最简单的基于FFMPEG+SDL的音视频播放器

一、概述

        在《最简单的基于FFMPEG+SDL的音频播放器》记录一中，我们实现了音频的播放。更早前，我们在《最简单的基于FFMPEG+SDL的视频播放器》记录一和二中，实现了视频的播放。在实现视频播放的时候，我们设置了一个延迟40ms，否则视频就会以解码的速度去播放，很快速。在音频播放器中，我们有两个地方控制了播放音频的速度。第一个是采样率(http://wiki.libsdl.org/SDL_AudioSpec)，采样率决定每秒送多少样本数据帧到音频设备，采样率和播放速度成正比。第二个是调用SDL_PauseAudio(0)播放音频后，有一个循环等待缓冲区数据播放完毕，才能进行下一帧数据帧的播放。

                //Play                    SDL_PauseAudio(0);                  while (audio_len > 0)//Wait until finish                        SDL_Delay(1);//ms  

这两点就保证了音频的正常播放，而不用视频播放人为的去设定一个延迟时间保证播放速度。

        这一次，我们将同时完成视频播放和音频播放。但我们将采用不同的方式去实现音频播放，核心内容跟”音频播放器“是一样的，只是我们采用了队列来保存解复用后的包，这样将解复用和解码两个过程分开，更加清晰，也有利于将来扩展更多的功能。

二、主要内容

1、首先、我们先描述一下我们将要用于保存包的队列，该结构体如下：

/*包队列*/typedef struct PacketQueue {AVPacketList *first_pkt, *last_pkt;int nb_packets;//包个数int size;//包大小SDL_mutex *mutex;//互斥量SDL_cond *cond;//条件变量} PacketQueue;

AVPacketList：AVPacketList是FFMPEG内置的结构体，是包的一个链表，结构体如下：

typedef struct AVPacketList {     AVPacket pkt;     struct AVPacketList *next;  } AVPacketList;

nb_packets:队列中包数量

size:队列中所有包的大小，以字节为单位

mutex和cond：互斥量和条件变量，因为音频播放是在另外一个线程中（想想那个callback），所以对于队列这个公共资源的读写需要互斥。如果对于互斥量和条件变量不熟悉可以参考生产者-消费者问题。

2、关于该队列的一些操作如下：

2.1、初始化队列

/*初始化队列*/void packet_queue_init(PacketQueue *q) {memset(q, 0, sizeof(PacketQueue));//将队列所在内存用0填充q->mutex = SDL_CreateMutex();//创建互斥量和条件变量q->cond = SDL_CreateCond();}

2.2、入队

/*入队操作*/int packet_queue_put(PacketQueue *q, AVPacket *pkt) {AVPacketList *pkt1;if (av_dup_packet(pkt) < 0) {//将pkt的内存复制到独立的内存中return -1;}pkt1 = (AVPacketList*)av_malloc(sizeof(AVPacketList));if (!pkt1) return -1;pkt1->pkt = *pkt;pkt1->next = NULL;SDL_LockMutex(q->mutex);//先锁住队列，然后再入队if (!q->last_pkt)q->first_pkt = pkt1;elseq->last_pkt->next = pkt1;q->last_pkt = pkt1;q->nb_packets++;q->size += pkt1->pkt.size;SDL_CondSignal(q->cond);//唤醒被该条件变量阻塞的线程SDL_UnlockMutex(q->mutex);//解锁资源return 0;}

2.3、出队

/*出队操作q:     队列pkt:   出队的包block: 是否人工阻塞return: -1 退出  0 阻塞  1 成功*/int packet_queue_get(PacketQueue *q, AVPacket *pkt, int block){AVPacketList *pkt1;int ret;SDL_LockMutex(q->mutex);//锁住队列，再出队for (;;) {if (quit) {//退出，quit是全局变量，暂时可不管ret = -1;break;}pkt1 = q->first_pkt;if (pkt1) {//正常出队q->first_pkt = pkt1->next;if (!q->first_pkt)q->last_pkt = NULL;q->nb_packets--;q->size -= pkt1->pkt.size;*pkt = pkt1->pkt;av_free(pkt1);ret = 1;break;}else if (!block) {//阻塞ret = 0;break;}else {SDL_CondWait(q->cond, q->mutex);//阻塞线程等待条件变量激活并解锁，收到激活信号后，再次上锁}}SDL_UnlockMutex(q->mutex);//解锁资源return ret;}

程序中的quit是一个全局变量，保证视频播放完成后可以正常退出，否则进程将无法结束。代码如下：

//当SDL退出时，设置quit=1SDL_PollEvent(&event);  switch(event.type) {  case SDL_QUIT:    quit = 1;     ....

