duff 设备不工作

Question

我已经尝试实现 Duff 设备，但它不起作用。它不复制任何东西。我已经实现了原始版本和更清晰的版本：

void duff_function(int *a, int *b, int length)
{
    int n = (length + 7) / 8;
    switch(length % 8)
    {
        case 0: *a++ = *b++;    // I dereference, copy, and then increment the pointer, * > ++
        case 7: *a++ = *b++;
        case 6: *a++ = *b++;
        case 5: *a++ = *b++;
        case 4: *a++ = *b++;
        case 3: *a++ = *b++;
        case 2: *a++ = *b++;
        case 1: *a++ = *b++;
    }
    while ( --n > 0)
    {
        *a++ = *b++;
        *a++ = *b++;
        *a++ = *b++;
        *a++ = *b++;
        *a++ = *b++;
        *a++ = *b++;
        *a++ = *b++;
        *a++ = *b++;
    }

}

void send(int *to, int *from, int count)
{
        int n=(count+7)/8;
        switch(count%8){
           case 0: do{ *to = *from++;
           case 7:     *to = *from++;
           case 6:     *to = *from++;
           case 5:     *to = *from++;
           case 4:     *to = *from++;
           case 3:     *to = *from++;
           case 2:     *to = *from++;
           case 1:     *to = *from++;
                    }while( --n>0);
        }  
}

int main()
{
    int a[10] = {21, 34, 12, 64, 13, 9, 56, 54, 90, 1};
    int b[10] = {22};
    for (int i = 0; i < 10; ++i)
    {
       printf("%d, ", a[i]);
    }
    printf("\n");
    duff_function(a, b, 10);
    //send(a, b, 10);

    for(int i = 0; i < 10; ++i)
        printf("%d, ", b[i]);
    printf("\n");
    return 0;
}

输出为：

21, 34, 12, 64, 13, 9, 56, 54, 90, 1, 
22, 0, 0, 0, 0, 0, 0, 0, 0, 0,

它不复制任何东西。在原始版本中，由于某种原因它没有增加指向的指针，但我认为我应该增加它（我在我的函数中这样做）。

编辑据报道，我的代码中存在错误：我已经更改了它：

send(b, a, 10);

但现在输出是：

1, 0, 0, 0, 0, 0, 0, 0, 0, 0,

编辑 2 使代码工作的最后编辑：

duff_function(b, a);

Answer 1

您正在从 b 复制到 a，但您想从 a 复制到 b。

将每个 *a++ = *b++; 更改为 *b++ = *a++;

你也可以只做memcpy(b, a, length * sizeof *a);

Answer 2

这个函数：

void send(int *to, int *from, int count)
{
    int n=(count+7)/8;
    switch(count%8)
    {
       case 0: do{ *to = *from++;
       case 7:     *to = *from++;
       case 6:     *to = *from++;
       case 5:     *to = *from++;
       case 4:     *to = *from++;
       case 3:     *to = *from++;
       case 2:     *to = *from++;
       case 1:     *to = *from++;
                }while( --n>0);
    }  
}

包含几个问题。

1) the while statement 
   places the majority of the switch case statements 
   at a different scope level from the first case statement.

2) stepping through the code
   int n = (10+7)%8 = 2
   switch(2)
      (jumping into the center of a do/while loop is 
      a very poor program practice)
      do{
      case 2: *to = *from++;
      case 1: *to = *from++; // which overlays the prior line
      } while( --n >0 ) // on first pass decrement 'n' to 1
      case 7:   overlays prior *to with from[2]
      case 6:   overlays prior *t0 with from[3]
      case 5:   ... from[4]
      case 4:   ... from[5]
      case 3:   ... from[6]
      case 2:   ... from[7]
      case 1"   ... from[8]
         while( --n >0 ) fails, exiting loop

      final result, *to = from[8]

这可能不是您想做的

duff 设备不工作

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