ALSA（高级Linux声音架构）：一 简单例子

ALSA是Advanced Linux Sound Architecture，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

更多内容可以参考这篇文章：
http://mysuperbaby.iteye.com/blog/932729

下面是两个简单例子，分别实现播放、录音的功能

/******************************************************************ALSA 简单的 playback 例子******************************************************************/#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 playback. */    /* 打开 PCM playback 设备 */    rc = snd_pcm_open(&handle, "default",    SND_PCM_STREAM_PLAYBACK, 0);    if (rc < 0) {        fprintf(stderr,                "unable to open pcm device: %s\n",                snd_strerror(rc));        exit(1);    }    /* Allocate a hardware parameters object. */    /* 分配一个硬件参数结构体 */    snd_pcm_hw_params_alloca(&params);    /* Fill it in with default values. */    /* 使用默认参数 */    snd_pcm_hw_params_any(handle, params);    /* Set the desired hardware parameters. */    /* 设置硬件参数 */    /* Interleaved mode */    snd_pcm_hw_params_set_access(handle, params,                                SND_PCM_ACCESS_RW_INTERLEAVED);    /* Signed 16-bit little-endian format */    /* 数据格式为 16位 小端 */    snd_pcm_hw_params_set_format(handle, params,                                SND_PCM_FORMAT_S16_LE);    /* Two channels (stereo) */    /* 两个声道 */    snd_pcm_hw_params_set_channels(handle, params, 2);    /* 44100 bits/second sampling rate (CD quality) */    /* 采样率为 44100 */    val = 44100;    snd_pcm_hw_params_set_rate_near(handle, params,              &val, &dir);    /* Set period size to 32 frames. */    /* 设置一个周期为 32 帧 */    frames = 32;    snd_pcm_hw_params_set_period_size_near(handle,          params, &frames, &dir);    /* Write the parameters to the driver */    /* 把前面设置好的参数写入到playback设备 */    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 */    /* 得到一个周期的数据长度 */    snd_pcm_hw_params_get_period_size(params, &frames,                &dir);    /* 因为我们是16位 两个通道，所以要 *2*2 也就是 *4 */    size = frames * 4; /* 2 bytes/sample, 2 channels */    buffer = (char *) malloc(size);    /* We want to loop for 5 seconds */    /* 得到一个周期的时间长度 */    snd_pcm_hw_params_get_period_time(params,                &val, &dir);    /* 5 seconds in microseconds divided by    * period time */    loops = 5000000 / val;    while (loops > 0) {        loops--;        /* 从标准输入中获取数据 */        rc = read(0, buffer, size);        if (rc == 0) {            fprintf(stderr, "end of file on input\n");            break;        } else if (rc != size) {            fprintf(stderr,                "short read: read %d bytes\n", rc);        }        /* 播放这些数据 */        rc = snd_pcm_writei(handle, buffer, frames);        if (rc == -EPIPE) {            /* EPIPE means underrun */            fprintf(stderr, "underrun occurred\n");            snd_pcm_prepare(handle);        } else if (rc < 0) {            fprintf(stderr,                    "error from writei: %s\n",                    snd_strerror(rc));        }  else if (rc != (int)frames) {            fprintf(stderr,                    "short write, write %d frames\n", rc);        }    }    snd_pcm_drain(handle);    snd_pcm_close(handle);    free(buffer);    return 0;}

/*********************************************************************ALSA 简单的录音功能 capture 捕获*********************************************************************//* 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). */    /* 打开 PCM capture 捕获设备 */    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. */    /* 分配一个硬件参数结构体 */    snd_pcm_hw_params_alloca(&params);    /* Fill it in with default values. */    /* 使用默认参数 */    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 */    /* 16位 小端 */    snd_pcm_hw_params_set_format(handle, params,                                  SND_PCM_FORMAT_S16_LE);    /* Two channels (stereo) */    /* 双通道 */    snd_pcm_hw_params_set_channels(handle, params, 2);    /* 44100 bits/second sampling rate (CD quality) */    /* 采样率 */    val = 44100;    snd_pcm_hw_params_set_rate_near(handle, params,                                      &val, &dir);    /* Set period size to 32 frames. */    /* 一个周期有 32 帧 */    frames = 32;    snd_pcm_hw_params_set_period_size_near(handle,                                  params, &frames, &dir);    /* Write the parameters to the driver */    /* 参数生效 */    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 */    /* 得到一个周期的数据大小 */    snd_pcm_hw_params_get_period_size(params,                                          &frames, &dir);    /* 16位 双通道，所以要 *4 */    size = frames * 4; /* 2 bytes/sample, 2 channels */    buffer = (char *) malloc(size);    /* We want to loop for 5 seconds */    /* 等到一个周期的时间长度 */    snd_pcm_hw_params_get_period_time(params,                                             &val, &dir);    loops = 5000000 / val;    while (loops > 0) {        loops--;        /* 捕获数据 */        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);        }        /* 写入到标准输出中去 */        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;}

可以尝试运行capture程序并把输出定向到某个文件，然后运行playback播放该文件里的声音数据：

./capture > sound.raw./playback < sound.raw