剩下的事情就和原来一样了，读文件，读取流信息，解复用，解码......，特别的地方就是如何将解复用的包入队，和从队列取包，然后解码。

main函数源代码如下：

<pre name="code" class="cpp">int main(int argc, char *argv[]) {struct SwsContext * sws_ctx = NULL;AVFormatContext *pFormatCtx = NULL;int             i, videoStream, audioStream;AVPacket        packet;int             frameFinished;AVCodecContext  *pCodecCtxOrig = NULL;AVCodecContext  *pCodecCtx = NULL;AVCodec         *pCodec = NULL;AVFrame         *pFrame = NULL;AVCodecContext  *aCodecCtxOrig = NULL;AVCodecContext  *aCodecCtx = NULL;AVCodec         *aCodec = NULL;SDL_Overlay     *bmp;SDL_Surface     *screen;SDL_Rect        rect;SDL_Event       event;SDL_AudioSpec   wanted_spec, spec;// Register all formats and codecsav_register_all();if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER)) {fprintf(stderr, "Could not initialize SDL - %s\n", SDL_GetError());exit(1);}//文件路径char* filepath = "2.mp4";// Open video fileif (avformat_open_input(&pFormatCtx, filepath, NULL, NULL) != 0)return -1; // Couldn't open file// Retrieve stream informationif (avformat_find_stream_info(pFormatCtx, NULL) < 0)return -1; // Couldn't find stream information// Find the first video stream and audio streamvideoStream = -1;audioStream = -1;for (i = 0; i < pFormatCtx->nb_streams; i++) {if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO &&videoStream < 0) {videoStream = i;}if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO &&audioStream < 0) {audioStream = i;}}if (videoStream == -1)return -1; // Didn't find a video streamif (audioStream == -1)return -1;//find the codeccontextaCodecCtxOrig = pFormatCtx->streams[audioStream]->codec;//get the decoderaCodec = avcodec_find_decoder(aCodecCtxOrig->codec_id);if (!aCodec) {fprintf(stderr, "Unsupported codec!\n");return -1;}// Copy contextaCodecCtx = avcodec_alloc_context3(aCodec);if (avcodec_copy_context(aCodecCtx, aCodecCtxOrig) != 0) {fprintf(stderr, "Couldn't copy codec context");return -1; // Error copying codec context}// Set audio settings from codec infowanted_spec.freq =   44100;wanted_spec.format = AUDIO_S16SYS;wanted_spec.channels = aCodecCtx->channels;wanted_spec.silence = 0;wanted_spec.samples = SDL_AUDIO_BUFFER_SIZE;wanted_spec.callback = audio_callback;wanted_spec.userdata = aCodecCtx;//openaudio deviceif (SDL_OpenAudio(&wanted_spec, &spec) < 0) {fprintf(stderr, "SDL_OpenAudio: %s\n", SDL_GetError());return -1;}//open the audio decoderavcodec_open2(aCodecCtx, aCodec, NULL);packet_queue_init(&audioq);//play audioSDL_PauseAudio(0);//------------------------------------------------------------//video part// Get a pointer to the codec context for the video streampCodecCtxOrig = pFormatCtx->streams[videoStream]->codec;// Find the decoder for the video streampCodec = avcodec_find_decoder(pCodecCtxOrig->codec_id);if (pCodec == NULL) {fprintf(stderr, "Unsupported codec!\n");return -1; // Codec not found}// Copy contextpCodecCtx = avcodec_alloc_context3(pCodec);if (avcodec_copy_context(pCodecCtx, pCodecCtxOrig) != 0) {fprintf(stderr, "Couldn't copy codec context");return -1; // Error copying codec context}// Open codecif (avcodec_open2(pCodecCtx, pCodec, NULL) < 0)return -1; // Could not open codec// Allocate video framepFrame = av_frame_alloc();// Make a screen to put our videoscreen = SDL_SetVideoMode(pCodecCtx->width, pCodecCtx->height, 0, 0);if (!screen) {fprintf(stderr, "SDL: could not set video mode - exiting\n");exit(1);}// Allocate a place to put our YUV image on that screenbmp = SDL_CreateYUVOverlay(pCodecCtx->width,pCodecCtx->height,SDL_YV12_OVERLAY,//YVU模式screen);// initialize SWS context for software scalingsws_ctx = sws_getContext(pCodecCtx->width,pCodecCtx->height,pCodecCtx->pix_fmt,pCodecCtx->width,pCodecCtx->height,PIX_FMT_YUV420P,SWS_BILINEAR,NULL,NULL,NULL);// Read frames and save first five frames to diskwhile (av_read_frame(pFormatCtx, &packet) >= 0) {// Is this a packet from the video stream?if (packet.stream_index == videoStream) {// Decode video frameavcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);// Did we get a video frame?if (frameFinished) {SDL_LockYUVOverlay(bmp);AVPicture pict;pict.data[0] = bmp->pixels[0];pict.data[1] = bmp->pixels[2];pict.data[2] = bmp->pixels[1];pict.linesize[0] = bmp->pitches[0];pict.linesize[1] = bmp->pitches[2];pict.linesize[2] = bmp->pitches[1];// Convert the image into YUV format that SDL usessws_scale(sws_ctx, (uint8_t const * const *)pFrame->data,pFrame->linesize, 0, pCodecCtx->height,pict.data, pict.linesize);SDL_UnlockYUVOverlay(bmp);rect.x = 0;rect.y = 0;rect.w = pCodecCtx->width;rect.h = pCodecCtx->height;SDL_DisplayYUVOverlay(bmp, &rect);av_free_packet(&packet);SDL_Delay(20);//延迟一下，防止播放太快}}else if (packet.stream_index == audioStream) {packet_queue_put(&audioq, &packet);//音频包入队}else {// Free the packet that was allocated by av_read_frameav_free_packet(&packet);}//事件处理，SDL视频播放完成后，会触发SDL_QUIT事件SDL_PollEvent(&event);switch (event.type) {case SDL_QUIT:quit = 1;SDL_Quit();exit(0);break;default:break;}}//close the video and audio contextavcodec_close(pCodecCtxOrig);avcodec_close(pCodecCtx);avcodec_close(aCodecCtxOrig);avcodec_close(aCodecCtx);// Close the video fileavformat_close_input(&pFormatCtx);return 0;}

