我正在尝试制作视频播放器。我添加了一个线程来计时视频应该在屏幕上显示多长时间。我正在尝试在主线程中解码视频和更新窗口；第二个线程将获取数据包，查看数据包应该显示多长时间，然后将数据包发送到主线程，然后等待时间过去。
出于某种原因，我收到此错误：
terminate called after throwing an instance of 'std::future_error'
what(): std::future_error: No associated state
是什么导致了错误？
我的代码：
extern "C"{
//FFmpeg libraries
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>
//SDL2 libraries
#include <SDL2/SDL.h>
}
// compatibility with newer API
#if LIBAVCODEC_VERSION_INT < AV_VERSION_INT(55,28,1)
#define av_frame_alloc avcodec_alloc_frame
#define av_frame_free avcodec_free_frame
#endif
//C++ libraries
#include <memory>
#include <stdio.h>
#include <iostream>
#include <chrono>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
typedef struct PacketQueue {
AVPacketList *first_pkt, *last_pkt;
} PacketQueue;
std::atomic<bool> quitting;
std::mutex mutex;
std::condition_variable convar;
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;
std::lock_guard<std::mutex> lock(mutex);
if (!q->last_pkt){
q->first_pkt = pkt1;
}else{
q->last_pkt->next = pkt1;
}
q->last_pkt = pkt1;
convar.notify_all();
return 0;
}
static int packet_queue_get(PacketQueue *q, AVPacket *pkt){
AVPacketList *pkt1;
int ret;
std::unique_lock<std::mutex> lk(mutex);
while(1){
if(quitting){
ret = -1;
break;
}
pkt1 = q->first_pkt;
if(pkt1){
q->first_pkt = pkt1->next;
if(!q->first_pkt){
q->last_pkt = NULL;
}
*pkt = pkt1->pkt;
av_free(pkt1);
ret = 1;
break;
}else {
convar.wait_for(lk, std::chrono::milliseconds(1));
}
}
return ret;
}
void videoTimerFunc(AVRational time_base, PacketQueue* videoq, std::promise<AVPacket> prms){
AVPacket pkt;
int64_t last_pts = 0;
int64_t frameDelay;
AVRational microseconds = {1, 1000000};
while(!quitting){
// Getting packet and check if there are more packets
if(!packet_queue_get(videoq, &pkt)){
// Close programme
quitting = true;
}else {
// Send packet and create timer
frameDelay = av_rescale_q(pkt.dts, time_base, microseconds) - last_pts;
last_pts = av_rescale_q(pkt.dts, time_base, microseconds);
prms.set_value(pkt);
std::this_thread::sleep_for(std::chrono::microseconds(frameDelay));
}
}
}
int main(int argc, char *argv[]){
AVFormatContext* FormatCtx = nullptr;
AVCodecContext* CodecCtxOrig = nullptr;
AVCodecContext* CodecCtx = nullptr;
AVCodec* Codec = nullptr;
int videoStream;
AVFrame* Frame = nullptr;
AVPacket packet;
struct SwsContext* SwsCtx = nullptr;
PacketQueue videoq;
std::promise<AVPacket> pktprms;
std::future<AVPacket> pktftr = pktprms.get_future();
int frameFinished;
int64_t lastPTS;
SDL_Event event;
SDL_Window* screen;
SDL_Renderer* renderer;
SDL_Texture* texture;
std::shared_ptr<Uint8> yPlane, uPlane, vPlane;
int uvPitch;
if (argc != 2) {
fprintf(stderr, "Usage: %s <file>\n", argv[0]);
return -1;
}
// Register all formats and codecs
av_register_all();
// Initialise SDL2
if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER)) {
fprintf(stderr, "Couldn't initialise SDL - %s\n", SDL_GetError());
return -1;
}
// Setting things up
quitting = false;
memset(&videoq, 0, sizeof(PacketQueue));
// Open video file
if(avformat_open_input(&FormatCtx, argv[1], NULL, NULL) != 0){
fprintf(stderr, "Couldn't open file\n");
