ALSA是Advanced Linux Sound Architecture，高階Linux聲音架構的簡稱,它在Linux作業系統上提供了音訊和MIDI（Musical Instrument Digital Interface，音樂裝置數字化介面）的支援

下面是兩個簡單例子，分別實現播放、錄音的功能

/******************************************************************
ALSA 簡單的 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. */
 

    /* 開啟 PCM playback 裝置 */
    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. */
 

    /* 分配一個硬體引數結構體 */
    snd_pcm_hw_params_alloca(&params);

    /* Fill it in with default values. */
    /* 使用預設引數 */
    snd_pcm_hw_params_any(handle, params);

    /* Set the desired hardware parameters. */
    /* 設定硬體引數 */

    /* Interleaved mode */
    snd_pcm_hw_params_set_access(handle, params,
                                SND_PCM_ACCESS_RW_INTERLEAVED);

    /* Signed 16-bit little-endian format */
    /* 資料格式為 16位 小端 */
    snd_pcm_hw_params_set_format(handle, params,
                                SND_PCM_FORMAT_S16_LE);

    /* Two channels (stereo) */
    /* 兩個聲道 */
    snd_pcm_hw_params_set_channels(handle, params, 2);

    /* 44100 bits/second sampling rate (CD quality) */
    /* 取樣率為 44100 */
    val = 44100;
    snd_pcm_hw_params_set_rate_near(handle, params,
              &val, &dir);

    /* Set period size to 32 frames. */
    /* 設定一個週期為 32 幀 */
    frames = 32;
    snd_pcm_hw_params_set_period_size_near(handle,
          params, &frames, &dir);

    /* Write the parameters to the driver */
    /* 把前面設定好的引數寫入到playback裝置 */
    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 */
    /* 得到一個週期的資料長度 */
    snd_pcm_hw_params_get_period_size(params, &frames,
                &dir);

    /* 因為我們是16位 兩個通道，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 */
    /* 得到一個週期的時間長度 */
    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--;
        /* 從標準輸入中獲取資料 */
        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);
        }

        /* 播放這些資料 */
        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 簡單的錄音功能 capture 捕獲
*********************************************************************/

/* 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). */
    /* 開啟 PCM capture 捕獲裝置 */
    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. */
    /* 分配一個硬體引數結構體 */
    snd_pcm_hw_params_alloca(&params);

    /* Fill it in with default values. */
    /* 使用預設引數 */
    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) */
    /* 雙通道 */
    snd_pcm_hw_params_set_channels(handle, params, 2);

    /* 44100 bits/second sampling rate (CD quality) */
    /* 取樣率 */
    val = 44100;
    snd_pcm_hw_params_set_rate_near(handle, params,
                                      &val, &dir);

    /* Set period size to 32 frames. */
    /* 一個週期有 32 幀 */
    frames = 32;
    snd_pcm_hw_params_set_period_size_near(handle,
                                  params, &frames, &dir);

    /* Write the parameters to the driver */
    /* 引數生效 */
    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 */
    /* 得到一個週期的資料大小 */
    snd_pcm_hw_params_get_period_size(params,
                                          &frames, &dir);
    /* 16位 雙通道，所以要 *4 */
    size = frames * 4; /* 2 bytes/sample, 2 channels */
    buffer = (char *) malloc(size);

    /* We want to loop for 5 seconds */
    /* 等到一個週期的時間長度 */
    snd_pcm_hw_params_get_period_time(params,
                                             &val, &dir);
    loops = 5000000 / val;

    while (loops > 0) {
        loops--;
        /* 捕獲資料 */
        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);
        }

        /* 寫入到標準輸出中去 */
        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;
}

可以嘗試執行capture程式並把輸出定向到某個檔案，然後執行playback播放該檔案裡的聲音資料：

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