ALSA用户空间编程

/**alsa play test
*ALSA用户空间编译，ALSA驱动的声卡在用户空间，不宜直接使用
*文件接口中，而应使用alsa-lib
*打开---->设置参数--->读写音频数据 ALSA全部使用alsa-lib中的API
*交叉编译
*export LD_LIBRARY_PATH=$PWD:$LD_LIBRARY_PATH
*arm-linux-gcc -o alsa_play alsa_play_test.c -L. -lasound
*需要交叉编译后的libasound.so库的支持
*
*/
#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设备句柄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. 打开PCM，最后一个参数为0意味着标准配置
    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结构体
    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. 初始化访问权限
    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. 初始化采样格式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. 设置采样率，如果硬件不支持我们设置的采样率，将使用最接近的
    //val = 44100,有些录音采样频率固定为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. 设置通道数量
    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. 设置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. 写音频数据到PCM设备
        while(ret = snd_pcm_writei(playback_handle, buffer, frames)<0)
        {
            usleep(2000);
            if (ret == -EPIPE)
            {
                  /* EPIPE means underrun */
                  fprintf(stderr, "underrun occurred\n");
                  //完成硬件参数设置，使设备准备好
                  snd_pcm_prepare(playback_handle);
            }
            else if (ret < 0)
            {
                  fprintf(stderr,
                      "error from writei: %s\n",
                      snd_strerror(ret));
            }
        }

    }
    //10. 关闭PCM设备句柄
    snd_pcm_close(playback_handle);

    return 0;
}