今天要在linux下搞音频编程，一般使用ALSA音频编程。ALSA是Advanced Linux Sound Architecture的缩写，高级Linux声音架构的简称,它在Linux操作系统上提供了音频和MIDI（Musical Instrument Digital Interface，音乐设备数字化接口）的支持。
首先介绍一下一些关于ALSA编程的知识：
1、GNU/Linux 系统下三大主流声卡驱动程序集：
Linux 有三个主流的声卡驱动程序集：OSS/Lite（也称为OSS/Free）、OSS/Full
（商业软件）、ALSA（自由软件）。
OSS/Lite 是现在linux内核中自带的声卡驱动程序集，最初由 Hannu Savolainen
开发。后来 Hannu 跑去开发 Open Sound System（也就是上面所说的OSS/Full）。
由于 Hannu 的“逃跑”，RH 资助 Alan Cox 增强 Hannu 开发的驱动程序，并使它们
完全模块化。现在 Alan Cox 是内核声卡驱程集的维护人。OSS/Lite 从kernel-2.0开
始并入内核，现在大家使用的声卡驱程默认都是OSS/Lite中的。
OSS/Full 是由 4Front Technologies 开发并销售的商业软件。它可以驱动很多
声卡并且可以用在很多 UNIX 系统中。OSS/Full 完全兼容以前基于 OSS／Lite 开发
的应用程序。作为一个商业软件，你虽然可以使用它，但是你得不到它的源代码，并且
你必须为此而付钱。
ALSA 是linux内核的下一代标准声卡驱动集。开始时 Jaroslav Kysela 等人为
Gravis UltraSound Card 开发驱动程序，后来该计划改名为 ALSA「先进的linux音频
体系」，因为他们认为 ALSA 比原来内核中的 OSS/Lite 驱动程序集更优秀，完全可以
代替 OSS/Lite。他们是对的，ALSA 支持的声卡比 OSS/Lite 多，完全兼容以前基于
OSS 开发的程序，SMP（多处理器） 与 线程安全设计，并且从 2.5 分支的内核开始，
ALSA 的驱动程序集开始并入内核，大家可以在今年的 2.6 版本的内核中看到使用它们。
2、为什么要使用 ALSA 开发音频程序
首先，ALSA 是 linux 以后声卡驱动程序的标准，OSS/Lite 迟早会从内核中去除。
开发基于 ALSA 的音频程序可以保证以后的兼容。
其次，我们简单比较一下开发基于 OSS 与 ALSA 的方法。
OSS 向应用程序提供了一系列的系统接口。开发基于 OSS 的应用程序需要使用
open,close,ioctl,read,write 等低级系统调用来完成音频设备的控制、音频流的输入
输出。
而 ALSA 则为应用程序开发人员提供了一个优秀的音频库。利用该音频库，开发
人员可以方便快捷地开发出自己的应用程序，细节则留给音频库内部处理。当然 ALSA
也提供了类似于 OSS 的系统接口，不过 ALSA 的开发者建议应用程序开发者使用音频
库而不是驱程API。
3、那么我该从何开始呢
第一步当然是安装 ALSA 驱动程序与音频库。
当前 ALSA 有两个分支，一个是以前的0.5版本，一个是现在的0.9。ALSA的开发者
已经不支持0.5版本了，所以我们要使用0.9。大家可以在 ALSA 的主页
www.alsa-project.org 上下载安装。这个页面上的信息对大家安装很有用：
www.alsa-project.org/alsa-doc ，建议先浏览一下。
第二步是参考文档与例子。
在 ALSA 的文档页面上有两篇为应用程序开发者提供的文章：
ALSA 0.9.0 HOWTO [ http://www.suse.de/~mana/alsa090_howto.html ]
Howto use the ALSA API [ http://www.op.net/~pbd/alsa-audio.html ]
当然，还有音频库API的在线参考：
http://www.alsa-project.org/alsa-doc/alsa-lib
在开发过程中大家肯定会遇到问题或者困难，这时请教跟讨论就少不了了。所以要
做的第三步就是订阅 ALSA 的邮件列表了： [email protected] 。或者
你 E 文不好的话，可以在这里跟别人讨论一下。不过不要期望过高，国内开发 ALSA
音频程序的人本来就少，能来这里的人就更少了。
要是你已经完成以上几步的话，那么你就应该开始开发了。
- ALSA设备文件结构
我们从alsa在linux中的设备文件结构开始我们的alsa之旅. 看看我的电脑中的alsa驱动的设备文件结构:
crw-rw—-+ 1 root audio 116, 8 2011-02-23 21:38 controlC0
crw-rw—-+ 1 root audio 116, 4 2011-02-23 21:38 midiC0D0
crw-rw—-+ 1 root audio 116, 7 2011-02-23 21:39 pcmC0D0c
crw-rw—-+ 1 root audio 116, 6 2011-02-23 21:56 pcmC0D0p
crw-rw—-+ 1 root audio 116, 5 2011-02-23 21:38 pcmC0D1p
crw-rw—-+ 1 root audio 116, 3 2011-02-23 21:38 seq
crw-rw—-+ 1 root audio 116, 2 2011-02-23 21:38 timer
$
我们可以看到以下设备文件:
controlC0 –> 用于声卡的控制，例如通道选择，混音，麦克风的控制等
midiC0D0 –> 用于播放midi音频
pcmC0D0c –〉 用于录音的pcm设备
pcmC0D0p –〉 用于播放的pcm设备
seq –〉 音序器
timer –〉 定时器
其中，C0D0代表的是声卡0中的设备0，pcmC0D0c最后一个c代表capture，pcmC0D0p最后一个p代表playback，这些都是alsa-driver中的命名规则。从上面的列表可以看出，我的声卡下挂了6个设备，根据声卡的实际能力，驱动实际上可以挂上更多种类的设备，在include/sound/core.h中，定义了以下设备类型：
#define SNDRV_DEV_TOPLEVEL ((__force snd_device_type_t) 0)
#define SNDRV_DEV_CONTROL ((__force snd_device_type_t) 1)
#define SNDRV_DEV_LOWLEVEL_PRE ((__force snd_device_type_t) 2)
