ALSA Audio API 使用指南（译文）

snd_pcm_t *pcm_handle;

bool device_open(int mode){
if (snd_pcm_open (&pcm_handle, “default” , mode , 0) < 0)
return false;
return true;
}

int bit_per_sample; //样本长度(bit)
int period_size; //周期长度(桢数)
int chunk_byte; //周期长度(字节数)
snd_pcm_hw_params_t *params; //定义参数变量

bool device_setparams()
{
snd_pcm_hw_params_t *hw_params;
snd_pcm_hw_params_malloc (&hw_params); //为参数变量分配空间
snd_pcm_hw_params_malloc (¶ms);
snd_pcm_hw_params_any ( pcm_handle, hw_params ); //参数初始化
snd_pcm_hw_params_set_access ( pcm_handle, hw_params,
SND_PCM_ACCESS_RW_INTERLEAVED); //设置为交错模式
snd_pcm_hw_params_set_format( pcm_handle, hw_params,
SND_FORMAT_S16_LE); //使用用16位样本
snd_pcm_hw_params_set_rate_near( pcm_handle, hw_params,
44100, 0); //设置采样率为44.1KHz
snd_pcm_hw_params_set_channels( pcm_handle, hw_params, 2); //设置为立体声
snd_pcm_hw_params_get_period_size( hw_params, &period_size); //获取周期长度
bit_per_sample = snd_pcm_hw_format_physical_width( hw_params.format );
//获取样本长度
chunk_byte = period_size * bit_per_sample * hw_params.channels / 8;
//计算周期长度（字节数(bytes) = 每周期的桢数 * 样本长度(bit) * 通道数 / 8 ）
snd_pcm_hw_params( pcm_handle, hw_params); //设置参数
params = hw_params; //保存参数，方便以后使用
snd_pcm_hw_params_free( hw_params); //释放参数变量空间
return true;

}

char *wave_buf;
int wave_buf_len;
bool device_capture( int dtime /*录音长度（单位：秒）*/){
　wave_buf_len = dtime * params.rate * bit_per_sample * params.channels / 8 ;
//计算音频数据长度（秒数 * 采样率 * 桢长度）
　char *data = wave_buf = (char*)malloc( wave_buf_len ); //分配空间

int r = 0;
while ( data ?C wave_buf <= wave_buf_len ?C chunk_size ){
r = snd_pcm_readi( pcm_handle, data , chunk_size);
if ( r>0 ) data += r * chunk_byte;
else
return false
}
return true;
}

bool device_play(){
char *data = wave_buf;
int r = 0;
while ( data ?C wave_buf <= wave_buf_len ?C chunk_size ){
r = snd_pcm_writei( pcm_handle, data , chunk_size);
if ( r>0 ) data += r * chunk_byte;
else
return false
}
return true;
}

#include <alsa/asoundlib.h>

bool device_open( int mode);
bool device_setparams();
bool device_capture( int dtime );
bool device_play();
char *wave_buf;
int wave_buf_len;
int bit_per_sample;
int period_size;
int chunk_byte;
int chunk_size;
snd_pcm_hw_params_t *params;

int main( int , char** ){
//录音
if (!device_open(SND_PCM_STREAM_CAPTURE ) return 1;
if (!device_setparams()) return 2;
if (!device_capture( 3 )) return 3; //录制3秒
snd_pcm_close( pcm_handle );

//播放
if (!device_open( SND_PCM_STREAM_PLAYBACK ) return 4;
if (!device_setparams()) return 5;
if (!device_play()) return 6;
snd_pcm_close( pcm_handle );

return 0;
}