视频部分的代码如果有不懂的，可以回去看看：最简单的视频播放器记录一最简单的视频播放器记录二

音频回调函数如下：

/*音频回调函数userdata:in 编解码上下文stream: out 播放的缓冲区len:    缓冲区大小*/void audio_callback(void *userdata, Uint8 *stream, int len) {AVCodecContext *aCodecCtx = (AVCodecContext *)userdata;int len1, audio_size;static uint8_t audio_buf[(MAX_AUDIO_FRAME_SIZE * 3) / 2];static unsigned int audio_buf_size = 0;static unsigned int audio_buf_index = 0;while (len > 0) {if (audio_buf_index >= audio_buf_size) {/* We have already sent all our data; get more */audio_size = audio_decode_frame(aCodecCtx, audio_buf, sizeof(audio_buf));//解码包if (audio_size < 0) {/* If error, output silence */audio_buf_size = 1024;memset(audio_buf, 0, audio_buf_size);}else {audio_buf_size = audio_size;}audio_buf_index = 0;}len1 = audio_buf_size - audio_buf_index;//音频数据长度if (len1 > len)//如果大于要播放的长度，则截取len1 = len;memcpy(stream, (uint8_t *)audio_buf + audio_buf_index, len1);//将要播放的内容拷贝至输出缓冲区                //记录剩余数据len -= len1;stream += len1;audio_buf_index += len1;}}

