来自 ALSA 的 pcm_min.c 示例的警告/错误。可能的问题？

Question

当我用

编译 ALSA 的 pcm_min.c 示例时

gcc -Wall -lasound pcm_min.c -o pcm_min

一切都很好，但是运行它，我得到了预期的白噪声，但我也得到了这个警告/错误：

Short write (expected 16384, wrote 7616)

来自最后一个 if 语句。

#include <alsa/asoundlib.h>

static char *device = "default";                        /* playback device */

snd_output_t *output = NULL;
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(err));
                        break;
                }
                if (frames > 0 && frames < (long)sizeof(buffer))
                        printf("Short write (expected %li, wrote %li)\n", (long)sizeof(buffer), frames);
        }

        snd_pcm_close(handle);
        return 0;
}

有人能看出为什么会出现这个警告/错误吗？

拥抱， 路易丝

Answer 1

这个例子我也遇到了一些问题。我稍微修改了一下，现在可以了。

#include <stdio.h>
#include <stdlib.h>
#include <alsa/asoundlib.h>

static char *device = "default"; /* playback device */
snd_output_t *output = NULL;
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;
    snd_pcm_uframes_t bufferSize, periodSize;

    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_S16_LE,
                                  SND_PCM_ACCESS_RW_INTERLEAVED,
                                  1, //channels
                                  44100, //sample rate
                                  1, //allow resampling
                                  500000) //required latency in us
          ) < 0) {
        printf("Playback open error: %s\n", snd_strerror(err));
        exit(EXIT_FAILURE);
    }

    if ((err = snd_pcm_prepare(handle)) < 0) {
        printf("Pcm prepare error: %s\n", snd_strerror(err));
        exit(EXIT_FAILURE);
    }

    if ((err = snd_pcm_get_params( handle, &bufferSize, &periodSize )) < 0) {
        printf("Pcm get params error: %s\n", snd_strerror(err));
        exit(EXIT_FAILURE);
    }
    printf("Buffer size:%d, Period size:%d\n", (int)bufferSize, (int)periodSize);

    for (i = 0; i < 16; i++) {
        frames = snd_pcm_writei(handle, buffer, periodSize);
        if (frames < 0)
            frames = snd_pcm_recover(handle, frames, 0);
        if (frames < 0) {
            printf("snd_pcm_writei failed: %s\n", snd_strerror(err));
            break;
        }
        if (frames > 0 && frames < (long)periodSize)
            printf("Short write (expected %li, wrote %li)\n", (long)sizeof(buffer), frames);
    }
    snd_pcm_close(handle);
    return 0;
}

Answer 2

当接收到信号或欠载时，snd_pcm_writei() 函数的返回值可能小于 sizeof(buffer)。在您的情况下，您似乎正在混合字节和帧。调用的最后一个参数是缓冲区中的帧数。由于您传递的是缓冲区中的字节数，因此您会看到欠载。