linuxALSA应用编程笔记
ALSA是linux的主流音频软件架构，包括驱动和应用程序两个部分，根据官网和自己查找的资料总结了一些知识，更详细的资料参考alsa官网。
alsa c开发参考文档地址：ALSA project - the C library reference: Index, Preamble and License
ALSA 官方 Examples主要有：
- /test/pcm.c
- /test/pcm_min.c
- /test/latency.c
- /test/rawmidi.c
- /test/timer.c
在使用例子之前需要安装ALSA库，Ubuntu开发安装方法：
- 进入ALSA主页：AlsaProject Download alsa-lib库解压出来
- 编译alsa-lib库 生成makefile ./configure，编译文件 make
- 安装alsa库 sudo make install （注意使用sudo超级权限）
- 编译例子使用(gcc -o 文件名 文件名.c -lasound)命令
ALSA在linux中的架构图
示例1 pcm.c创建一个正弦波输入到声卡里面，提供了几种写的方式
struct transfer_method {
const char *name;
snd_pcm_access_t access;
int (*transfer_loop)(snd_pcm_t *handle,
signed short *samples,
snd_pcm_channel_area_t *areas);
};
static struct transfer_method transfer_methods[] = {
{ "write", SND_PCM_ACCESS_RW_INTERLEAVED, write_loop },
{ "write_and_poll", SND_PCM_ACCESS_RW_INTERLEAVED, write_and_poll_loop },
{ "async", SND_PCM_ACCESS_RW_INTERLEAVED, async_loop },
{ "async_direct", SND_PCM_ACCESS_MMAP_INTERLEAVED, async_direct_loop },
{ "direct_interleaved", SND_PCM_ACCESS_MMAP_INTERLEAVED, direct_loop },
{ "direct_noninterleaved", SND_PCM_ACCESS_MMAP_NONINTERLEAVED, direct_loop },
{ "direct_write", SND_PCM_ACCESS_MMAP_INTERLEAVED, direct_write_loop },
{ NULL, SND_PCM_ACCESS_RW_INTERLEAVED, NULL }
};这个文件提供了几个具体写音频数据格式的函数，播放音乐可以使用这些接口具体参考pcm.c文件
播放声音参考pcm_min.c文件：
/*
* This extra small demo sends a random samples to your speakers.
*/
#include "../include/asoundlib.h"
static char *device = "default"; /* playback device */
unsigned char buffer[16*1024]; /* some random data */
int main(void)
{
int err;
unsigned int i;
snd_pcm_t *handle;
snd_pcm_sframes_t frames;
for (i = 0; i < sizeof(buffer); i++)
buffer[i] = random() & 0xff;
if ((err = snd_pcm_open(&handle, device, SND_PCM_STREAM_PLAYBACK, 0)) < 0) {
printf("Playback open error: %s\n", snd_strerror(err));
exit(EXIT_FAILURE);
}
if ((err = snd_pcm_set_params(handle,
SND_PCM_FORMAT_U8,
SND_PCM_ACCESS_RW_INTERLEAVED,
1,
48000,
1,
500000)) < 0) { /* 0.5sec */
printf("Playback open error: %s\n", snd_strerror(err));
exit(EXIT_FAILURE);
}
for (i = 0; i < 16; i++) {
frames = snd_pcm_writei(handle, buffer, sizeof(buffer));
if (frames < 0)
frames = snd_pcm_recover(handle, frames, 0);
if (frames < 0) {
printf("snd_pcm_writei failed: %s\n", snd_strerror(frames));
break;
}
if (frames > 0 && frames < (long)sizeof(buffer))
printf("Short write (expected %li, wrote %li)\n", (long)sizeof(buffer), frames);
}
/* pass the remaining samples, otherwise they're dropped in close */
err = snd_pcm_drain(handle);
if (err < 0)
printf("snd_pcm_drain failed: %s\n", snd_strerror(err));
snd_pcm_close(handle);
return 0;
}编译此文件: gcc -o pcm_min pcm_min.c -lasound -lm
执行文件：./pcm_min
这个文件产生一个随机噪声，写到声卡buffer里面播放声音。
录制声音参考 latency.c
/*
* Latency test program
*
* Author: Jaroslav Kysela
*
* Author of bandpass filter sweep effect:
* Maarten de Boer
*
* This small demo program can be used for measuring latency between
* capture and playback. This latency is measured from driver (diff when
* playback and capture was started). Scheduler is set to SCHED_RR.