音频解码函数如下：

/*aCodecCtx:编解码上下文audio_buf:out 音频缓冲区buf_size: 缓冲区大小return: -1 错误， data_size  音频原始数据大小*/int audio_decode_frame(AVCodecContext *aCodecCtx, uint8_t *audio_buf, int buf_size) {static AVPacket pkt;static uint8_t *audio_pkt_data = NULL;static int audio_pkt_size = 0;static AVFrame frame;int len1;//解码消费了包多少字节数据        int data_size = 0;//新版中avcodec_decode_audio4()解码后输出的音频采样数据格式为AV_SAMPLE_FMT_FLTP（float, planar）而不再是AV_SAMPLE_FMT_S16（signed 16 bits）。因此         //无法直接使用SDL进行播放，必须使用SwrContext对音频采样数据进行转换之后，再进行输出播放。        //输出参数设置        uint64_t out_channel_layout = AV_CH_LAYOUT_STEREO;//立体声         int out_channels = av_get_channel_layout_nb_channels(out_channel_layout);//根据通道布局类型获取通道数          int out_nb_samples = 1024;//单个通道样本个数，需要根据音频封装格式动态改变，不然某些格式的音频文件播放会有杂音          AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16;//采样格式，SDL可以播放此种格式           int out_sample_rate = 44100;//输出时采样率,CD一般为44100HZ            //获取声道布局,一些codec的channel_layout可能会丢失，这里需要重新获取一次           uint64_t in_channel_layout = av_get_default_channel_layout(aCodecCtx->channels);          //输出缓存           uint8_t *out_buffer_audio = (uint8_t *)av_malloc(MAX_AUDIO_FRAME_SIZE * 2);//*2是保证输出缓存大于输入数据大小         //音频格式转换设置          struct SwrContext *au_convert_ctx;         au_convert_ctx = swr_alloc();         au_convert_ctx = swr_alloc_set_opts(au_convert_ctx, out_channel_layout, out_sample_fmt, out_sample_rate,in_channel_layout, aCodecCtx->sample_fmt, aCodecCtx->sample_rate, 0, NULL);       swr_init(au_convert_ctx);       for (;;) {       while (audio_pkt_size > 0) { //不停的解码，直到一个包的数据都被解码完毕         int got_frame = 0;        len1 = avcodec_decode_audio4(aCodecCtx, &frame, &got_frame, &pkt);        if (len1 < 0) { /* if error, skip frame */           audio_pkt_size = 0;           break;          }        audio_pkt_data += len1;//剩余数据起始位置        audio_pkt_size -= len1;//剩余的字节数        data_size = 0; if (got_frame) { //动态改变输出样本数，保证有些文件不会出现杂音           if (out_nb_samples != frame.nb_samples){out_nb_samples = frame.nb_samples;}           //计算原始数据所需空间           data_size = av_samples_get_buffer_size(NULL,aCodecCtx->channels,out_nb_samples,out_sample_fmt,1);          //转换格式，否则SDL不能播放           swr_convert(au_convert_ctx, &out_buffer_audio, MAX_AUDIO_FRAME_SIZE, (const uint8_t **)frame.data, frame.nb_samples);           assert(data_size <= buf_size);           memcpy(audio_buf, out_buffer_audio, data_size);//将数据拷贝到输出缓冲区           av_free(au_convert_ctx);         }        if (data_size <= 0) { /* No data yet, get more frames */          continue;         }         /* We have data, return it and come back for more later */         return data_size;        }       if (pkt.data) av_free_packet(&pkt);        if (quit) { //退出          return -1;           } //从队列获取包        if (packet_queue_get(&audioq, &pkt, 1) < 0) {return -1;}        audio_pkt_data = pkt.data;//<span style="font-family:Arial,Helvetica,sans-serif">audio_pkt_data 指向数据地址</span>        audio_pkt_size = pkt.size;//<span style="font-family:Arial,Helvetica,sans-serif">audio_pkt_size=包大小</span>}}

运行结果：

        运行后，可以看到视频和音频都可以播放了。但是视频和音频并不同步，因为视频我们采用的是延迟20ms，所以看起来有点偏快。音频是正常的，因为有采样率控制着播放的速度。如果视频延迟时间再慢一点，就会影响到音频的播放，听起来就会有卡顿。        