#define SNDRV_DEV_LOWLEVEL_NORMAL ((__force snd_device_type_t) 0x1000)
#define SNDRV_DEV_PCM ((__force snd_device_type_t) 0x1001)
#define SNDRV_DEV_RAWMIDI ((__force snd_device_type_t) 0x1002)
#define SNDRV_DEV_TIMER ((__force snd_device_type_t) 0x1003)
#define SNDRV_DEV_SEQUENCER ((__force snd_device_type_t) 0x1004)
#define SNDRV_DEV_HWDEP ((__force snd_device_type_t) 0x1005)
#define SNDRV_DEV_INFO ((__force snd_device_type_t) 0x1006)
#define SNDRV_DEV_BUS ((__force snd_device_type_t) 0x1007)
#define SNDRV_DEV_CODEC ((__force snd_device_type_t) 0x1008)
#define SNDRV_DEV_JACK ((__force snd_device_type_t) 0x1009)
#define SNDRV_DEV_LOWLEVEL ((__force snd_device_type_t) 0x2000)
通常，我们更关心的是pcm和control这两种设备。
5.一些例子，这些例子在官方文档也有，请自行查阅：
1.）显示一些PCM的类型和格式:
#include <iostream>
#include <alsa/asoundlib.h>
int main()
{
std::cout << "ALSA library version: " << SND_LIB_VERSION_STR << std::endl;
std::cout << "PCM stream types: " << std::endl;
for (int val=0; val <= SND_PCM_STREAM_LAST; ++val)
std::cout << snd_pcm_stream_name((snd_pcm_stream_t)val) << std::endl;
std::cout << std::endl;
std::cout << "PCM access types: " << std::endl;
for (int val=0; val <= SND_PCM_ACCESS_LAST; ++val)
std::cout << snd_pcm_access_name((snd_pcm_access_t)val) << std::endl;
std::cout << std::endl;
std::cout << "PCM subformats: " << std::endl;
for (int val=0; val <= SND_PCM_SUBFORMAT_LAST; ++val)
std::cout << snd_pcm_subformat_name((snd_pcm_subformat_t)val) << " (" << snd_pcm_subformat_description((snd_pcm_subformat_t)val) << ")" << std::endl;
std::cout << std::endl;
std::cout << "PCM states: " << std::endl;
for (int val=0; val <= SND_PCM_STATE_LAST; ++val)
std::cout << snd_pcm_state_name((snd_pcm_state_t)val) << std::endl;
std::cout << std::endl;
std::cout << "PCM formats: " << std::endl;
for (int val=0; val <= SND_PCM_FORMAT_LAST; ++val)
std::cout << snd_pcm_format_name((snd_pcm_format_t)val) << " (" << snd_pcm_format_description((snd_pcm_format_t)val) << ")" << std::endl;
std::cout << std::endl;
}
2.）打开PCM设备和设置参数
#include <iostream>
#include <alsa/asoundlib.h>
int main()
{
int rc;
snd_pcm_t* handle;
snd_pcm_hw_params_t* params;
unsigned int val, val2;
int dir;
snd_pcm_uframes_t frames;
if ( (rc = snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0)) < 0)
{
std::cerr << "unable to open pcm devices: " << snd_strerror(rc) << std::endl;
exit(1);
}
snd_pcm_hw_params_alloca(¶ms);
snd_pcm_hw_params_any(handle, params);
snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
snd_pcm_hw_params_set_channels(handle, params, 2);
val = 44100;
snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir);
if ( (rc = snd_pcm_hw_params(handle, params)) < 0)
{
std::cerr << "unable to set hw parameters: " << snd_strerror(rc) << std::endl;
exit(1);
}
std::cout << "PCM handle name = " << snd_pcm_name(handle) << std::endl;