*
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*
*/
#include
#include
#include
#include
#include
#include
#include "../include/asoundlib.h"
#include
#include
char *pdevice = "hw:0,0";
char *cdevice = "hw:0,0";
snd_pcm_format_t format = SND_PCM_FORMAT_S16_LE;
int rate = 22050;
int channels = 2;
int buffer_size = 0; /* auto */
int period_size = 0; /* auto */
int latency_min = 32; /* in frames / 2 */
int latency_max = 2048; /* in frames / 2 */
int loop_sec = 30; /* seconds */
int block = 0; /* block mode */
int use_poll = 0;
int resample = 1;
unsigned long loop_limit;
snd_output_t *output = NULL;
int setparams_stream(snd_pcm_t *handle,
snd_pcm_hw_params_t *params,
const char *id)
{
int err;
unsigned int rrate;
err = snd_pcm_hw_params_any(handle, params);
if (err < 0) {
printf("Broken configuration for %s PCM: no configurations available: %s\n", snd_strerror(err), id);
return err;
}
err = snd_pcm_hw_params_set_rate_resample(handle, params, resample);
if (err < 0) {
printf("Resample setup failed for %s (val %i): %s\n", id, resample, snd_strerror(err));
return err;
}
err = snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
if (err < 0) {
printf("Access type not available for %s: %s\n", id, snd_strerror(err));
return err;
}
err = snd_pcm_hw_params_set_format(handle, params, format);
if (err < 0) {
printf("Sample format not available for %s: %s\n", id, snd_strerror(err));
return err;
}
err = snd_pcm_hw_params_set_channels(handle, params, channels);
if (err < 0) {
printf("Channels count (%i) not available for %s: %s\n", channels, id, snd_strerror(err));
return err;
}
rrate = rate;
err = snd_pcm_hw_params_set_rate_near(handle, params, &rrate, 0);
if (err < 0) {
printf("Rate %iHz not available for %s: %s\n", rate, id, snd_strerror(err));
return err;
}
if ((int)rrate != rate) {
printf("Rate doesn't match (requested %iHz, get %iHz)\n", rate, err);
return -EINVAL;
}
return 0;
}
int setparams_bufsize(snd_pcm_t *handle,
snd_pcm_hw_params_t *params,
snd_pcm_hw_params_t *tparams,
snd_pcm_uframes_t bufsize,
const char *id)
{
int err;
snd_pcm_uframes_t periodsize;
snd_pcm_hw_params_copy(params, tparams);
periodsize = bufsize * 2;
err = snd_pcm_hw_params_set_buffer_size_near(handle, params, &periodsize);
if (err < 0) {
printf("Unable to set buffer size %li for %s: %s\n", bufsize * 2, id, snd_strerror(err));
return err;
}
if (period_size > 0)
periodsize = period_size;
else
periodsize /= 2;
err = snd_pcm_hw_params_set_period_size_near(handle, params, &periodsize, 0);
if (err < 0) {
printf("Unable to set period size %li for %s: %s\n", periodsize, id, snd_strerror(err));
return err;
}
return 0;
}
int setparams_set(snd_pcm_t *handle,
snd_pcm_hw_params_t *params,
snd_pcm_sw_params_t *swparams,
const char *id)
{
int err;
snd_pcm_uframes_t val;
err = snd_pcm_hw_params(handle, params);
if (err < 0) {
printf("Unable to set hw params for %s: %s\n", id, snd_strerror(err));
return err;
}
err = snd_pcm_sw_params_current(handle, swparams);
if (err < 0) {
printf("Unable to determine current swparams for %s: %s\n", id, snd_strerror(err));
return err;
}
err = snd_pcm_sw_params_set_start_threshold(handle, swparams, 0x7fffffff);
