linux alsa pcm 播放声音

/**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;
}