同步音频的原理可以参考：  本文是在  的基础上面继续进行将视频和音频同步到外部时钟的工作，同时也包含了修正音频解码的问题。

/* * SDL_Lesson.c * *  Created on: Aug 12, 2014 *      Author: clarck */#include 
     
      #include 
      
       #include "SDL.h"#include "SDL_thread.h"#include "SDL_events.h"#include "../include/logger.h"#include "../ffmpeg/include/libavcodec/avcodec.h"#include "../ffmpeg/include/libavformat/avformat.h"#include "../ffmpeg/include/libavutil/pixfmt.h"#include "../ffmpeg/include/libswscale/swscale.h"#include "../ffmpeg/include/libswresample/swresample.h"#define SDL_AUDIO_BUFFER_SIZE 1024#define MAX_AUDIOQ_SIZE (5 * 16 * 1024)#define MAX_VIDEOQ_SIZE (5 * 256 * 1024)#define AV_SYNC_THRESHOLD 0.01#define AV_NOSYNC_THRESHOLD 10.0#define SAMPLE_CORRECTION_PERCENT_MAX 10#define AUDIO_DIFF_AVG_NB 20#define FF_ALLOC_EVENT   (SDL_USEREVENT)#define FF_REFRESH_EVENT (SDL_USEREVENT + 1)#define FF_QUIT_EVENT (SDL_USEREVENT + 2)#define VIDEO_PICTURE_QUEUE_SIZE 1#define DEFAULT_AV_SYNC_TYPE AV_SYNC_VIDEO_MASTER#define AVCODEC_MAX_AUDIO_FRAME_SIZE 192000 // 1 second of 48khz 32bit audiotypedef struct PacketQueue {    AVPacketList *first_pkt, *last_pkt;    int nb_packets;    int size;    SDL_mutex *mutex;    SDL_cond *cond;} PacketQueue;typedef struct VideoPicture {    SDL_Window *screen;    SDL_Renderer *renderer;    SDL_Texture *bmp;    AVFrame* rawdata;    int width, height; /*source height & width*/    int allocated;    double pts;} VideoPicture;typedef struct VideoState {    char filename[1024];    AVFormatContext *ic;    int videoStream, audioStream;    AVStream *audio_st;    AVFrame *audio_frame;    PacketQueue audioq;    unsigned int audio_buf_size;    unsigned int audio_buf_index;    AVPacket audio_pkt;    uint8_t *audio_pkt_data;    int audio_pkt_size;    uint8_t *audio_buf;    DECLARE_ALIGNED(16,uint8_t,audio_buf2) [AVCODEC_MAX_AUDIO_FRAME_SIZE * 4];    enum AVSampleFormat audio_src_fmt;    enum AVSampleFormat audio_tgt_fmt;    int audio_src_channels;    int audio_tgt_channels;    int64_t audio_src_channel_layout;    int64_t audio_tgt_channel_layout;    int audio_src_freq;    int audio_tgt_freq;    struct SwrContext *swr_ctx;    AVStream *video_st;    PacketQueue videoq;    VideoPicture pictq[VIDEO_PICTURE_QUEUE_SIZE];    int pictq_size, pictq_rindex, pictq_windex;    SDL_mutex *pictq_mutex;    SDL_cond *pictq_cond;    SDL_Thread *parse_tid;    SDL_Thread *audio_tid;    SDL_Thread *video_tid;    AVIOContext *io_ctx;    struct SwsContext *sws_ctx;    double audio_clock;    int av_sync_type;    double external_clock;/*external clock base*/    int64_t external_clock_time;    int audio_hw_buf_size;    double audio_diff_cum;/*used of AV difference average computation*/    double audio_diff_avg_coef;    double audio_diff_threshold;    int audio_diff_avg_count;    double frame_timer;    double frame_last_pts;    double frame_last_delay;    double video_current_pts; ///
       
        
         
          mutex = SDL_CreateMutex();    q->cond = SDL_CreateCond();}int packet_queue_put(PacketQueue *q, AVPacket *pkt) {    AVPacketList *pkt1;    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;    } else {        q->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;}static int packet_queue_get(PacketQueue *q, AVPacket *pkt, int block) {    AVPacketList *pkt1;    int ret;    SDL_LockMutex(q->mutex);    for (;;) {        if (global_video_state->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;}double get_audio_clock(VideoState *is) {    double pts;    int hw_buf_size, bytes_per_sec, n;    pts = is->audio_clock; /* maintained in the audio thread */    hw_buf_size = is->audio_buf_size - is->audio_buf_index;    bytes_per_sec = 0;    n = is->audio_st->codec->channels * 2;    if (is->audio_st) {        bytes_per_sec = is->audio_st->codec->sample_rate * n;    }    if (bytes_per_sec) {        pts -= (double) hw_buf_size / bytes_per_sec;    }    return pts;}double get_video_clock(VideoState *is) {    double delta;    delta = (av_gettime() - is->video_current_pts_time) / 1000000.0;    return is->video_current_pts + delta;}double get_external_clock(VideoState *is) {    return