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 <stdio.h>
#include <tchar.h> 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){}
如果获取不到4096个字节的音频数据，就从头接着播放了，当然这意味着读到末尾了，但是剩下的音频数据没有做处理？
疑问：
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）
int pcm_buffer_size = 4096;//每次读取4096字节的数据，同时也是音频帧大小