std::cout << "PCM state = " << snd_pcm_state_name(snd_pcm_state(handle)) << std::endl;
snd_pcm_hw_params_get_access(params, (snd_pcm_access_t *)&val);
std::cout << "access type = " << snd_pcm_access_name((snd_pcm_access_t)val) << std::endl;
snd_pcm_hw_params_get_format(params, (snd_pcm_format_t*)(&val));
std::cout << "format = '" << snd_pcm_format_name((snd_pcm_format_t)val) << "' (" << snd_pcm_format_description((snd_pcm_format_t)val) << ")" << std::endl;
snd_pcm_hw_params_get_subformat(params, (snd_pcm_subformat_t *)&val);
std::cout << "subformat = '" <<
snd_pcm_subformat_name((snd_pcm_subformat_t)val) << "' (" << snd_pcm_subformat_description((snd_pcm_subformat_t)val) << ")" << std::endl;
snd_pcm_hw_params_get_channels(params, &val);
std::cout << "channels = " << val << std::endl;
snd_pcm_hw_params_get_rate(params, &val, &dir);
std::cout << "rate = " << val << " bps" << std::endl;
snd_pcm_hw_params_get_period_time(params, &val, &dir);
std::cout << "period time = " << val << " us" << std::endl;
snd_pcm_hw_params_get_period_size(params, &frames, &dir);
std::cout << "period size = " << static_cast<int>(frames) << " frames" << std::endl;
snd_pcm_hw_params_get_buffer_time(params, &val, &dir);
std::cout << "buffer time = " << val << " us" << std::endl;
snd_pcm_hw_params_get_buffer_size(params, (snd_pcm_uframes_t *) &val);
std::cout << "buffer size = " << val << " frames" << std::endl;
snd_pcm_hw_params_get_periods(params, &val, &dir);
std::cout << "periods per buffer = " << val << " frames" << std::endl;
snd_pcm_hw_params_get_rate_numden(params, &val, &val2);
std::cout << "exact rate = " << val/val2 << " bps" << std::endl;
val = snd_pcm_hw_params_get_sbits(params);
std::cout << "significant bits = " << val << std::endl;
snd_pcm_hw_params_get_tick_time(params, &val, &dir);
std::cout << "tick time = " << val << " us" << std::endl;
val = snd_pcm_hw_params_is_batch(params);
std::cout << "is batch = " << val << std::endl;
val = snd_pcm_hw_params_is_block_transfer(params);
std::cout << "is block transfer = " << val << std::endl;
val = snd_pcm_hw_params_is_double(params);
std::cout << "is double = " << val << std::endl;
val = snd_pcm_hw_params_is_half_duplex(params);
std::cout << "is half duplex = " << val << std::endl;
val = snd_pcm_hw_params_is_joint_duplex(params);
std::cout << "is joint duplex = " << val << std::endl;
val = snd_pcm_hw_params_can_overrange(params);
std::cout << "can overrange = " << val << std::endl;
val = snd_pcm_hw_params_can_mmap_sample_resolution(params);
std::cout << "can mmap = " << val << std::endl;
val = snd_pcm_hw_params_can_pause(params);
std::cout << "can pause = " << val << std::endl;
val = snd_pcm_hw_params_can_resume(params);
std::cout << "can resume = " << val << std::endl;
val = snd_pcm_hw_params_can_sync_start(params);
std::cout << "can sync start = " << val << std::endl;
snd_pcm_close(handle);
return 0;
}
3.）一个简单的声音播放程序
#include <iostream>