av_gettime() / 1000000.0;}double get_master_clock(VideoState *is) {    if (is->av_sync_type == AV_SYNC_VIDEO_MASTER) {        return get_video_clock(is);    } else if (is->av_sync_type == AV_SYNC_AUDIO_MASTER) {        return get_audio_clock(is);    } else {        return get_external_clock(is);    }}/* Add or subtract samples to get a better sync, return new audio buffer size */int synchronize_audio(VideoState *is, short *samples, int samples_size,        double pts) {    int n;    double ref_clock;    n = 2 * is->audio_st->codec->channels;    if (is->av_sync_type != AV_SYNC_AUDIO_MASTER) {        double diff, avg_diff;        int wanted_size, min_size, max_size;        //int nb_samples;        ref_clock = get_master_clock(is);        diff = get_audio_clock(is) - ref_clock;        if (diff < AV_NOSYNC_THRESHOLD) {            // accumulate the diffs            is->audio_diff_cum = diff                    + is->audio_diff_avg_coef * is->audio_diff_cum;            if (is->audio_diff_avg_count < AUDIO_DIFF_AVG_NB) {                is->audio_diff_avg_count++;            } else {                avg_diff = is->audio_diff_cum * (1.0 - is->audio_diff_avg_coef);                if (fabs(avg_diff) >= is->audio_diff_threshold) {                    wanted_size = samples_size                            + ((int) (diff * is->audio_st->codec->sample_rate)                                    * n);                    min_size = samples_size                            * ((100 - SAMPLE_CORRECTION_PERCENT_MAX) / 100);                    max_size = samples_size                            * ((100 + SAMPLE_CORRECTION_PERCENT_MAX) / 100);                    if (wanted_size < min_size) {                        wanted_size = min_size;                    } else if (wanted_size > max_size) {                        wanted_size = max_size;                    }                    if (wanted_size < samples_size) {                        /* remove samples */                        samples_size = wanted_size;                    } else if (wanted_size > samples_size) {                        uint8_t *samples_end, *q;                        int nb;                        /* add samples by copying final sample*/                        nb = (samples_size - wanted_size);                        samples_end = (uint8_t *) samples + samples_size - n;                        q = samples_end + n;                        while (nb > 0) {                            memcpy(q, samples_end, n);                            q += n;                            nb -= n;                        }                        samples_size = wanted_size;                    }                }            }        } else {            /* difference is TOO big; reset diff stuff */            is->audio_diff_avg_count = 0;            is->audio_diff_cum = 0;        }    }    return samples_size;}int audio_decode_frame(VideoState *is, double *pts_ptr) {    int len1, len2, decoded_data_size;    AVPacket *pkt = &is->audio_pkt;    int got_frame = 0;    int64_t dec_channel_layout;    int wanted_nb_samples, resampled_data_size, n;    double pts;    for (;;) {        while (is->audio_pkt_size > 0) {            if (!is->audio_frame) {                if (!(is->audio_frame = avcodec_alloc_frame())) {                    return AVERROR(ENOMEM);                }            } else                avcodec_get_frame_defaults(is->audio_frame);            len1 = avcodec_decode_audio4(is->audio_st->codec, is->audio_frame,                    &got_frame, pkt);            if (len1 < 0) {                // error, skip the frame                is->audio_pkt_size = 0;                break;            }            is->audio_pkt_data += len1;            is->audio_pkt_size -= len1;            if (!got_frame)                continue;            /* 计算解码出来的桢需要的缓冲大小 */            decoded_data_size = av_samples_get_buffer_size(NULL,                    is->audio_frame->channels, is->audio_frame->nb_samples,                    is->audio_frame->format, 1);            dec_channel_layout =                    (is->audio_frame->channel_layout                            && is->audio_frame->channels                                    == av_get_channel_layout_nb_channels(                                            is->audio_frame->channel_layout)) ?                            