PCM设备的hw_params结构定义如下：

struct snd_pcm_hw_params { unsigned int flags; unsigned int msbits; unsigned int lsbits; unsigned int mmap_offset; unsigned int reserved[6]; unsigned int rmask; unsigned int cmask; unsigned int info; unsigned int msb_hi; unsigned int msb_lo; unsigned int rate_num; unsigned int rate_den; unsigned int fifo_size; unsigned int reserved1[15]; unsigned int period_bytes; unsigned int period_frames; unsigned int period_size; unsigned int buffer_bytes; unsigned int buffer_frames; unsigned int buffer_size; unsigned int tick_time; unsigned int reserved2[5]; };

其中，period_size和buffer_size是分别对应每个周期和缓冲区的大小（以帧为单位）。这些值很大程度上是由硬件决定的，ALSA只是提供了一些限制，以确保在物理设备中可以实现所请求的参数。也就是说，如果请求过大的period_size或buffer_size，硬件将不会接受它们。

例如，以下代码片段演示如何使用ALSA API设置PCM设备的hw_params：

snd_pcm_t *handle; snd_pcm_hw_params_t *params; int dir; unsigned int val; int rc;

/* Open PCM device for recording (capture). */ 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. */ snd_pcm_hw_params_alloca(¶ms);

/* Fill it in with default values. */ 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 */ snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16