alsa 测试小程序

具体还不是很理解，下边是参考代码，也不知道下边的程序怎么用

alsa api 参考文档：http://www.alsa-project.org/alsa-doc/alsa-lib/index.html

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下面是一个ALSA录音的测试程序，只能录5秒。

编译方法： cc test.c -lasound

如果编译说找不到asound.h需要先安装一下必要的库

缺少一个库：

apt-get install libasound2-dev

编译之后用法如下：./test > /test.wav

遇到的问题：
1.运行时遇到的问题是:alsa的设备节点都建在了/dev下面而不是/dev/snd下面，导致程序出错。
可以用下面3条命令手动建立节点：
mknod dev/snd/controlC0 c 116 0
mknod dev/snd/pcmC0D0c c 116 24
mknod dev/snd/pcmC0D0p c 116 16

2.由于缺少动态库，导致很多接口都找不到。但是实际上这些动态库里面用的方法静态库里面都有，所以干脆改了程序直接调用静态库。

附录：

/*

Example 4 - Simple sound recording

This example reads from the default PCM device
and writes to standard output for 5 seconds of data.

*/

/* Use the newer ALSA API */
#define ALSA_PCM_NEW_HW_PARAMS_API

#include <alsa/asoundlib.h>

int main() {
  long loops;
  int rc;
  int size;
  snd_pcm_t *handle;
  snd_pcm_hw_params_t *params;
  unsigned int val;
  int dir;
  snd_pcm_uframes_t frames;
  char *buffer;
  
  /* Open PCM device for recording (capture). */
  rc = snd_pcm_open(&handle, "default",
                    SND_PCM_STREAM_CAPTURE, 0);
  if (rc < 0) {
    fprintf(stderr, 
            "unable to open pcm device: %s\n",
            snd_strerror(rc));
    exit(1);
  }
  
  /* Allocate a hardware parameters object. */
  snd_pcm_hw_params_alloca(¶ms);

  /* Fill it in with default values. */
  snd_pcm_hw_params_any(handle, params);

  /* Set the desired hardware parameters. */

  /* Interleaved mode */
  snd_pcm_hw_params_set_access(handle, params,
                      SND_PCM_ACCESS_RW_INTERLEAVED);

  /* Signed 16-bit little-endian format */
  snd_pcm_hw_params_set_format(handle, params,
                              SND_PCM_FORMAT_S16_LE);

  /* Two channels (stereo) */
  snd_pcm_hw_params_set_channels(handle, params, 2);

  /* 44100 bits/second sampling rate (CD quality) */
  val = 44100;
  snd_pcm_hw_params_set_rate_near(handle, params, 
                                  &val, &dir);

  /* Set period size to 32 frames. */
  frames = 32;
  snd_pcm_hw_params_set_period_size_near(handle, 
                              params, &frames, &dir);

  /* Write the parameters to the driver */
  rc = snd_pcm_hw_params(handle, params);
  if (rc < 0) {
    fprintf(stderr,
            "unable to set hw parameters: %s\n",
            snd_strerror(rc));
    exit(1);
  }

  /* Use a buffer large enough to hold one period */
  snd_pcm_hw_params_get_period_size(params,
                                      &frames, &dir);
  size = frames * 4; /* 2 bytes/sample, 2 channels */
  buffer = (char *) malloc(size);

  /* We want to loop for 5 seconds */
  snd_pcm_hw_params_get_period_time(params,
                                         &val, &dir);
  loops = 5000000 / val;

  while (loops > 0) {
    loops--;
    rc = snd_pcm_readi(handle, buffer, frames);
    if (rc == -EPIPE) {
      /* EPIPE means overrun */
      fprintf(stderr, "overrun occurred\n");
      snd_pcm_prepare(handle);
    } else if (rc < 0) {
      fprintf(stderr,
              "error from read: %s\n", 
              snd_strerror(rc));
    } else if (rc != (int)frames) {
      fprintf(stderr, "short read, read %d frames\n", rc);
    }
    rc = write(1, buffer, size);
    if (rc != size)
      fprintf(stderr, 
              "short write: wrote %d bytes\n", rc);
  }

  snd_pcm_drain(handle);
  snd_pcm_close(handle);
  free(buffer);

  return 0;
}

3. 其他：oss测试方法如下。
录音 cat /dev/oss > test.wav
播放 cat /mnt/test.wav > /dev/oss