is->audio_frame->channel_layout :                            av_get_default_channel_layout(                                    is->audio_frame->channels);            wanted_nb_samples = is->audio_frame->nb_samples;            if (is->audio_frame->format != is->audio_src_fmt                    || dec_channel_layout != is->audio_src_channel_layout                    || is->audio_frame->sample_rate != is->audio_src_freq                    || (wanted_nb_samples != is->audio_frame->nb_samples                            && !is->swr_ctx)) {                if (is->swr_ctx)                    swr_free(&is->swr_ctx);                is->swr_ctx = swr_alloc_set_opts(NULL,                        is->audio_tgt_channel_layout, is->audio_tgt_fmt,                        is->audio_tgt_freq, dec_channel_layout,                        is->audio_frame->format, is->audio_frame->sample_rate,                        0, NULL);                if (!is->swr_ctx || swr_init(is->swr_ctx) < 0) {                    fprintf(stderr, "swr_init() failed\n");                    break;                }                is->audio_src_channel_layout = dec_channel_layout;                is->audio_src_channels = is->audio_st->codec->channels;                is->audio_src_freq = is->audio_st->codec->sample_rate;                is->audio_src_fmt = is->audio_st->codec->sample_fmt;            }            /* 这里我们可以对采样数进行调整，增加或者减少，一般可以用来做声画同步 */            if (is->swr_ctx) {                const uint8_t **in =                        (const uint8_t **) is->audio_frame->extended_data;                uint8_t *out[] = { is->audio_buf2 };                if (wanted_nb_samples != is->audio_frame->nb_samples) {                    if (swr_set_compensation(is->swr_ctx,                            (wanted_nb_samples - is->audio_frame->nb_samples)                                    * is->audio_tgt_freq                                    / is->audio_frame->sample_rate,                            wanted_nb_samples * is->audio_tgt_freq                                    / is->audio_frame->sample_rate) < 0) {                        fprintf(stderr, "swr_set_compensation() failed\n");                        break;                    }                }                len2 = swr_convert(is->swr_ctx, out,                        sizeof(is->audio_buf2) / is->audio_tgt_channels                                / av_get_bytes_per_sample(is->audio_tgt_fmt),                        in, is->audio_frame->nb_samples);                if (len2 < 0) {                    fprintf(stderr, "swr_convert() failed\n");                    break;                }                if (len2                        == sizeof(is->audio_buf2) / is->audio_tgt_channels                                / av_get_bytes_per_sample(is->audio_tgt_fmt)) {                    fprintf(stderr,                            "warning: audio buffer is probably too small\n");                    swr_init(is->swr_ctx);                }                is->audio_buf = is->audio_buf2;                resampled_data_size = len2 * is->audio_tgt_channels                        * av_get_bytes_per_sample(is->audio_tgt_fmt);            } else {                resampled_data_size = decoded_data_size;                is->audio_buf = is->audio_frame->data[0];            }            pts = is->audio_clock;            *pts_ptr = pts;            n = 2 * is->audio_st->codec->channels;            is->audio_clock += (double) resampled_data_size                    / (double) (n * is->audio_st->codec->sample_rate);            // We have data, return it and come back for more later            return resampled_data_size;        }        if (pkt->data)            av_free_packet(pkt);        memset(pkt, 0, sizeof(*pkt));        if (is->quit)            return -1;        if (packet_queue_get(&is->audioq, pkt, 1) < 0)            return -1;        is->audio_pkt_data = pkt->data;        is->audio_pkt_size = pkt->size;        /* if update, update the audio clock w/pts */        if (pkt->pts != AV_NOPTS_VALUE) {            is->audio_clock = av_q2d(is->audio_st->time_base) * pkt->pts;        }    }    return 0;}void audio_callback(void *userdata, Uint8 *stream, int len) {    VideoState *is = (VideoState *) userdata;    int len1, audio_data_size;    double pts;    /*   len是由SDL传入的SDL缓冲区的大小，如果这个缓冲未满，我们就一直往里填充数据 */    while (len > 0) {        /*  audio_buf_index 和 audio_buf_size 标示我们自己用来放置解码出来的数据的缓冲区，*/        /*   这些数据待copy到SDL缓冲区， 当audio_buf_index >= audio_buf_size的时候意味着我*/        /*   们的缓冲为空，没有数据可供copy，这时候需要调用audio_decode_frame来解码出更         /*   多的桢数据 */        if (is->audio_buf_index >= is->audio_buf_size) {            audio_data_size = audio_decode_frame(is, &pts);            /* audio_data_size < 0 标示没能解码出数据，我们默认播放静音 */            if (audio_data_size < 0) {                /* silence */                is->audio_buf_size = 1024;                /* 清零，静音 */                memset(is->audio_buf, 0, is->audio_buf_size);            } else {                audio_data_size = synchronize_audio(is, (int16_t *) is->audio_buf,                        audio_data_size, pts);                is->audio_buf_size = audio_data_size;            }            is->audio_buf_index = 0;        }        /*  查看stream可用空间，决定一次copy多少数据，剩下的下次继续copy */        len1 = is->audio_buf_size - is->audio_buf_index;        if (len1 > len) {            len1 = len;        }        memcpy(stream, (uint8_t *) is->audio_buf + is->audio_buf_index, len1);        len -= len1;        stream += len1;        is->audio_buf_index += len1;    }}static Uint32 sdl_refresh_timer_cb(Uint32 interval, void *opaque) {    SDL_Event event;    event.type = FF_REFRESH_EVENT;    event.user.data1 = opaque;    SDL_PushEvent(&event);    return 0;}static void schedule_refresh(VideoState *is, int delay) {    SDL_AddTimer(delay, sdl_refresh_timer_cb, is);}int decode_interrupt_cb(void *opaque) {    return (global_video_state && global_video_state->quit);}void video_display(VideoState *is) {    SDL_Rect rect;    VideoPicture *vp;    float aspect_ratio;    vp = &is->pictq[is->pictq_rindex];    if (vp->bmp) {        if (is->video_st->codec->sample_aspect_ratio.num == 0) {            aspect_ratio = 0;        } else {            aspect_ratio = av_q2d(is->video_st->codec->sample_aspect_ratio)                    * is->video_st->codec->width / is->video_st->codec->height;        }        if (aspect_ratio <= 0.0) {            aspect_ratio = (float) is->video_st->codec->width                    / (float) is->video_st->codec->height;        }        rect.x = 0;        rect.y = 0;        rect.w = vp->width;        rect.h = vp->height;        SDL_UpdateYUVTexture(vp->bmp, &rect, vp->rawdata->data[0],                vp->rawdata->linesize[0], vp->rawdata->data[1],                vp->rawdata->linesize[1], vp->rawdata->data[2],                vp->rawdata->linesize[2]);        SDL_RenderClear(vp->renderer);        SDL_RenderCopy(vp->renderer, vp->bmp, &rect, &rect);        SDL_RenderPresent(vp->renderer);    }}void video_refresh_timer(void *userdata) {    VideoState *is = (VideoState *) userdata;    VideoPicture *vp;    double actual_delay, delay, sync_threshold, ref_clock, diff;    if (is->video_st) {        if (is->pictq_size == 0) {            schedule_refresh(is, 1);        } else {            vp = &is->pictq[is->pictq_rindex];            is->video_current_pts = vp->pts;            is->video_current_pts_time = av_gettime();            delay = vp->pts - is->frame_last_pts; /* the pts from last time */            if (delay <= 0 || delay >= 1.0) {                /* if incorrect delay, use previous one */                delay = is->frame_last_delay;            }            /* save for next time */            is->frame_last_delay = delay;            is->frame_last_pts = vp->pts;            /* update delay to sync to audio */            ref_clock = get_audio_clock(is);            diff = vp->pts - ref_clock;            /* update delay to sync to audio if not master source */            if (is->av_sync_type != AV_SYNC_VIDEO_MASTER) {                ref_clock = get_master_clock(is);                diff = vp->pts - ref_clock;                /* Skip or repeat the frame. Take delay into account                 FFPlay still doesn't "know if this is the best guess." */                sync_threshold =                        (delay > AV_SYNC_THRESHOLD) ? delay : AV_SYNC_THRESHOLD;                if (fabs(diff) < AV_NOSYNC_THRESHOLD) {                    if (diff <= -sync_threshold) {                        delay = 0;                    } else if (diff >= sync_threshold) {                        delay = 2 * delay;                    }                }            }            is->frame_timer += delay;            /* computer the REAL delay */            actual_delay = is->frame_timer - (av_gettime() / 1000000.0);            if (actual_delay < 0.010) {                /* Really it should skip the picture instead */                actual_delay = 0.010;            }            schedule_refresh(is, (int) (actual_delay * 1000 + 0.5));            /* show the picture! */            video_display(is);            /* update queue for next picture! */            if (++is->pictq_rindex == VIDEO_PICTURE_QUEUE_SIZE) {                is->pictq_rindex = 0;            }            SDL_LockMutex(is->pictq_mutex);            is->pictq_size--;            SDL_CondSignal(is->pictq_cond);            SDL_UnlockMutex(is->pictq_mutex);        }    } else {        schedule_refresh(is, 100);    }}void alloc_picture(void *userdata) {    VideoState *is = (VideoState *) userdata;    VideoPicture *vp;    vp = &is->pictq[is->pictq_windex];    if (vp->bmp) {        // we already have one make another, bigger/smaller        SDL_DestroyTexture(vp->bmp);    }    if(vp->rawdata) {        av_free(vp->rawdata);    }    // Allocate a place to put our YUV image on that screen    vp->screen = SDL_CreateWindow("My Player Window", SDL_WINDOWPOS_UNDEFINED,            SDL_WINDOWPOS_UNDEFINED, is->video_st->codec->width,            is->video_st->codec->height,            SDL_WINDOW_FULLSCREEN | SDL_WINDOW_OPENGL);    vp->renderer = SDL_CreateRenderer(vp->screen, -1, 0);    vp->bmp = SDL_CreateTexture(vp->renderer, SDL_PIXELFORMAT_YV12,                SDL_TEXTUREACCESS_STREAMING, is->video_st->codec->width, is->video_st->codec->height);    vp->width = is->video_st->codec->width;    vp->height = is->video_st->codec->height;    AVFrame* pFrameYUV = avcodec_alloc_frame();    if (pFrameYUV == NULL)        return;    int numBytes = avpicture_get_size(PIX_FMT_YUV420P, vp->width,            vp->height);    uint8_t* buffer = (uint8_t *) av_malloc(numBytes * sizeof(uint8_t));    avpicture_fill((AVPicture *) pFrameYUV, buffer, PIX_FMT_YUV420P,            vp->width, vp->height);    vp->rawdata = pFrameYUV;    SDL_LockMutex(is->pictq_mutex);    vp->allocated = 1;    SDL_CondSignal(is->pictq_cond);    SDL_UnlockMutex(is->pictq_mutex);}int queue_picture(VideoState *is, AVFrame *pFrame, double pts) {    VideoPicture *vp;    //int dst_pic_fmt    AVPicture pict;    /* wait unitl we have space for a new pic */    SDL_LockMutex(is->pictq_mutex);    while (is->pictq_size >= VIDEO_PICTURE_QUEUE_SIZE && !is->quit) {        SDL_CondWait(is->pictq_cond, is->pictq_mutex);    }    SDL_UnlockMutex(is->pictq_mutex);    if (is->quit)        return -1;    // windex is set to 0 initially    vp = &is->pictq[is->pictq_windex];    /* allocate or resize the buffer ! */    if (!vp->bmp || vp->width != is->video_st->codec->width            || vp->height != is->video_st->codec->height) {        SDL_Event event;        vp->allocated = 0;        /* we have to do it in the main thread */        event.type = FF_ALLOC_EVENT;        event.user.data1 = is;        SDL_PushEvent(&event);        /* wait until we have a picture allocated */        SDL_LockMutex(is->pictq_mutex);        while (!vp->allocated && !is->quit) {            SDL_CondWait(is->pictq_cond, is->pictq_mutex);        }    }    SDL_UnlockMutex(is->pictq_mutex);    if (is->quit) {        return -1;    }    /* We have a place to put our picture on the queue */    if (vp->rawdata) {        // Convert the image into YUV format that SDL uses        sws_scale(is->sws_ctx, (uint8_t const * const *) pFrame->data,                pFrame->linesize, 0, is->video_st->codec->height,                vp->rawdata->data, vp->rawdata->linesize);        vp->pts = pts;        /* now we inform our display thread that we have a pic ready */        if (++is->pictq_windex == VIDEO_PICTURE_QUEUE_SIZE) {            is->pictq_windex = 0;        }        SDL_LockMutex(is->pictq_mutex);        is->pictq_size++;        SDL_UnlockMutex(is->pictq_mutex);    }    return 0;}double synchronize_video(VideoState *is, AVFrame *src_frame, double pts) {    double frame_delay;    if (pts != 0) {        /* if we have pts, set video clock to it */        is->video_clock = pts;    } else {        /* if we aren't given a pts, set it to the clock */        pts = is->video_clock;    }    /* update the video clock */    frame_delay = av_q2d(is->video_st->codec->time_base);    /* if we are repeating a frame, adjust clock accordingly */    frame_delay += src_frame->repeat_pict * (frame_delay * 0.5);    is->video_clock += frame_delay;    return pts;}uint64_t global_video_pkt_pts = AV_NOPTS_VALUE;/* These are called whenever we allocate a frame * buffer. We use this to store the global_pts in * a frame at the time it is allocated. */int our_get_buffer(struct AVCodecContext *c, AVFrame *pic) {    int ret = avcodec_default_get_buffer(c, pic);    uint64_t *pts = av_malloc(sizeof(uint64_t));    *pts = global_video_pkt_pts;    pic->opaque = pts;    return ret;}void our_release_buffer(struct AVCodecContext *c, AVFrame *pic) {    if (pic)        av_freep(&pic->opaque);    avcodec_default_release_buffer(c, pic);}int video_thread(void *arg) {    VideoState *is = (VideoState *) arg;    AVPacket pkt1, *packet = &pkt1;    int frameFinished;    AVFrame *pFrame;    double pts;    pFrame = av_frame_alloc();    for (;;) {        if (packet_queue_get(&is->videoq, packet, 1) < 0) {            // means we quit getting packets            break;        }        pts = 0;        // Save global pts to be stored in pFrame in first call        global_video_pkt_pts = packet->pts;        // Decode video frame        avcodec_decode_video2(is->video_st->codec, pFrame, &frameFinished,                packet);        if (packet->dts == AV_NOPTS_VALUE && pFrame->opaque                && *(uint64_t*) pFrame->opaque != AV_NOPTS_VALUE) {            pts = *(uint64_t *) pFrame->opaque;        } else if (packet->dts != AV_NOPTS_VALUE) {            pts = packet->dts;        } else {            pts = 0;        }        pts *= av_q2d(is->video_st->time_base);        // Did we get a video frame?        if (frameFinished) {            pts = synchronize_video(is, pFrame, pts);            if (queue_picture(is, pFrame, pts) < 0) {                break;            }        }        av_free_packet(packet);    }    av_free(pFrame);    return 0;}int audio_stream_component_open(VideoState *is, int stream_index) {    AVFormatContext *ic = is->ic;    AVCodecContext *codecCtx;    AVCodec *codec;    SDL_AudioSpec wanted_spec, spec;    int64_t wanted_channel_layout = 0;    int wanted_nb_channels;    /*  SDL支持的声道数为 1, 2, 4, 6 */    /*  后面我们会使用这个数组来纠正不支持的声道数目 */    const int next_nb_channels[] = { 0, 0, 1, 6, 2, 6, 4, 6 };    if (stream_index < 0 || stream_index >= ic->nb_streams) {        return -1;    }    codecCtx = ic->streams[stream_index]->codec;    wanted_nb_channels = codecCtx->channels;    if (!wanted_channel_layout            || wanted_nb_channels                    != av_get_channel_layout_nb_channels(                            wanted_channel_layout)) {        wanted_channel_layout = av_get_default_channel_layout(                wanted_nb_channels);        wanted_channel_layout &= ~AV_CH_LAYOUT_STEREO_DOWNMIX;    }    wanted_spec.channels = av_get_channel_layout_nb_channels(            wanted_channel_layout);    wanted_spec.freq = codecCtx->sample_rate;    if (wanted_spec.freq <= 0 || wanted_spec.channels <= 0) {        fprintf(stderr, "Invalid sample rate or channel count!