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参考资料：
FFmpeg和SDL2播放mp4_sdl 播放mp4 声音-CSDN博客
SimplePlayer/SimplePlayer.c at master · David1840/SimplePlayer · GitHub
在前面的学习中，通过获得的AVFrame进行了播放画面，
[FFmpeg学习]初级的SDL播放mp4测试-CSDN博客
播放音频原理类似，也是获取AVFrame的信息，
extern "C" {
#include <libavutil/log.h>
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswresample/swresample.h>
}
// Simplest FFmpeg Sync Player.cpp : 定义控制台应用程序的入口点。
//#include <stdio.h>
#include <SDL_types.h>
#include "SDL.h"static Uint8* audio_chunk;
static Uint32 audio_len;
static Uint8* audio_pos;#define MAX_AUDIO_FRAME_SIZE 19200//音频设备需要更多数据的时候会调用该回调函数
void read_audio_data(void* udata, Uint8* stream, int len) {fprintf(stderr, "stream addr:%p, audio_len:%d, len:%d\n",stream,audio_len,len);//首先使用SDL_memset()将stream中的数据设置为0SDL_memset(stream, 0, len);if (audio_len == 0)return;len = (len > audio_len ? audio_len : len);SDL_MixAudio(stream, audio_pos, len, SDL_MIX_MAXVOLUME);audio_pos += len;audio_len -= len;
}#undef main
int main(int argc, char* argv[]) {const char* file = "test.mp4";AVFormatContext* pFormatCtx = NULL; //for opening multi-media fileint i, audioStream = -1;AVCodecParameters* pCodecParameters = NULL; //codec contextAVCodecContext* pCodecCtx = NULL;const AVCodec* pCodec = NULL; // the codecerAVFrame* pFrame = NULL;AVPacket* packet;uint8_t* out_buffer;int64_t in_channel_layout;struct SwrContext* au_convert_ctx;if (avformat_open_input(&pFormatCtx, file, NULL, NULL) != 0) {SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Failed to open video file!");return -1; // Couldn't open file}audioStream = av_find_best_stream(pFormatCtx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);if (audioStream == -1) {SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Din't find a video stream!");return -1;// Didn't find a video stream}// Get a pointer to the codec context for the video streampCodecParameters = pFormatCtx->streams[audioStream]->codecpar;// Find the decoder for the video streampCodec = avcodec_find_decoder(pCodecParameters->codec_id);if (pCodec == NULL) {SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Unsupported codec!\n");return -1; // Codec not found}// Copy contextpCodecCtx = avcodec_alloc_context3(pCodec);if (avcodec_parameters_to_context(pCodecCtx, pCodecParameters) != 0) {SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Couldn't copy codec context");return -1;// Error copying codec context}// Open codecif (avcodec_open2(pCodecCtx, pCodec, NULL) < 0) {SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Failed to open decoder!\n");return -1; // Could not open codec}packet = (AVPacket*)av_malloc(sizeof(AVPacket));av_init_packet(packet);pFrame = av_frame_alloc();uint64_t out_channel_layout = AV_CH_LAYOUT_STEREO;//输出声道int out_nb_samples = 1024;enum AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16;//输出格式S16int out_sample_rate = 44100;int out_channels = av_get_channel_layout_nb_channels(out_channel_layout);int out_buffer_size = av_samples_get_buffer_size(NULL, out_channels, out_nb_samples, out_sample_fmt, 1);out_buffer = (uint8_t*)av_malloc(MAX_AUDIO_FRAME_SIZE * 2);//Initif (SDL_Init(SDL_INIT_AUDIO | SDL_INIT_TIMER)) {printf("Could not initialize SDL - %s\n", SDL_GetError());return -1;}SDL_AudioSpec spec;spec.freq = out_sample_rate;spec.format = AUDIO_S16SYS;spec.channels = out_channels;spec.silence = 0;spec.samples = out_nb_samples;spec.callback = read_audio_data;spec.userdata = pCodecCtx;if (SDL_OpenAudio(&spec, NULL) < 0) {printf("can't open audio.\n");return -1;}in_channel_layout = av_get_default_channel_layout(pCodecCtx->channels);printf("in_channel_layout --->%d\n", in_channel_layout);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, pCodecCtx->sample_fmt, pCodecCtx->sample_rate, 0, NULL);swr_init(au_convert_ctx);SDL_PauseAudio(0);while (av_read_frame(pFormatCtx, packet) >= 0) {if (packet->stream_index == audioStream) {avcodec_send_packet(pCodecCtx, packet);while (avcodec_receive_frame(pCodecCtx, pFrame) == 0) {swr_convert(au_convert_ctx, &out_buffer, MAX_AUDIO_FRAME_SIZE, (const uint8_t**)pFrame->data,pFrame->nb_samples); // 转换音频}audio_chunk = (Uint8*)out_buffer;audio_len = out_buffer_size;audio_pos = audio_chunk;while (audio_len > 0) {SDL_Delay(1);//延迟播放}}av_packet_unref(packet);}swr_free(&au_convert_ctx);SDL_Quit();return 0;
}
在windows上，如果有方法废弃的错误，
错误 C4996 'AVCodecContext::channels': 被声明为已否决
可以设置SDL检查为否，来解决
代码流程里，需要注意一下audio_callback是怎么回调的，
audio_callback函数是由SDL音频系统在需要更多音频数据以填充音频缓冲区时自动调用的。当SDL音频系统开始播放音频时，它会周期性地调用音频回调函数以获取新的音频数据。音频回调函数的调用频率取决于音频采样率和缓冲区大小。
在main函数中，我们初始化SDL音频系统并设置了音频回调函数audio_callback：
if (SDL_Init(SDL_INIT_AUDIO) < 0) {fprintf(stderr, "Could not initialize SDL - %s\n", SDL_GetError());exit(1);
}// 设置音频回调函数
SDL_AudioSpec desired;
desired.freq = audioCodecContext->sample_rate;
desired.format = AUDIO_S16SYS; // 使用16位有符号小端字节序样本
desired.channels = audioCodecContext->channels;
desired.samples = 4096; // 音频缓冲区大小，可以根据需要调整
desired.callback = audio_callback;
desired.userdata = NULL;// 打开音频设备
SDL_AudioDeviceID deviceId = SDL_OpenAudioDevice(NULL, 0, &desired, NULL, SDL_AUDIO_ALLOW_ANY_CHANGE);
if (deviceId <= 0) {fprintf(stderr, "Failed to open audio device: %s\n", SDL_GetError());return -1;
}// 开始播放音频
SDL_PauseAudioDevice(deviceId, 0);当我们调用SDL_OpenAudioDevice函数时，SDL音频系统会注册音频回调函数audio_callback。当我们调用SDL_PauseAudioDevice(deviceId, 0)开始播放音频时，SDL音频系统会根据音频参数（如采样率、通道数和缓冲区大小）定期调用audio_callback函数。
在audio_callback函数中，我们需要根据音频缓冲区的需求提供音频数据。这通常涉及从文件或实时流中解码音频数据，并将其传递给SDL音频系统。在我们的示例中，我们从全局变量audio_buffer中读取音频数据，该变量由swr_convert函数填充。
总之，audio_callback函数是由SDL音频系统在需要更多音频数据以填充音频缓冲区时自动触发的。您无需手动调用此函数，SDL音频系统会负责调用它。只需确保在回调函数中提供正确的音频数据即可。
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