ALSA用戶空間編程

/**alsa play test 
*ALSA用戶空間編譯，ALSA驅動的聲卡在用戶空間，不宜直接使用 
*文件接口中，而應使用alsa-lib 
*打開---->設置參數--->讀寫音頻數據 ALSA全部使用alsa-lib中的API 
*交叉編譯 
*export LD_LIBRARY_PATH=$PWD:$LD_LIBRARY_PATH 
*arm-linux-gcc -o alsa_play alsa_play_test.c -L. -lasound 
*需要交叉編譯後的libasound.so庫的支持 
* 
*/  
#include <stdio.h>  
#include <stdlib.h>  
#include "alsa/asoundlib.h"  
  
int main(int argc, char *argv[])  
{  
    int i;  
    int ret;  
    int buf[128];  
    unsigned int val;  
    int dir=0;  
    char *buffer;  
    int size;  
    snd_pcm_uframes_t frames;  
    snd_pcm_uframes_t periodsize;  
    snd_pcm_t *playback_handle;//PCM設備句柄pcm.h  
    snd_pcm_hw_params_t *hw_params;//硬件信息和PCM流配置  
    if (argc != 2) {  
        printf("error: alsa_play_test [music name]\n");  
        exit(1);  
    }  
    printf("play song %s by wolf\n", argv[1]);  
    FILE *fp = fopen(argv[1], "rb");  
    if(fp == NULL)  
    return 0;  
    fseek(fp, 100, SEEK_SET);  
      
    //1. 打開PCM，最後一個參數爲0意味着標準配置  
    ret = snd_pcm_open(&playback_handle, "default", SND_PCM_STREAM_PLAYBACK, 0);  
    if (ret < 0) {  
        perror("snd_pcm_open");  
        exit(1);  
    }  
      
    //2. 分配snd_pcm_hw_params_t結構體  
    ret = snd_pcm_hw_params_malloc(&hw_params);  
    if (ret < 0) {  
        perror("snd_pcm_hw_params_malloc");  
        exit(1);  
    }  
    //3. 初始化hw_params  
    ret = snd_pcm_hw_params_any(playback_handle, hw_params);  
    if (ret < 0) {  
        perror("snd_pcm_hw_params_any");  
        exit(1);  
    }  
    //4. 初始化訪問權限  
    ret = snd_pcm_hw_params_set_access(playback_handle, hw_params, SND_PCM_ACCESS_RW_INTERLEAVED);  
    if (ret < 0) {  
        perror("snd_pcm_hw_params_set_access");  
        exit(1);  
    }  
    //5. 初始化採樣格式SND_PCM_FORMAT_U8,8位  
    ret = snd_pcm_hw_params_set_format(playback_handle, hw_params, SND_PCM_FORMAT_U8);  
    if (ret < 0) {  
        perror("snd_pcm_hw_params_set_format");  
        exit(1);  
    }  
    //6. 設置採樣率，如果硬件不支持我們設置的採樣率，將使用最接近的  
    //val = 44100,有些錄音採樣頻率固定爲8KHz  
      
  
    val = 8000;  
    ret = snd_pcm_hw_params_set_rate_near(playback_handle, hw_params, &val, &dir);  
    if (ret < 0) {  
        perror("snd_pcm_hw_params_set_rate_near");  
        exit(1);  
    }  
    //7. 設置通道數量  
    ret = snd_pcm_hw_params_set_channels(playback_handle, hw_params, 2);  
    if (ret < 0) {  
        perror("snd_pcm_hw_params_set_channels");  
        exit(1);  
    }  
      
    /* Set period size to 32 frames. */  
    frames = 32;  
    periodsize = frames * 2;  
    ret = snd_pcm_hw_params_set_buffer_size_near(playback_handle, hw_params, &periodsize);  
    if (ret < 0)   
    {  
         printf("Unable to set buffer size %li : %s\n", frames * 2, snd_strerror(ret));  
           
    }  
          periodsize /= 2;  
  
    ret = snd_pcm_hw_params_set_period_size_near(playback_handle, hw_params, &periodsize, 0);  
    if (ret < 0)   
    {  
        printf("Unable to set period size %li : %s\n", periodsize,  snd_strerror(ret));  
    }  
                                    
    //8. 設置hw_params  
    ret = snd_pcm_hw_params(playback_handle, hw_params);  
    if (ret < 0) {  
        perror("snd_pcm_hw_params");  
        exit(1);  
    }  
      
     /* Use a buffer large enough to hold one period */  
    snd_pcm_hw_params_get_period_size(hw_params, &frames, &dir);  
                                  
    size = frames * 2; /* 2 bytes/sample, 2 channels */  
    buffer = (char *) malloc(size);  
    fprintf(stderr,  
            "size = %d\n",  
            size);  
      
    while (1)   
    {  
        ret = fread(buffer, 1, size, fp);  
        if(ret == 0)   
        {  
              fprintf(stderr, "end of file on input\n");  
              break;  
        }   
        else if (ret != size)   
        {  
        }  
        //9. 寫音頻數據到PCM設備  
        while(ret = snd_pcm_writei(playback_handle, buffer, frames)<0)  
        {  
            usleep(2000);  
            if (ret == -EPIPE)  
            {  
                  /* EPIPE means underrun */  
                  fprintf(stderr, "underrun occurred\n");  
                  //完成硬件參數設置，使設備準備好  
                  snd_pcm_prepare(playback_handle);  
            }   
            else if (ret < 0)   
            {  
                  fprintf(stderr,  
                      "error from writei: %s\n",  
                      snd_strerror(ret));  
            }    
        }  
          
    }         
    //10. 關閉PCM設備句柄  
    snd_pcm_close(playback_handle);  
      
    return 0;  
}