本文主要是介绍ffmpeg源码分析二：main函数和transcode函数，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
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_AVDEVICEavdevice_register_all();
#endifavfilter_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_PTHREADSif ((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_PTHREADSfree_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_PTHREADSfree_input_threads();
#endifif (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;
}
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