if (err < 0) {
printf("Unable to set start threshold mode for %s: %s\n", id, snd_strerror(err));
return err;
}
if (!block)
val = 4;
else
snd_pcm_hw_params_get_period_size(params, &val, NULL);
err = snd_pcm_sw_params_set_avail_min(handle, swparams, val);
if (err < 0) {
printf("Unable to set avail min for %s: %s\n", id, snd_strerror(err));
return err;
}
err = snd_pcm_sw_params(handle, swparams);
if (err < 0) {
printf("Unable to set sw params for %s: %s\n", id, snd_strerror(err));
return err;
}
return 0;
}
//设定软件硬件参数
int setparams(snd_pcm_t *phandle, snd_pcm_t *chandle, int *bufsize)
{
int err, last_bufsize = *bufsize;
snd_pcm_hw_params_t *pt_params, *ct_params; /* templates with rate, format and channels */
snd_pcm_hw_params_t *p_params, *c_params;
snd_pcm_sw_params_t *p_swparams, *c_swparams;
snd_pcm_uframes_t p_size, c_size, p_psize, c_psize;
unsigned int p_time, c_time;
unsigned int val;
snd_pcm_hw_params_alloca(&p_params);
snd_pcm_hw_params_alloca(&c_params);
snd_pcm_hw_params_alloca(&pt_params);
snd_pcm_hw_params_alloca(&ct_params);
snd_pcm_sw_params_alloca(&p_swparams);
snd_pcm_sw_params_alloca(&c_swparams);
if ((err = setparams_stream(phandle, pt_params, "playback")) < 0) {
printf("Unable to set parameters for playback stream: %s\n", snd_strerror(err));
exit(0);
}
if ((err = setparams_stream(chandle, ct_params, "capture")) < 0) {
printf("Unable to set parameters for playback stream: %s\n", snd_strerror(err));
exit(0);
}
if (buffer_size > 0) {
*bufsize = buffer_size;
goto __set_it;
}
__again:
if (buffer_size > 0)
return -1;
if (last_bufsize == *bufsize)
*bufsize += 4;
last_bufsize = *bufsize;
if (*bufsize > latency_max)
return -1;
__set_it:
if ((err = setparams_bufsize(phandle, p_params, pt_params, *bufsize, "playback")) < 0) {
printf("Unable to set sw parameters for playback stream: %s\n", snd_strerror(err));
exit(0);
}
if ((err = setparams_bufsize(chandle, c_params, ct_params, *bufsize, "capture")) < 0) {
printf("Unable to set sw parameters for playback stream: %s\n", snd_strerror(err));
exit(0);
}
snd_pcm_hw_params_get_period_size(p_params, &p_psize, NULL);
if (p_psize > (unsigned int)*bufsize)
*bufsize = p_psize;
snd_pcm_hw_params_get_period_size(c_params, &c_psize, NULL);
if (c_psize > (unsigned int)*bufsize)
*bufsize = c_psize;
snd_pcm_hw_params_get_period_time(p_params, &p_time, NULL);
snd_pcm_hw_params_get_period_time(c_params, &c_time, NULL);
if (p_time != c_time)
goto __again;
snd_pcm_hw_params_get_buffer_size(p_params, &p_size);
if (p_psize * 2 < p_size) {
snd_pcm_hw_params_get_periods_min(p_params, &val, NULL);
if (val > 2) {
printf("playback device does not support 2 periods per buffer\n");
exit(0);
}
goto __again;
}
snd_pcm_hw_params_get_buffer_size(c_params, &c_size);
if (c_psize * 2 < c_size) {
snd_pcm_hw_params_get_periods_min(c_params, &val, NULL);
if (val > 2 ) {
printf("capture device does not support 2 periods per buffer\n");
exit(0);
}
goto __again;
}
if ((err = setparams_set(phandle, p_params, p_swparams, "playback")) < 0) {
printf("Unable to set sw parameters for playback stream: %s\n", snd_strerror(err));
exit(0);
}
