使用alsalib时的奇怪C行为

Question

当我尝试使用ALSA库时,我获得了C代码的奇怪行为.我使用此代码生成设备的sid

snd_mixer_selem_id_t*
getSid(){
  snd_mixer_selem_id_t *sid;
  snd_mixer_selem_id_alloca(&sid);
  snd_mixer_selem_id_set_index(sid,0);
  snd_mixer_selem_id_set_name(sid, selem_name);

  return sid;
}

然后我尝试通过访问此功能

snd_mixer_t *handle = getHandle();
snd_mixer_selem_id_t *sid = getSid();

snd_mixer_elem_t *elem = snd_mixer_find_selem(handle,sid);

getHandle()是一个获得卡处理程序的等效函数,但这个工作正常.奇怪的是,如果我在使用函数之前直接使用snd_mixer_find_selem函数代码就可以了.我不得不说我是C的新手,这是我的第一个项目之一,所以这可能是初学者对指针的错误.

错误消息是 simple.c:282: snd_mixer_selem_get_playback_volume_range: Assertion 'elem' failed.

那么有谁知道问题是什么？

这是我使用的代码的最小示例:

#include <alsa/asoundlib.h>

static const char *selem_name = "Master";
static const char *card = "default";

snd_mixer_t*
getHandle(){
  snd_mixer_t *handle;

  snd_mixer_open(&handle, 0);
  snd_mixer_attach(handle, card);
  snd_mixer_selem_register(handle, NULL, NULL);
  snd_mixer_load(handle);

  return handle;
}

snd_mixer_selem_id_t*
getSid(){
  snd_mixer_selem_id_t *sid;
  snd_mixer_selem_id_alloca(&sid);
  snd_mixer_selem_id_set_index(sid,0);
  snd_mixer_selem_id_set_name(sid, selem_name);

  return sid;
}

void
incMasterVol(long step){ // Step is a value in [-100,100]
  long min,max,curr;

  snd_mixer_t *handle = getHandle();

/*snd_mixer_open(&handle, 0);
  snd_mixer_attach(handle, card);
  snd_mixer_selem_register(handle, NULL, NULL);
  snd_mixer_load(handle);*/

  snd_mixer_selem_id_t *sid;
  snd_mixer_selem_id_alloca(&sid);
  snd_mixer_selem_id_set_index(sid,0);
  snd_mixer_selem_id_set_name(sid, selem_name);

  snd_mixer_elem_t *elem = snd_mixer_find_selem(handle,sid);

  snd_mixer_selem_get_playback_volume_range(elem, &min, &max);
  snd_mixer_selem_get_playback_volume(elem, SND_MIXER_SCHN_SIDE_LEFT, &curr);
  curr = curr * 100/max;
  if(curr + step > 100)
    curr = 100;
  else if(curr + step < 0)
    curr = 0;
  else
    curr += step;
  snd_mixer_selem_set_playback_volume_all(elem, curr * max / 100);

  snd_mixer_close(handle);
}

void
toggleMasterVol(){
 int swiVal;
 snd_mixer_t *handle = getHandle();
 snd_mixer_elem_t *elem = snd_mixer_find_selem(handle,getSid());

 snd_mixer_selem_get_playback_switch(elem, SND_MIXER_SCHN_SIDE_LEFT, &swiVal);
 snd_mixer_selem_set_playback_switch_all(elem, !swiVal);

 snd_mixer_close(handle);
}

int
main(int argc, char *argv[]){
  incMasterVol(5);
}

Answer 1

我认为这源于alloca()系统调用

alloca在堆栈框架中分配,所以当getSid()返回时,你的已分配变量被释放,你最终得到一个dandling指针...(当你把它放在main函数中时,它工作,因为alloca在主激活记录上分配空间)

这只是一个建议,但我认为你应该使用malloc(在堆上分配空间,所以动态内存被分配,直到你调用free()它)

尝试用snd_mixer_selem_id_malloc():)