return -1; // Couldn't open file
}
// Retrieve stream information
if(avformat_find_stream_info(FormatCtx, NULL) < 0){
fprintf(stderr, "Couldn't find stream information\n");
// Close the video file
avformat_close_input(&FormatCtx);
return -1; // Couldn't find stream information
}
// Find the video stream
videoStream = av_find_best_stream(FormatCtx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
if(videoStream < 0){
fprintf(stderr, "Couldn't find video stream\n");
// Close the video file
avformat_close_input(&FormatCtx);
return -1; // Didn't find a video stream
}
// Get a pointer to the codec context for the video stream
CodecCtxOrig = FormatCtx->streams[videoStream]->codec;
// Find the decoder for the video stream
Codec = avcodec_find_decoder(CodecCtxOrig->codec_id);
if(Codec == NULL){
fprintf(stderr, "Unsupported codec\n");
// Close the codec
avcodec_close(CodecCtxOrig);
// Close the video file
avformat_close_input(&FormatCtx);
return -1; // Codec not found
}
// Copy context
CodecCtx = avcodec_alloc_context3(Codec);
if(avcodec_copy_context(CodecCtx, CodecCtxOrig) != 0){
fprintf(stderr, "Couldn't copy codec context");
// Close the codec
avcodec_close(CodecCtxOrig);
// Close the video file
avformat_close_input(&FormatCtx);
return -1; // Error copying codec context
}
// Open codec
if(avcodec_open2(CodecCtx, Codec, NULL) < 0){
fprintf(stderr, "Couldn't open codec\n");
// Close the codec
avcodec_close(CodecCtx);
avcodec_close(CodecCtxOrig);
// Close the video file
avformat_close_input(&FormatCtx);
return -1; // Could not open codec
}
// Allocate video frame
Frame = av_frame_alloc();
// Make a screen to put our video
screen = SDL_CreateWindow("Video Player", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, CodecCtx->width, CodecCtx->height, 0);
if(!screen){
fprintf(stderr, "SDL: could not create window - exiting\n");
quitting = true;
// Clean up SDL2
SDL_Quit();
// Free the YUV frame
av_frame_free(&Frame);
// Close the codec
avcodec_close(CodecCtx);
avcodec_close(CodecCtxOrig);
// Close the video file
avformat_close_input(&FormatCtx);
return -1;
}
renderer = SDL_CreateRenderer(screen, -1, 0);
if(!renderer){
fprintf(stderr, "SDL: could not create renderer - exiting\n");
quitting = true;
// Clean up SDL2
SDL_DestroyWindow(screen);
SDL_Quit();
// Free the YUV frame
av_frame_free(&Frame);
// Close the codec
avcodec_close(CodecCtx);
avcodec_close(CodecCtxOrig);
// Close the video file
avformat_close_input(&FormatCtx);
return -1;
}
// Allocate a place to put our YUV image on that screen
texture = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_YV12, SDL_TEXTUREACCESS_STREAMING, CodecCtx->width, CodecCtx->height);
if(!texture){
fprintf(stderr, "SDL: could not create texture - exiting\n");
quitting = true;
// Clean up SDL2
SDL_DestroyRenderer(renderer);
SDL_DestroyWindow(screen);
SDL_Quit();
// Free the YUV frame
av_frame_free(&Frame);
// Close the codec
avcodec_close(CodecCtx);
avcodec_close(CodecCtxOrig);
// Close the video file
avformat_close_input(&FormatCtx);
return -1;
}
// Initialise SWS context for software scaling
SwsCtx = sws_getContext(CodecCtx->width, CodecCtx->height, CodecCtx->pix_fmt,