if ((err = setparams_set(chandle, c_params, c_swparams, "capture")) < 0) {
printf("Unable to set sw parameters for playback stream: %s\n", snd_strerror(err));
exit(0);
}
if ((err = snd_pcm_prepare(phandle)) < 0) {
printf("Prepare error: %s\n", snd_strerror(err));
exit(0);
}
snd_pcm_dump(phandle, output);
snd_pcm_dump(chandle, output);
fflush(stdout);
return 0;
}
void showstat(snd_pcm_t *handle, size_t frames)
{
int err;
snd_pcm_status_t *status;
snd_pcm_status_alloca(&status);
if ((err = snd_pcm_status(handle, status)) < 0) {
printf("Stream status error: %s\n", snd_strerror(err));
exit(0);
}
printf("*** frames = %li ***\n", (long)frames);
snd_pcm_status_dump(status, output);
}
void showlatency(size_t latency)
{
double d;
latency *= 2;
d = (double)latency / (double)rate;
printf("Trying latency %li frames, %.3fus, %.6fms (%.4fHz)\n", (long)latency, d * 1000000, d * 1000, (double)1 / d);
}
void showinmax(size_t in_max)
{
double d;
printf("Maximum read: %li frames\n", (long)in_max);
d = (double)in_max / (double)rate;
printf("Maximum read latency: %.3fus, %.6fms (%.4fHz)\n", d * 1000000, d * 1000, (double)1 / d);
}
void gettimestamp(snd_pcm_t *handle, snd_timestamp_t *timestamp)
{
int err;
snd_pcm_status_t *status;
snd_pcm_status_alloca(&status);
if ((err = snd_pcm_status(handle, status)) < 0) {
printf("Stream status error: %s\n", snd_strerror(err));
exit(0);
}
snd_pcm_status_get_trigger_tstamp(status, timestamp);
}
void setscheduler(void)
{
struct sched_param sched_param;
if (sched_getparam(0, &sched_param) < 0) {
printf("Scheduler getparam failed...\n");
return;
}
sched_param.sched_priority = sched_get_priority_max(SCHED_RR);
if (!sched_setscheduler(0, SCHED_RR, &sched_param)) {
printf("Scheduler set to Round Robin with priority %i...\n", sched_param.sched_priority);
fflush(stdout);
return;
}
printf("!!!Scheduler set to Round Robin with priority %i FAILED!!!\n", sched_param.sched_priority);
}
long timediff(snd_timestamp_t t1, snd_timestamp_t t2)
{
signed long l;
t1.tv_sec -= t2.tv_sec;
l = (signed long) t1.tv_usec - (signed long) t2.tv_usec;
if (l < 0) {
t1.tv_sec--;
l = 1000000 + l;
l %= 1000000;
}
return (t1.tv_sec * 1000000) + l;
}
long readbuf(snd_pcm_t *handle, char *buf, long len, size_t *frames, size_t *max)
{
long r;
if (!block) {
do {
r = snd_pcm_readi(handle, buf, len);
} while (r == -EAGAIN);
if (r > 0) {
*frames += r;
if ((long)*max < r)
*max = r;
}
// printf("read = %li\n", r);
} else {
int frame_bytes = (snd_pcm_format_width(format) / 8) * channels;
do {
r = snd_pcm_readi(handle, buf, len);
if (r > 0) {
buf += r * frame_bytes;
len -= r;
*frames += r;
if ((long)*max < r)
*max = r;
}
// printf("r = %li, len = %li\n", r, len);
} while (r >= 1 && len > 0);
}
// showstat(handle, 0);
return r;
}
long writebuf(snd_pcm_t *handle, char *buf, long len, size_t *frames)
{
long r;
int frame_bytes = (snd_pcm_format_width(format) / 8) * channels;
while (len > 0) {
r = snd_pcm_writei(handle, buf, len);
if (r == -EAGAIN)
continue;
// printf("write = %li\n", r);
if (r < 0)
return r;
// showstat(handle, 0);
buf += r * frame_bytes;
len -= r;
*frames += r;
}
return 0;
}
#define FILTERSWEEP_LFO_CENTER 2000.