\n");        return -1;    }    wanted_spec.format = AUDIO_S16SYS; // 具体含义请查看“SDL宏定义”部分    wanted_spec.silence = 0;            // 0指示静音    wanted_spec.samples = SDL_AUDIO_BUFFER_SIZE;  // 自定义SDL缓冲区大小    wanted_spec.callback = audio_callback;        // 音频解码的关键回调函数    wanted_spec.userdata = is;                    // 传给上面回调函数的外带数据    /*  打开音频设备，这里使用一个while来循环尝试打开不同的声道数(由上面 */    /*  next_nb_channels数组指定）直到成功打开，或者全部失败 */    while (SDL_OpenAudio(&wanted_spec, &spec) < 0) {        fprintf(stderr, "SDL_OpenAudio (%d channels): %s\n",                wanted_spec.channels, SDL_GetError());        wanted_spec.channels = next_nb_channels[FFMIN(7, wanted_spec.channels)];        if (!wanted_spec.channels) {            fprintf(stderr,                    "No more channel combinations to tyu, audio open failed\n");            return -1;        }        wanted_channel_layout = av_get_default_channel_layout(                wanted_spec.channels);    }    /* 检查实际使用的配置（保存在spec,由SDL_OpenAudio()填充） */    if (spec.format != AUDIO_S16SYS) {        fprintf(stderr, "SDL advised audio format %d is not supported!\n",                spec.format);        return -1;    }    if (spec.channels != wanted_spec.channels) {        wanted_channel_layout = av_get_default_channel_layout(spec.channels);        if (!wanted_channel_layout) {            fprintf(stderr, "SDL advised channel count %d is not supported!\n",                    spec.channels);            return -1;        }    }    is->audio_hw_buf_size = spec.size;    /* 把设置好的参数保存到大结构中 */    is->audio_src_fmt = is->audio_tgt_fmt = AV_SAMPLE_FMT_S16;    is->audio_src_freq = is->audio_tgt_freq = spec.freq;    is->audio_src_channel_layout = is->audio_tgt_channel_layout =            wanted_channel_layout;    is->audio_src_channels = is->audio_tgt_channels = spec.channels;    codec = avcodec_find_decoder(codecCtx->codec_id);    if (!codec || (avcodec_open2(codecCtx, codec, NULL) < 0)) {        fprintf(stderr, "Unsupported codec!\n");        return -1;    }    ic->streams[stream_index]->discard = AVDISCARD_DEFAULT;    switch (codecCtx->codec_type) {    case AVMEDIA_TYPE_AUDIO:        is->audioStream = stream_index;        is->audio_st = ic->streams[stream_index];        is->audio_buf_size = 0;        is->audio_buf_index = 0;        /* averaging filter for audio sync */        is->audio_diff_avg_coef = exp(log(0.01 / AUDIO_DIFF_AVG_NB));        is->audio_diff_avg_count = 0;        /* Correct audio only if larger error than this */        is->audio_diff_threshold = 2.0 * SDL_AUDIO_BUFFER_SIZE                / codecCtx->sample_rate;        memset(&is->audio_pkt, 0, sizeof(is->audio_pkt));        packet_queue_init(&is->audioq);        SDL_PauseAudio(0); // 开始播放静音        break;    default:        break;    }    return 0;}int video_stream_component_open(VideoState *is, int stream_index) {    AVFormatContext *pFormatCtx = is->ic;    AVCodecContext *codecCtx;    AVCodec *codec;    if (stream_index < 0 || stream_index >= pFormatCtx->nb_streams) {        return -1;    }    // Get a pointer to the codec context for the video stream    codecCtx = pFormatCtx->streams[stream_index]->codec;    codec = avcodec_find_decoder(codecCtx->codec_id);    if (!codec || (avcodec_open2(codecCtx, codec, NULL) < 0)) {        fprintf(stderr, "Unsupported codec!\n");        return -1;    }    switch (codecCtx->codec_type) {    case AVMEDIA_TYPE_VIDEO:        is->videoStream = stream_index;        is->video_st = pFormatCtx->streams[stream_index];        is->sws_ctx = sws_getContext(is->video_st->codec->width,                is->video_st->codec->height, is->video_st->codec->pix_fmt,                is->video_st->codec->width, is->video_st->codec->height,                AV_PIX_FMT_YUV420P, SWS_FAST_BILINEAR, NULL, NULL, NULL);        is->frame_timer = (double) av_gettime() / 1000000.0;        is->frame_last_delay = 40e-3;        is->video_current_pts_time = av_gettime();        packet_queue_init(&is->videoq);        is->video_tid = SDL_CreateThread(video_thread, "video_thread", is);        codecCtx->get_buffer = our_get_buffer;        codecCtx->release_buffer = our_release_buffer;        break;    default:        break;    }    return 0;}int decode_thread(void *arg) {    VideoState *is = (VideoState *) arg;    AVFormatContext *pFormatCtx = NULL;    AVPacket pkt1, *packet = &pkt1;    int video_index = -1;    int audio_index = -1;    int i;    is->videoStream = -1;    is->audioStream = -1;    AVIOInterruptCB interupt_cb;    global_video_state = is;    // will interrup blocking functions if we quit!    interupt_cb.callback = decode_interrupt_cb;    interupt_cb.opaque = is;    if (avio_open2(&is->io_ctx, is->filename, 0, &interupt_cb, NULL)) {        fprintf(stderr, "Cannot open I/O for %s\n", is->filename);        return -1;    }    //Open video file    if (avformat_open_input(&pFormatCtx, is->filename, NULL, NULL) != 0) {        return -1; //Couldn't open file    }    is->ic = pFormatCtx;    //Retrieve stream infomation    if (avformat_find_stream_info(pFormatCtx, NULL) < 0) {        return -1; // Couldn't find stream information    }    //Dump information about file onto standard error    av_dump_format(pFormatCtx, 0, is->filename, 0);    //Find the first video stream    for (i = 0; i < pFormatCtx->nb_streams; i++) {        if (pFormatCtx->streams[i]->codec->coder_type == AVMEDIA_TYPE_VIDEO                && video_index < 0) {            video_index = i;        }        if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO                && audio_index < 0) {            audio_index = i;        }    }    if (audio_index >= 0) {        /* 所有设置SDL音频流信息的步骤都在这个函数里完成 */        audio_stream_component_open(is, audio_index);    }    if (video_index >= 0) {        video_stream_component_open(is, video_index);    }    if (is->videoStream < 0 || is->audioStream <= 0) {        fprintf(stderr, "%s: could not open codec\n", is->filename);        goto fail;    }    //main decode loop    /* 读包的主循环， av_read_frame不停的从文件中读取数据包*/    for (;;) {        if (is->quit) {            break;        }        //seek  stuff goes here        /* 这里audioq.size是指队列中的所有数据包带的音频数据的总量或者视频数据总量，并不是包的数量 */        if (is->audioq.size > MAX_AUDIOQ_SIZE || is->videoq.size > MAX_VIDEOQ_SIZE) {            SDL_Delay(10);            continue;        }        if (av_read_frame(is->ic, packet) < 0) {            if (is->ic->pb->error == 0) {                SDL_Delay(100); /* no error; wait for user input */                continue;            } else {                break;            }        }        // Is this a packet from the video stream?        if (packet->stream_index == is->videoStream) {            packet_queue_put(&is->videoq, packet);        } else if (packet->stream_index == is->audioStream) {            packet_queue_put(&is->audioq, packet);        } else {            av_free_packet(packet);        }    }    /*all done - wait for it*/    while (!is->quit) {        SDL_Delay(100);    }    fail: if (1) {        SDL_Event event;        event.type = FF_QUIT_EVENT;        event.user.data1 = is;        SDL_PushEvent(&event);    }    return 0;}int main(int argc, char *argv[]) {    char *filename = argv[1];    SDL_Event event;    VideoState *is;    is = av_malloc(sizeof(VideoState));    // Register all formats and codecs    av_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);    }    av_strlcpy(is->filename, filename, sizeof(is->filename));    is->pictq_mutex = SDL_CreateMutex();    is->pictq_cond = SDL_CreateCond();    schedule_refresh(is, 40);    is->av_sync_type = DEFAULT_AV_SYNC_TYPE;    is->parse_tid = SDL_CreateThread(decode_thread, "parse_thread", is);    if (!is->parse_tid) {        av_free(is);        return -1;    }    for (;;) {        SDL_WaitEvent(&event);        switch (event.type) {        case FF_QUIT_EVENT:        case SDL_QUIT:            SDL_CondSignal(is->audioq.cond);            SDL_CondSignal(is->videoq.cond);            is->quit = 1;            SDL_Quit();            return 0;            break;        case FF_ALLOC_EVENT:            alloc_picture(event.user.data1);            break;        case FF_REFRESH_EVENT:            video_refresh_timer(event.user.data1);            break;        }    }    return 0;}