为什么 ALSA snd_pcm_hw_params_get_buffer_size 会对本地堆栈帧分配的大小做出反应？

Question

发行版：Debian 9.11

编译器：gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516

ALSA 版本：高级 Linux 声音架构驱动版本 k4.9.0-11-amd64.

请考虑以下可重现的最小示例（现已更新，具有更多错误处理）：

/**
 * Trying to set some parameters for ALSA.
 *
 * Code from: http://equalarea.com/paul/alsa-audio.html
 *
 */

#include <alsa/asoundlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <math.h>

#define PCM_DEVICE "default"

int main(int argc, char **argv)
{
    unsigned int err;
    snd_pcm_t *pcm_handle;
    snd_pcm_hw_params_t *params;

    if((err = snd_pcm_open(&pcm_handle, PCM_DEVICE, SND_PCM_STREAM_PLAYBACK, 0)) < 0) {
        printf("Error: Can't open \"%s\" PCM device. %s\n", PCM_DEVICE, snd_strerror(err));

        return 1;
    }

    snd_pcm_hw_params_alloca(&params);

    snd_pcm_hw_params_any(pcm_handle, params);

    if((err = snd_pcm_hw_params_set_access(pcm_handle, params, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0) {
        printf("Error: Can't set interleaved mode. %s\n", snd_strerror(err));

        return 1;
    }

    if((err = snd_pcm_hw_params_set_format(pcm_handle, params, SND_PCM_FORMAT_FLOAT)) < 0) {
        printf("Error: Can't set format. %s\n", snd_strerror(err));

        return 1;
    }

    int channels = 2;
    if((err = snd_pcm_hw_params_set_channels(pcm_handle, params, channels)) < 0) {
        printf("Error: Can't set channels number. %s\n", snd_strerror(err));

        return 1;
    }

    int rate = 44100;
    if((err = snd_pcm_hw_params_set_rate_near(pcm_handle, params, &rate, 0)) < 0) {
        printf("Error: Can't set rate. %s\n", snd_strerror(err));

        return 1;
    }

    printf("PCM name: '%s'\n", snd_pcm_name(pcm_handle));
    printf("PCM state: %s\n", snd_pcm_state_name(snd_pcm_state(pcm_handle)));

    int ch;
    if((err = snd_pcm_hw_params_get_channels(params, &ch)) < 0) {
        printf("Error: Unable to get the number of channels: %s\n", snd_strerror(err));

        return 1;
    }

    if(ch > 1) {
        printf("Channels: %i, stereo\n", ch);
    }
    else if(ch == 1) {
        printf("Channels: %i, mono\n", ch);
    }
    else {
        printf("Error: Unknown number of channels (%d).\n", ch);

        return 1;
    }

    if((err = snd_pcm_hw_params_get_rate(params, &ch, 0)) < 0) {
        printf("Error: Unable to get the rate: %s\n", snd_strerror(err));

        return 1;
    }

    printf("Rate: %d bps\n", ch);

    int dir;
    snd_pcm_uframes_t period_size = 1024;

    printf("Attempting to set the period size to %d\n", period_size);

    if((err = snd_pcm_hw_params_set_period_size_near(pcm_handle, params, &period_size, &dir)) < 0) {
        printf("Error: Unable to set the period size: %s\n", snd_strerror(err));

        return 1;
    }

    printf("Attempting to get the period size.\n");

    if((err = snd_pcm_hw_params_get_period_size(params, &period_size, 0)) < 0) {
        printf("Error: Unable to get the period size: %s\n", snd_strerror(err));

        return 1;
    }

    printf("Period size is now: %zd\n", period_size);

    int period_time = -1;

    printf("Attempting to get the period time.\n");

    if((err = snd_pcm_hw_params_get_period_time(params, &period_time, NULL)) < 0) {
        printf("Error: Unable to get the period time: %s\n", snd_strerror(err));

        return 1;
    }

    printf("Period time: %d\n", period_time);

    int buffer_size = 4096;

    printf("Attempting to set the buffer size to: %d\n", buffer_size);

    if((err = snd_pcm_hw_params_set_buffer_size(pcm_handle, params, buffer_size)) < 0) {
        printf("Error: Unable to set the buffer size: %s\n", snd_strerror(err));

        return 1;
    }

    printf("Finalizing hw params.\n");

    if((err = snd_pcm_hw_params(pcm_handle, params)) < 0) {
        printf("Error: Can't set harware parameters. %s\n", snd_strerror(err));

        return 1;
    }

    snd_pcm_uframes_t temp;

    printf("Attempting to get the buffer size.\n");

    if((err = snd_pcm_hw_params_get_buffer_size(params, &temp)) < 0) {
        printf("Error: Unable to get the buffer size: %s\n", snd_strerror(err));

        return 1;
    }

    printf("Buffer size is now: %d\n", temp);

    if((err = snd_pcm_drain(pcm_handle)) < 0) {
        printf("Error: Unable to drain the pcm handle: %s\n", snd_strerror(err));

        return 1;
    }

    if((err = snd_pcm_close(pcm_handle)) < 0) {
        printf("Error: Unable to close the pcm handle: %s\n", snd_strerror(err));

        return 1;
    }

    return 0;
}

当 char test[5] 存在时，我得到这个输出：

PCM name: 'default'
PCM state: OPEN
Channels: 2, stereo
Rate: 44100 bps
Attempting to set the period size to 1024
Attempting to get the period size.
Period size is now: 1024
Attempting to get the period time.
Period time: -1
Attempting to set the buffer size to: 4096
Finalizing hw params.
Attempting to get the buffer size.
Buffer size is now: 523774

注意 "Buffer size is now: 523774"。预期输出如下：

但是，如果我将 test[5] 更改为 test[4] 或更少（或完全删除该语句），重新编译，然后运行，我得到：

PCM name: 'default'
PCM state: OPEN
Channels: 2, stereo
Rate: 44100 bps
Attempting to set frames to 1024
Frames is now: 1024
Attempting to set the buffer size to: 4096
Buffer size is now: 4096

我一定是在滥用 ALSA API，在做一些我没有意识到的奇怪事情，或者 ALSA API 坏了。

为什么它会对这个 5 字节的微不足道的堆栈分配做出反应？

请注意，高于 5 的值也会产生此问题，例如字对齐数字 32。我认为与堆栈对齐或类似的事情无关。

编译器注释，如何复现：

Copy/paste 最小例子变成 broken_alsa.c

然后发出：

$ gcc broken_alsa.c -lasound
$ ./a.out

删除 char temp[5]（或将其设置为 4），它应该可以工作。

Answer 1

调用snd_pcm_hw_params_set_period_size_near()时，将dir初始化为零。

为什么 ALSA snd_pcm_hw_params_get_buffer_size 会对本地堆栈帧分配的大小做出反应？

Why does ALSA snd_pcm_hw_params_get_buffer_size react to the size of local stack frame allocations?

alsa