#define FILTERSWEEP_LFO_DEPTH 1800.
#define FILTERSWEEP_LFO_FREQ 0.2
#define FILTER_BANDWIDTH 50
/* filter the sweep variables */
float lfo,dlfo,fs,fc,BW,C,D,a0,a1,a2,b1,b2,*x[3],*y[3];
void applyeffect(char* buffer,int r)
{
short* samples = (short*) buffer;
int i;
for (i=0;i2.*M_PI) lfo -= 2.*M_PI;
C = 1./tan(M_PI*BW/fs);
D = 2.*cos(2*M_PI*fc/fs);
a0 = 1./(1.+C);
a1 = 0;
a2 = -a0;
b1 = -C*D*a0;
b2 = (C-1)*a0;
for (chn=0;chn= 4 ? err : 4;
if (latency_max < latency_min)
latency_max = latency_min;
break;
case 'M':
err = atoi(optarg) / 2;
latency_max = latency_min > err ? latency_min : err;
break;
case 'f':
format = snd_pcm_format_value(optarg);
if (format == SND_PCM_FORMAT_UNKNOWN) {
printf("Unknown format, setting to default S16_LE\n");
format = SND_PCM_FORMAT_S16_LE;
}
break;
case 'c':
err = atoi(optarg);
channels = err >= 1 && err < 1024 ? err : 1;
break;
case 'r':
err = atoi(optarg);
rate = err >= 4000 && err < 200000 ? err : 44100;
break;
case 'B':
err = atoi(optarg);
buffer_size = err >= 32 && err < 200000 ? err : 0;
break;
case 'E':
err = atoi(optarg);
period_size = err >= 32 && err < 200000 ? err : 0;
break;
case 's':
err = atoi(optarg);
loop_sec = err >= 1 && err <= 100000 ? err : 30;
break;
case 'b':
block = 1;
break;
case 'p':
use_poll = 1;
break;
case 'e':
effect = 1;
break;
case 'n':
resample = 0;
break;
}
}
if (morehelp) {
help();
return 0;
}
//把输出重定向到stdout
err = snd_output_stdio_attach(&output, stdout, 0);
if (err < 0) {
printf("Output failed: %s\n", snd_strerror(err));
return 0;
}
loop_limit = loop_sec * rate;
latency = latency_min - 4;
buffer = malloc((latency_max * snd_pcm_format_width(format) / 8) * 2);
//设定进程调度相关
setscheduler();
printf("Playback device is %s\n", pdevice);
printf("Capture device is %s\n", cdevice);
printf("Parameters are %iHz, %s, %i channels, %s mode\n", rate, snd_pcm_format_name(format), channels, block ? "blocking" : "non-blocking");
printf("Poll mode: %s\n", use_poll ? "yes" : "no");
printf("Loop limit is %lu frames, minimum latency = %i, maximum latency = %i\n", loop_limit, latency_min * 2, latency_max * 2);
//打开播放设备
if ((err = snd_pcm_open(&phandle, pdevice, SND_PCM_STREAM_PLAYBACK, block ? 0 : SND_PCM_NONBLOCK)) < 0) {
printf("Playback open error: %s\n", snd_strerror(err));
return 0;
}
//打开录音设备
if ((err = snd_pcm_open(&chandle, cdevice, SND_PCM_STREAM_CAPTURE, block ? 0 : SND_PCM_NONBLOCK)) < 0) {
printf("Record open error: %s\n", snd_strerror(err));
return 0;
}
/* initialize the filter sweep variables */
if (effect) {
