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我有以下代码，以便能够访问结构数组中的众多字段（为简单起见，我将其减少为两个）。最终指针计算的正确咒语是什么， *(ptr + offset) = data; 因为我总是得到：

错误：从类型 'int32_t' {aka 'int'} 分配给类型 'struct osc_in_data' 时类型不兼容

#define NumHarmonics   10

int32_t data1;
int32_t data2;



struct osc_in_data                                                           
{
    int32_t     LevelAttackRate;                                                
    int64_t     LevelPeakLevel;                                                 
    int32_t     LevelDecayRate;                                                 
} OscControl[NumHarmonics];



void SetADSRvalues(int32_t offset, int32_t data)
{
    int32_t harmonic;
    struct osc_in_data *ptr;
    for (harmonic = 0; harmonic < NumHarmonics; harmonic++)
    {
       ptr = &OscControl[harmonic];
       *(ptr + offset) = data;
    }
} 


SetADSRvalues(offsetof(struct osc_in_data, LevelAttackRate), data1)
SetADSRvalues(offsetof(struct osc_in_data, LevelDecayRate), data2)
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2 回答 2

3

成员具有int32_t类型，因此指向它们的指针是int32_t*.

这offsetof(...)只是字节的偏移量。因此，您只需将指针指向要修改的成员的结构。然后使用普通加法将偏移量添加到指针上，并记住使用char*指针一次添加一个字节。然后只需将指针转换为正确的类型并取消引用并访问它。

void SetADSRvalues(size_t offset, int32_t data)
{
    for (size_t harmonic = 0; harmonic < NumHarmonics; harmonic++) {
          // take the pointer to the structure we want to modify
          void *base = &OscControl[harmonic];
          // arithmetic using void* pointers is invalid
          // so convert to `char*` pointer before arithmetic
          char *basechar = base;
          // then add the offset - just plain addition
          // results in the address of the member inside the struct
          void *memberpnt = basechar + offset;
          // the member is `int32_t`, so the pointer has to be `int32_t*`
          int32_t *memberpnt_int32 = memberpnt;
          // finally set the value
          *memberpnt_int32 = data;

          // or a oneliner version:
          *(int32_t*)((char*)&OscControl[harmonic] + offset) = data;
    }
}
于 2019-10-30T10:48:37.313 回答
3

The offsetoff function tells you the distance in bytes between 2 memory adresses inside the same structure.

With below code you are telling,

   *(ptr + offset) = data;

do pointer arithmetic on struct osc_in_data pointer, that is same as ptr[offset] = data;

Instead what you can try is.

memcpy((char *)ptr + offset, data, sizeof data);
于 2019-10-30T10:48:58.300 回答