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参考雷神的代码：

/*** 最简单的SDL2播放音频的例子（SDL2播放PCM）* Simplest Audio Play SDL2 (SDL2 play PCM)** 本程序使用SDL2播放PCM音频采样数据。SDL实际上是对底层绘图* API（Direct3D，OpenGL）的封装，使用起来明显简单于直接调用底层* API。** 函数调用步骤如下:** [初始化]* SDL_Init(): 初始化SDL。* SDL_OpenAudio(): 根据参数（存储于SDL_AudioSpec）打开音频设备。* SDL_PauseAudio(): 播放音频数据。** [循环播放数据]* SDL_Delay(): 延时等待播放完成。** This software plays PCM raw audio data using SDL2.* SDL is a wrapper of low-level API (DirectSound).* Use SDL is much easier than directly call these low-level API.** The process is shown as follows:** [Init]* SDL_Init(): Init SDL.* SDL_OpenAudio(): Opens the audio device with the desired*                  parameters (In SDL_AudioSpec).* SDL_PauseAudio(): Play Audio.** [Loop to play data]* SDL_Delay(): Wait for completetion of playback.*/#include 
   
    #include 
    
     extern "C"{#include "SDL.h"};Uint32  audio_len;//音频数据大小Uint8  *audio_pos;//指向音频数据的指针/**回调函数(由系统调用)*  函数声明：typedef void (SDLCALL * SDL_AudioCallback) *            (void *userdata, Uint8 * stream, int len);*  This function is called when the audio device needs more data.**  userdata: An application-specific parameter saved in the SDL_AudioSpec structure(SDL_AudioSpec结构中的用户自定义数据，一般情况下可以不用)*  stream:   A pointer to the audio data buffer.(该指针指向需要填充的音频缓冲区)*  len:      The length of that buffer in bytes.(音频缓冲区的大小,以字节为单位)**  Once the callback returns, the buffer will no longer be valid.*  Stereo samples are stored in a LRLRLR ordering.**  You can choose to avoid callbacks and use SDL_QueueAudio() instead, if*  you like. Just open your audio device with a NULL callback.*/void  fill_audio(void *userdata, Uint8 *stream, int len){	//SDL2中必须首先使用SDL_memset()将stream中的数据设置为0	SDL_memset(stream, 0, len);	if (audio_len == 0)		/*  Only  play  if  we  have  data  left  */	{		return;	}	len = (len > audio_len ? audio_len : len);	/*  Mix  as  much  data  as  possible  */	/**	*  函数声明：extern DECLSPEC void SDLCALL 	*  SDL_MixAudio(Uint8 * dst, const Uint8 * src, Uint32 len, int volume);	*  This takes two audio buffers of the playing audio format and mixes	*  them, performing addition, volume adjustment, and overflow clipping.	*  The volume ranges from 0 - 128, and should be set to ::SDL_MIX_MAXVOLUME	*  for full audio volume.  Note this does not change hardware volume.	*  This is provided for convenience -- you can mix your own audio data.	*	*  #define SDL_MIX_MAXVOLUME 128	*/	SDL_MixAudio(stream, audio_pos, len, SDL_MIX_MAXVOLUME);	audio_pos += len;	audio_len -= len;}int main(int argc, char* argv[]){	//初始化SDL	if (SDL_Init(SDL_INIT_AUDIO))	{		printf("Could not initialize SDL - %s\n", SDL_GetError());		return -1;	}	//SDL_AudioSpec初始化	SDL_AudioSpec wanted_spec;	wanted_spec.freq = 44100;			//音频数据的采样率（常用的有48000,44100等）	wanted_spec.format = AUDIO_S16SYS;//音频数据的格式	wanted_spec.channels = 2;			//声道数（例如单声道取值为1，立体声取值为2）	wanted_spec.silence = 0;			//设置静音的值	wanted_spec.samples = 1024;		//音频缓冲区中的采样个数（要求必须是2的n次方）	wanted_spec.callback = fill_audio;//填充音频缓冲区的回调函数	//打开音频	if (SDL_OpenAudio(&wanted_spec, NULL) < 0)	{		printf("can't open audio.\n");		return -1;	}	FILE *fp_pcm = fopen("..\\FFmpeg_PCM\\output.pcm", "rb");	if (fp_pcm == NULL)	{		printf("cannot open this file\n");		return -1;	}	int pcm_buffer_size = 4096;//每次读取4096字节的数据，同时也是音频帧大小	char *pcm_buffer = (char *)malloc(pcm_buffer_size);	int data_count = 0;	//播放音频数据	SDL_PauseAudio(0);	while (true)	{		int ret = fread(pcm_buffer, 1, pcm_buffer_size, fp_pcm);		if (ret != pcm_buffer_size)		{			//这里有可能是会有剩余音频数据的，不知道这样改对不对？			audio_pos = (Uint8 *)pcm_buffer;			audio_len = ret;			while (audio_len > 0)			{				SDL_Delay(1);			}			//退出			break;						//循环播放			fseek(fp_pcm, 0, SEEK_SET);			fread(pcm_buffer, 1, pcm_buffer_size, fp_pcm);			data_count = 0;		}		printf("Now Playing %10d Bytes data.\n", data_count);		data_count += pcm_buffer_size;				audio_pos = (Uint8 *)pcm_buffer;		//Audio buffer length 		audio_len = pcm_buffer_size;				while (audio_len > 0)//Wait until finish		{			SDL_Delay(1);		}	}	free(pcm_buffer);	fclose(fp_pcm);	SDL_Quit();	return 0;}
    
   

雷神代码中if (fread(pcm_buffer, 1, pcm_buffer_size, fp) != pcm_buffer_size){}

疑问：

wanted_spec.silence = 0; //设置静音的值

int pcm_buffer_size = 4096;//每次读取4096字节的数据，同时也是音频帧大小