CodecCtx->width, CodecCtx->height, PIX_FMT_YUV420P, SWS_BILINEAR, NULL, NULL, NULL);
if(!SwsCtx){
fprintf(stderr, "Couldn't create sws context\n");
quitting = true;
// Clean up SDL2
SDL_DestroyTexture(texture);
SDL_DestroyRenderer(renderer);
SDL_DestroyWindow(screen);
SDL_Quit();
// Free the YUV frame
av_frame_free(&Frame);
// Close the codec
avcodec_close(CodecCtx);
avcodec_close(CodecCtxOrig);
// Close the video file
avformat_close_input(&FormatCtx);
return -1;
}
// set up YV12 pixel array (12 bits per pixel)
yPlane = std::shared_ptr<Uint8>((Uint8 *)::operator new (CodecCtx->width * CodecCtx->height, std::nothrow));
uPlane = std::shared_ptr<Uint8>((Uint8 *)::operator new (CodecCtx->width * CodecCtx->height / 4, std::nothrow));
vPlane = std::shared_ptr<Uint8>((Uint8 *)::operator new (CodecCtx->width * CodecCtx->height / 4, std::nothrow));
uvPitch = CodecCtx->width / 2;
if (!yPlane || !uPlane || !vPlane) {
fprintf(stderr, "Could not allocate pixel buffers - exiting\n");
quitting = true;
// Clean up SDL2
SDL_DestroyTexture(texture);
SDL_DestroyRenderer(renderer);
SDL_DestroyWindow(screen);
SDL_Quit();
// Free the YUV frame
av_frame_free(&Frame);
// Close the codec
avcodec_close(CodecCtx);
avcodec_close(CodecCtxOrig);
// Close the video file
avformat_close_input(&FormatCtx);
return -1;
}
std::thread videoTimerThread(videoTimerFunc, FormatCtx->streams[videoStream]->time_base, &videoq, std::move(pktprms));
while (!quitting) {
// Check for more packets
if(av_read_frame(FormatCtx, &packet) >= 0){
// Check what stream it belongs to
if (packet.stream_index == videoStream) {
packet_queue_put(&videoq, &packet);
}else{
// Free the packet that was allocated by av_read_frame
av_free_packet(&packet);
}
}
// Check if its time to update
if(pktftr.wait_for(std::chrono::milliseconds(1)) == std::future_status::ready){
// Getting packet
packet = pktftr.get();
// Decode video frame
avcodec_decode_video2(CodecCtx, Frame, &frameFinished, &packet);
// Did we get a video frame?
if (frameFinished) {
AVPicture pict;
pict.data[0] = yPlane.get();
pict.data[1] = uPlane.get();
pict.data[2] = vPlane.get();
pict.linesize[0] = CodecCtx->width;
pict.linesize[1] = uvPitch;
pict.linesize[2] = uvPitch;
// Convert the image into YUV format that SDL uses
sws_scale(SwsCtx, (uint8_t const * const *) Frame->data, Frame->linesize, 0, CodecCtx->height, pict.data, pict.linesize);
SDL_UpdateYUVTexture(texture, NULL, yPlane.get(), CodecCtx->width, uPlane.get(), uvPitch, vPlane.get(), uvPitch);
SDL_RenderClear(renderer);
SDL_RenderCopy(renderer, texture, NULL, NULL);
SDL_RenderPresent(renderer);
}
// Free the packet that was allocated by av_read_frame
av_free_packet(&packet);
}
SDL_PollEvent(&event);
switch (event.type) {
case SDL_QUIT:
quitting = true;
break;
default:
break;
}
}
videoTimerThread.join();
//SDL2 clean up
SDL_DestroyTexture(texture);
SDL_DestroyRenderer(renderer);
SDL_DestroyWindow(screen);
SDL_Quit();
// Free the YUV frame
av_frame_free(&Frame);
// Free Sws
sws_freeContext(SwsCtx);
// Close the codec
avcodec_close(CodecCtx);
avcodec_close(CodecCtxOrig);
// Close the video file
avformat_close_input(&FormatCtx);
return 0;
}