        所以，如果还要继续优化，那么视频播放应该在另外的线程中进行。而且还要考虑视频和音频的同步，比如将视频同步到音频。

       最后再将整个源代码贴上来：

extern"C"{#include <libavcodec/avcodec.h>#include <libavformat/avformat.h>#include <libswscale/swscale.h>#include "include/sdl/SDL.h"#include "include/sdl/SDL_thread.h"#include "include/libavutil/time.h"#include "include/libavutil/avstring.h"#include "libswresample/swresample.h"}#pragma comment(lib, "lib/avformat.lib")#pragma comment(lib, "lib/avcodec.lib")#pragma comment(lib, "lib/avutil.lib")#pragma comment(lib, "lib/swscale.lib")#pragma comment(lib, "lib/swresample.lib")#pragma comment(lib, "lib/SDL.lib")#pragma comment(lib, "lib/SDLmain.lib")#include <stdio.h>#include<stdlib.h>#include<string.h>#include <assert.h>#define SDL_AUDIO_BUFFER_SIZE 1024#define MAX_AUDIO_FRAME_SIZE  192000/*包队列*/typedef struct PacketQueue {AVPacketList *first_pkt, *last_pkt;int nb_packets;//包个数int size;//包大小SDL_mutex *mutex;//互斥量SDL_cond *cond;//条件量} PacketQueue;PacketQueue audioq;//音频包队列int quit = 0;//是否退出void packet_queue_init(PacketQueue *q) {memset(q, 0, sizeof(PacketQueue));q->mutex = SDL_CreateMutex();q->cond = SDL_CreateCond();}int packet_queue_put(PacketQueue *q, AVPacket *pkt) {AVPacketList *pkt1;if (av_dup_packet(pkt) < 0) {return -1;}pkt1 = (AVPacketList*)av_malloc(sizeof(AVPacketList));if (!pkt1)return -1;pkt1->pkt = *pkt;pkt1->next = NULL;SDL_LockMutex(q->mutex);if (!q->last_pkt)q->first_pkt = pkt1;elseq->last_pkt->next = pkt1;q->last_pkt = pkt1;q->nb_packets++;q->size += pkt1->pkt.size;SDL_CondSignal(q->cond);SDL_UnlockMutex(q->mutex);return 0;}int packet_queue_get(PacketQueue *q, AVPacket *pkt, int block){AVPacketList *pkt1;int ret;SDL_LockMutex(q->mutex);for (;;) {if (quit) {ret = -1;break;}pkt1 = q->first_pkt;if (pkt1) {q->first_pkt = pkt1->next;if (!q->first_pkt)q->last_pkt = NULL;q->nb_packets--;q->size -= pkt1->pkt.size;*pkt = pkt1->pkt;av_free(pkt1);ret = 1;break;}else if (!block) {ret = 0;break;}else {SDL_CondWait(q->cond, q->mutex);}}SDL_UnlockMutex(q->mutex);return ret;}int audio_decode_frame(AVCodecContext *aCodecCtx, uint8_t *audio_buf, int buf_size) {static AVPacket pkt;static uint8_t *audio_pkt_data = NULL;static int audio_pkt_size = 0;static AVFrame frame;int len1, data_size = 0;//音频转换数据格式，否则输出是杂音//---------------------------------------------------------------------------//输出参数设置  uint64_t out_channel_layout = AV_CH_LAYOUT_STEREO;//立体声  int out_channels = av_get_channel_layout_nb_channels(out_channel_layout);//根据通道布局类型获取通道数int out_nb_samples = 1024;//单个通道样本个数，需要根据音频封装格式动态改变，不然不同格式的文件音频播放速度会不同AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16;//采样格式  int out_sample_rate = 44100;//输出时采样率,CD一般为44100HZ  //获取声道布局,一些codec的channel_layout可能会丢失，这里需要重新获取一次uint64_t in_channel_layout = av_get_default_channel_layout(aCodecCtx->channels);//输出缓存uint8_t *out_buffer_audio = (uint8_t *)av_malloc(MAX_AUDIO_FRAME_SIZE * 2);//*2是保证输出缓存大于输入数据大小  //音频格式转换设置   struct SwrContext *au_convert_ctx;au_convert_ctx = swr_alloc();au_convert_ctx = swr_alloc_set_opts(au_convert_ctx, out_channel_layout, out_sample_fmt, out_sample_rate,in_channel_layout, aCodecCtx->sample_fmt, aCodecCtx->sample_rate, 0, NULL);swr_init(au_convert_ctx);for (;;) {while (audio_pkt_size > 0) {int got_frame = 0;len1 = avcodec_decode_audio4(aCodecCtx, &frame, &got_frame, &pkt);if (len1 < 0) {/* if error, skip frame */audio_pkt_size = 0;break;}audio_pkt_data += len1;audio_pkt_size -= len1;data_size = 0;if (got_frame) {//FIX:FLAC,MP3,AAC Different number of samples  if (out_nb_samples != frame.nb_samples){out_nb_samples = frame.nb_samples;}data_size = av_samples_get_buffer_size(NULL,aCodecCtx->channels,out_nb_samples,out_sample_fmt,1);//转换格式，否则是杂音swr_convert(au_convert_ctx, &out_buffer_audio, MAX_AUDIO_FRAME_SIZE, (const uint8_t **)frame.data, frame.nb_samples);assert(data_size <= buf_size);memcpy(audio_buf, out_buffer_audio, data_size);av_free(au_convert_ctx);}if (data_size <= 0) {/* No data yet, get more frames */continue;}/* We have data, return it and come back for more later */return data_size;}if (pkt.data)av_free_packet(&pkt);if (quit) {return -1;}if (packet_queue_get(&audioq, &pkt, 1) < 0) {return -1;}audio_pkt_data = pkt.data;audio_pkt_size = pkt.size;}}void audio_callback(void *userdata, Uint8 *stream, int len) {AVCodecContext *aCodecCtx = (AVCodecContext *)userdata;int len1, audio_size;static uint8_t audio_buf[(MAX_AUDIO_FRAME_SIZE * 3) / 2];static unsigned int audio_buf_size = 0;static unsigned int audio_buf_index = 0;while (len > 0) {if (audio_buf_index >= audio_buf_size) {/* We have already sent all our data; get more */audio_size = audio_decode_frame(aCodecCtx, audio_buf, sizeof(audio_buf));if (audio_size < 0) {/* If error, output silence */audio_buf_size = 1024; // arbitrary?memset(audio_buf, 0, audio_buf_size);}else {audio_buf_size = audio_size;}audio_buf_index = 0;}len1 = audio_buf_size - audio_buf_index;if (len1 > len)len1 = len;memcpy(stream, (uint8_t *)audio_buf + audio_buf_index, len1);len -= len1;stream += len1;audio_buf_index += len1;    SDL_Delay(1);}}int main(int argc, char *argv[]) {struct SwsContext * sws_ctx = NULL;AVFormatContext *pFormatCtx = NULL;int             i, videoStream, audioStream;AVPacket        packet;int             frameFinished;AVCodecContext  *pCodecCtxOrig = NULL;AVCodecContext  *pCodecCtx = NULL;AVCodec         *pCodec = NULL;AVFrame         *pFrame = NULL;AVCodecContext  *aCodecCtxOrig = NULL;AVCodecContext  *aCodecCtx = NULL;AVCodec         *aCodec = NULL;SDL_Overlay     *bmp;SDL_Surface     *screen;SDL_Rect        rect;SDL_Event       event;SDL_AudioSpec   wanted_spec, spec;// Register all formats and codecsav_register_all();if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER)) {fprintf(stderr, "Could not initialize SDL - %s\n", SDL_GetError());exit(1);}//文件路径char* filepath = "2.mp4";// Open video fileif (avformat_open_input(&pFormatCtx, filepath, NULL, NULL) != 0)return -1; // Couldn't open file// Retrieve stream informationif (avformat_find_stream_info(pFormatCtx, NULL) < 0)return -1; // Couldn't find stream information// Find the first video stream and audio streamvideoStream = -1;audioStream = -1;for (i = 0; i < pFormatCtx->nb_streams; i++) {if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO &&videoStream < 0) {videoStream = i;}if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO &&audioStream < 0) {audioStream = i;}}if (videoStream == -1)return -1; // Didn't find a video streamif (audioStream == -1)return -1;//find the codeccontextaCodecCtxOrig = pFormatCtx->streams[audioStream]->codec;//get the decoderaCodec = avcodec_find_decoder(aCodecCtxOrig->codec_id);if (!aCodec) {fprintf(stderr, "Unsupported codec!