参考雷神的代码：
1/** 2* 最简单的SDL2播放音频的例子（SDL2播放PCM） 3* Simplest Audio Play SDL2 (SDL2 play PCM) 4* 5* 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 6* API（Direct3D，OpenGL）的封装，使用起来明显简单于直接调用底层 7* API。 8* 9* 函数调用步骤如下: 10* 11* [初始化] 12* SDL_Init(): 初始化SDL。 13* SDL_OpenAudio(): 根据参数（存储于SDL_AudioSpec）打开音频设备。 14* SDL_PauseAudio(): 播放音频数据。 15* 16* [循环播放数据] 17* SDL_Delay(): 延时等待播放完成。 18* 19* This software plays PCM raw audio data using SDL2. 20* SDL is a wrapper of low-level API (DirectSound). 21* Use SDL is much easier than directly call these low-level API. 22* 23* The process is shown as follows: 24* 25* [Init] 26* SDL_Init(): Init SDL. 27* SDL_OpenAudio(): Opens the audio device with the desired 28* parameters (In SDL_AudioSpec). 29* SDL_PauseAudio(): Play Audio. 30* 31* [Loop to play data] 32* SDL_Delay(): Wait for completetion of playback. 33*/ 34 35#include <stdio.h> 36#include <tchar.h> 37 38extern "C" 39{ 40#include "SDL.h" 41}; 42 43Uint32 audio_len;//音频数据大小 44Uint8 *audio_pos;//指向音频数据的指针 45 46 47/**回调函数(由系统调用) 48* 函数声明：typedef void (SDLCALL * SDL_AudioCallback) 49* (void *userdata, Uint8 * stream, int len); 50* This function is called when the audio device needs more data. 51* 52* 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) 53* stream: A pointer to the audio data buffer.(该指针指向需要填充的音频缓冲区) 54* len: The length of that buffer in bytes.(音频缓冲区的大小,以字节为单位) 55* 56* Once the callback returns, the buffer will no longer be valid. 57* Stereo samples are stored in a LRLRLR ordering. 58* 59* You can choose to avoid callbacks and use SDL_QueueAudio() instead, if 60* you like. Just open your audio device with a NULL callback. 61*/ 62 63void fill_audio(void *userdata, Uint8 *stream, int len) 64{ 65 //SDL2中必须首先使用SDL_memset()将stream中的数据设置为0 66 SDL_memset(stream, 0, len); 67 if (audio_len == 0) /* Only play if we have data left */ 68 { 69 return; 70 } 71 len = (len > audio_len ? audio_len : len); /* Mix as much data as possible */ 72 73 /** 74 * 函数声明：extern DECLSPEC void SDLCALL 75 * SDL_MixAudio(Uint8 * dst, const Uint8 * src, Uint32 len, int volume); 76 * This takes two audio buffers of the playing audio format and mixes 77 * them, performing addition, volume adjustment, and overflow clipping. 78 * The volume ranges from 0 - 128, and should be set to ::SDL_MIX_MAXVOLUME 79 * for full audio volume. Note this does not change hardware volume. 80 * This is provided for convenience -- you can mix your own audio data. 81 * 82 * #define SDL_MIX_MAXVOLUME 128 83 */ 84 85 SDL_MixAudio(stream, audio_pos, len, SDL_MIX_MAXVOLUME); 86 audio_pos += len; 87 audio_len -= len; 88} 89 90int main(int argc, char* argv[]) 91{ 92 //初始化SDL 93 if (SDL_Init(SDL_INIT_AUDIO)) 94 { 95 printf("Could not initialize SDL - %s\n", SDL_GetError()); 96 return -1; 97 } 98 99 //SDL_AudioSpec初始化 100 SDL_AudioSpec wanted_spec; 101 wanted_spec.freq = 44100; //音频数据的采样率（常用的有48000,44100等） 102 wanted_spec.format = AUDIO_S16SYS;//音频数据的格式 103 wanted_spec.channels = 2; //声道数（例如单声道取值为1，立体声取值为2） 104 wanted_spec.silence = 0; //设置静音的值 105 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） 106 wanted_spec.callback = fill_audio;//填充音频缓冲区的回调函数 107 108 //打开音频 109 if (SDL_OpenAudio(&wanted_spec, NULL) < 0) 110 { 111 printf("can't open audio.\n"); 112 return -1; 113 } 114 115 FILE *fp_pcm = fopen("..\\FFmpeg_PCM\\output.pcm", "rb"); 116 if (fp_pcm == NULL) 117 { 118 printf("cannot open this file\n"); 119 return -1; 120 } 121 122 int pcm_buffer_size = 4096;//每次读取4096字节的数据，同时也是音频帧大小 123 char *pcm_buffer = (char *)malloc(pcm_buffer_size); 124 int data_count = 0; 125 126 //播放音频数据 127 SDL_PauseAudio(0); 128 129 while (true) 130 { 131 int ret = fread(pcm_buffer, 1, pcm_buffer_size, fp_pcm); 132 if (ret != pcm_buffer_size) 133 { 134 //这里有可能是会有剩余音频数据的，不知道这样改对不对？ 135 audio_pos = (Uint8 *)pcm_buffer; 136 audio_len = ret; 137 while (audio_len > 0) 138 { 139 SDL_Delay(1); 140 } 141 142 //退出 143 break; 144 145 //循环播放 146 fseek(fp_pcm, 0, SEEK_SET); 147 fread(pcm_buffer, 1, pcm_buffer_size, fp_pcm); 148 data_count = 0; 149 } 150 printf("Now Playing %10d Bytes data.\n", data_count); 151 data_count += pcm_buffer_size; 152 153 154 audio_pos = (Uint8 *)pcm_buffer; 155 //Audio buffer length 156 audio_len = pcm_buffer_size; 157 158 159 while (audio_len > 0)//Wait until finish 160 { 161 SDL_Delay(1); 162 } 163 } 164 free(pcm_buffer); 165 fclose(fp_pcm); 166 SDL_Quit(); 167 return 0; 168}
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