->logfile) {                    fclose(ost->logfile);                    ost->logfile = NULL;                }                av_freep(&ost->st->codec->subtitle_header);                av_freep(&ost->forced_kf_pts);                av_freep(&ost->apad);                av_dict_free(&ost->opts);                av_dict_free(&ost->swr_opts);                av_dict_free(&ost->resample_opts);            }        }    }    return ret;}