OpenWrt：使用ALSA实现边录边播

ALSA是Linux系统中的高级音频架构（Advanced Linux Sound Architecture）。目前已经成为了linux的主流音频体系结构，想了解更多的关于ALSA的知识，详见：http://www.alsa-project.org
在内核设备驱动层，ALSA提供了alsa-driver。同时在应用层，ALSA为我们提供了alsa-lib，应用程序只要调用alsa-lib提供的API，即可以完成对底层音频硬件的控制。ALSA也包括一系列实用工具如aplay（播放音频文件）、arecord（录制音频文件）、amixer（调节混音器设置）。

一.Ubuntu

在Ubuntu中，首先要确保已经安装了ALSA。可以用以下指令安装：
sudo apt-get install libasound2-dev
PS：centos 需换一个包：yum install alsa-lib-devel
安装后就可以引用libasound库了。实际上alsa-lib源码可编译出libasound.a
下面是实现代码
TestChat.cpp

#include <stdio.h>
#include <stdlib.h>
#include <alsa/asoundlib.h>snd_pcm_t *open_sound_dev(snd_pcm_stream_t type)
{int err;snd_pcm_t *handle;if ((err = snd_pcm_open (&handle, "default", type, 0)) < 0) {printf("open error: %s\n", snd_strerror(err));return NULL;}if ((err = snd_pcm_set_params(handle,SND_PCM_FORMAT_S16_LE,SND_PCM_ACCESS_RW_INTERLEAVED,1,16000,1,500000)) < 0) {    /* 0.5sec */printf("set params error: %s\n", snd_strerror(err));return NULL;}return handle;
}void close_sound_dev(snd_pcm_t *handle)
{snd_pcm_close (handle);
}snd_pcm_t *open_playback(void)
{return open_sound_dev(SND_PCM_STREAM_PLAYBACK);
}snd_pcm_t *open_capture(void)
{return open_sound_dev(SND_PCM_STREAM_CAPTURE);
}int main (int argc, char *argv[])
{int err;char buf[128];snd_pcm_t *playback_handle;snd_pcm_t *capture_handle;playback_handle = open_playback();if (!playback_handle){fprintf (stderr, "cannot open for playback\n");return -1;}capture_handle = open_capture();if (!capture_handle){fprintf (stderr, "cannot open for capture\n");return -1;}if ((err = snd_pcm_prepare (playback_handle)) < 0) {fprintf (stderr, "cannot prepare audio interface for use (%s)\n",snd_strerror (err));return -1;}if ((err = snd_pcm_prepare (capture_handle)) < 0) {fprintf (stderr, "cannot prepare audio interface for use (%s)\n",snd_strerror (err));return -1;}while (1) {if ((err = snd_pcm_readi (capture_handle, buf, 128)) != 128) {fprintf (stderr, "read from audio interface failed (%s)\n",snd_strerror (err));break;}if ((err = snd_pcm_writei (playback_handle, buf, 128)) != 128) {fprintf (stderr, "write to audio interface failed (%s)\n",snd_strerror (err));break;}}snd_pcm_close (playback_handle);snd_pcm_close (capture_handle);return 0;
}

编译
g++ TestCat.cpp -o TestChat -lasound

执行
./TestChat
就可以正常测试了
如果是VM虚拟机中Ubuntu，需要虚拟机列表中右键这个虚拟机，在弹出的这个快捷菜单中选择：可移动设备-》声卡-》连接，否则测试无声音。

二.OpenWrt