fs = (float) rate;
BW = FILTER_BANDWIDTH;
lfo = 0;
dlfo = 2.*M_PI*FILTERSWEEP_LFO_FREQ/fs;
x[0] = (float*) malloc(channels*sizeof(float));
x[1] = (float*) malloc(channels*sizeof(float));
x[2] = (float*) malloc(channels*sizeof(float));
y[0] = (float*) malloc(channels*sizeof(float));
y[1] = (float*) malloc(channels*sizeof(float));
y[2] = (float*) malloc(channels*sizeof(float));
}
while (1) {
frames_in = frames_out = 0;
if (setparams(phandle, chandle, &latency) < 0)
break;
showlatency(latency);
if ((err = snd_pcm_link(chandle, phandle)) < 0) {
printf("Streams link error: %s\n", snd_strerror(err));
exit(0);
}
if (snd_pcm_format_set_silence(format, buffer, latency*channels) < 0) {
fprintf(stderr, "silence error\n");
break;
}
if (writebuf(phandle, buffer, latency, &frames_out) < 0) {
fprintf(stderr, "write error\n");
break;
}
if (writebuf(phandle, buffer, latency, &frames_out) < 0) {
fprintf(stderr, "write error\n");
break;
}
if ((err = snd_pcm_start(chandle)) < 0) {
printf("Go error: %s\n", snd_strerror(err));
exit(0);
}
gettimestamp(phandle, &p_tstamp);
gettimestamp(chandle, &c_tstamp);
#if 0
printf("Playback:\n");
showstat(phandle, frames_out);
printf("Capture:\n");
showstat(chandle, frames_in);
#endif
ok = 1;
in_max = 0;
while (ok && frames_in < loop_limit) {
if (use_poll) {
/* use poll to wait for next event */
snd_pcm_wait(chandle, 1000);
}
if ((r = readbuf(chandle, buffer, latency, &frames_in, &in_max)) < 0)
ok = 0;
else {
if (effect)
applyeffect(buffer,r);
if (writebuf(phandle, buffer, r, &frames_out) < 0)
ok = 0;
}
}
if (ok)
printf("Success\n");
else
printf("Failure\n");
printf("Playback:\n");
showstat(phandle, frames_out);
printf("Capture:\n");
showstat(chandle, frames_in);
showinmax(in_max);
if (p_tstamp.tv_sec == c_tstamp.tv_sec &&
p_tstamp.tv_usec == c_tstamp.tv_usec)
printf("Hardware sync\n");
snd_pcm_drop(chandle);
snd_pcm_nonblock(phandle, 0);
snd_pcm_drain(phandle);
snd_pcm_nonblock(phandle, !block ? 1 : 0);
if (ok) {
#if 1
printf("Playback time = %li.%i, Record time = %li.%i, diff = %li\n",
p_tstamp.tv_sec,
(int)p_tstamp.tv_usec,
c_tstamp.tv_sec,
(int)c_tstamp.tv_usec,
timediff(p_tstamp, c_tstamp));
#endif
break;
}
snd_pcm_unlink(chandle);
snd_pcm_hw_free(phandle);
snd_pcm_hw_free(chandle);
}
snd_pcm_close(phandle);
snd_pcm_close(chandle);
return 0;
}
录音设备这里写得比较复杂可以参考我上传的书籍资料里面的课件。
参考资料：
《Linux高级程序设计第三版(中文)》 作者：杨宗德
《Linux环境编程图文指南》 作者：林世霖
ALSA官网:ALSA project - the C library reference: Index, Preamble and License
百问网：深圳百问网科技有限公司
alsa-lib以及pdf书籍资料链接：CSDN