\n");return -1;}// Copy contextaCodecCtx = avcodec_alloc_context3(aCodec);if (avcodec_copy_context(aCodecCtx, aCodecCtxOrig) != 0) {fprintf(stderr, "Couldn't copy codec context");return -1; // Error copying codec context}// Set audio settings from codec infowanted_spec.freq =   44100;wanted_spec.format = AUDIO_S16SYS;wanted_spec.channels = aCodecCtx->channels;wanted_spec.silence = 0;wanted_spec.samples = SDL_AUDIO_BUFFER_SIZE;wanted_spec.callback = audio_callback;wanted_spec.userdata = aCodecCtx;//openaudio deviceif (SDL_OpenAudio(&wanted_spec, &spec) < 0) {fprintf(stderr, "SDL_OpenAudio: %s\n", SDL_GetError());return -1;}//open the audio decoderavcodec_open2(aCodecCtx, aCodec, NULL);packet_queue_init(&audioq);//play audioSDL_PauseAudio(0);//------------------------------------------------------------//video part// Get a pointer to the codec context for the video streampCodecCtxOrig = pFormatCtx->streams[videoStream]->codec;// Find the decoder for the video streampCodec = avcodec_find_decoder(pCodecCtxOrig->codec_id);if (pCodec == NULL) {fprintf(stderr, "Unsupported codec!\n");return -1; // Codec not found}// Copy contextpCodecCtx = avcodec_alloc_context3(pCodec);if (avcodec_copy_context(pCodecCtx, pCodecCtxOrig) != 0) {fprintf(stderr, "Couldn't copy codec context");return -1; // Error copying codec context}// Open codecif (avcodec_open2(pCodecCtx, pCodec, NULL) < 0)return -1; // Could not open codec// Allocate video framepFrame = av_frame_alloc();// Make a screen to put our videoscreen = SDL_SetVideoMode(pCodecCtx->width, pCodecCtx->height, 0, 0);if (!screen) {fprintf(stderr, "SDL: could not set video mode - exiting\n");exit(1);}// Allocate a place to put our YUV image on that screenbmp = SDL_CreateYUVOverlay(pCodecCtx->width,pCodecCtx->height,SDL_YV12_OVERLAY,//YVU模式screen);// initialize SWS context for software scalingsws_ctx = sws_getContext(pCodecCtx->width,pCodecCtx->height,pCodecCtx->pix_fmt,pCodecCtx->width,pCodecCtx->height,PIX_FMT_YUV420P,SWS_BILINEAR,NULL,NULL,NULL);// Read frames and save first five frames to diskwhile (av_read_frame(pFormatCtx, &packet) >= 0) {// Is this a packet from the video stream?if (packet.stream_index == videoStream) {// Decode video frameavcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);// Did we get a video frame?if (frameFinished) {SDL_LockYUVOverlay(bmp);AVPicture pict;pict.data[0] = bmp->pixels[0];pict.data[1] = bmp->pixels[2];pict.data[2] = bmp->pixels[1];pict.linesize[0] = bmp->pitches[0];pict.linesize[1] = bmp->pitches[2];pict.linesize[2] = bmp->pitches[1];// Convert the image into YUV format that SDL usessws_scale(sws_ctx, (uint8_t const * const *)pFrame->data,pFrame->linesize, 0, pCodecCtx->height,pict.data, pict.linesize);SDL_UnlockYUVOverlay(bmp);rect.x = 0;rect.y = 0;rect.w = pCodecCtx->width;rect.h = pCodecCtx->height;SDL_DisplayYUVOverlay(bmp, &rect);av_free_packet(&packet);SDL_Delay(20);//延迟一下，防止播放太快}}else if (packet.stream_index == audioStream) {packet_queue_put(&audioq, &packet);//音频包入队}else {// Free the packet that was allocated by av_read_frameav_free_packet(&packet);}SDL_PollEvent(&event);switch (event.type) {case SDL_QUIT:quit = 1;SDL_Quit();exit(0);break;default:break;}}//close the video and audio contextavcodec_close(pCodecCtxOrig);avcodec_close(pCodecCtx);avcodec_close(aCodecCtxOrig);avcodec_close(aCodecCtx);// Close the video fileavformat_close_input(&pFormatCtx);return 0;}

参考链接：
http://dranger.com/ffmpeg/tutorial03.html