#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;
if ( (rc = snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0)) < 0)
{
std::cerr << "unable to open pcm device: " << snd_strerror(rc) << std::endl;
exit(1);
}
snd_pcm_hw_params_alloca(¶ms);
snd_pcm_hw_params_any(handle, params);
snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
snd_pcm_hw_params_set_channels(handle, params, 2);
val = 44100;
snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir);
frames = 32;
snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir);
if ( (rc = snd_pcm_hw_params(handle, params)) < 0)
{
std::cerr << "unable to set hw paramseters: " << snd_strerror(rc) << std::endl;
exit(1);
}
snd_pcm_hw_params_get_period_size(params, &frames, &dir);
size = frames * 4;
buffer = new char[size];
snd_pcm_hw_params_get_period_time(params, &val, &dir);
loops = 5000000 / val;
while (loops > 0) {
loops--;
if ( (rc = read(0, buffer, size)) == 0)
{
std::cerr << "end of file on input" << std::endl;
break;
}
else if (rc != size)
std::cerr << "short read: read " << rc << " bytes" << std::endl;
if ( (rc = snd_pcm_writei(handle, buffer, frames)) == -EPIPE)
{
std::cerr << "underrun occurred" << std::endl;
snd_pcm_prepare(handle);
}
else if (rc < 0)
std::cerr << "error from writei: " << snd_strerror(rc) << std::endl;
else if (rc != (int)frames)
std::cerr << "short write, write " << rc << " frames" << std::endl;
}
snd_pcm_drain(handle);
snd_pcm_close(handle);
free(buffer);
return 0;
}
4.）一个简单的记录声音的程序
#include <iostream>
#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;
if ( (rc = snd_pcm_open(&handle, "default", SND_PCM_STREAM_CAPTURE, 0)) < 0)
{
std::cerr << "unable to open pcm device: " << snd_strerror(rc) << std::endl;
exit(1);
}
snd_pcm_hw_params_alloca(¶ms);
snd_pcm_hw_params_any(handle, params);
snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
snd_pcm_hw_params_set_channels(handle, params, 2);
val = 44100;
snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir);
if ( (rc = snd_pcm_hw_params(handle, params)) < 0)
{
std::cerr << "unable to set hw parameters: " << snd_strerror(rc) << std::endl;
exit(1);
}
snd_pcm_hw_params_get_period_size(params, &frames, &dir);
size = frames * 4;
buffer = new char[size];
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)
{
std::cerr << "overrun occurred" << std::endl;
snd_pcm_prepare(handle);
}
else if (rc < 0)
std::cerr << "error from read: " << snd_strerror(rc) << std::endl;
else if ( rc != (int)frames)
std::cerr << "short read, read " << rc << " frames" << std::endl;
rc = write(1, buffer, size);
if (rc != size)
std::cerr << "short write: wrote " << rc << " bytes" << std::endl;
}
snd_pcm_drain(handle);
snd_pcm_close(handle);
free(buffer);
return 0;
}
注意：编译的时候记得加上参数，g++ xxx.cpp -o xxx -lasound；如果编译时出现如下错误：alsa/asoundlib.h: No such file or directory
缺少一个库：
apt-get install libasound2-dev
OK!