Question

我正在尝试按照1d结构类型条目的数组对1 d指针数组进行malloc。

在使每个指针指向1d数组的对应元素时，我遇到了问题。

指针存储在内存的开头，每个指针指向每个条目的地址。

下面是一个简单的说明：

--------------------------
ptr1|ptr2|ptr3|ptr4|ptr5
--------------------------
entry1
--------------------------
entry2
--------------------------
entry3
--------------------------
entry4
--------------------------
entry5
--------------------------


each pointer in the array of pointers points to entry 1- 5 accordingly.

这是代码，

#include <stdio.h>

#define MAX_LAST_NAME_SIZE 16
#define TABLE_SIZE 131072  //2^17

typedef struct __PHONE_BOOK_DETAIL {   //not really matter in this question
    char firstName[16];
    char city[16];
    char zip[5];

} __PHONE_BOOK_DETAIL;

typedef struct __PHONE_BOOK_ENTRY {    // the size is 16 + 8 +8 =32byte
    char lastName[MAX_LAST_NAME_SIZE];

    __PHONE_BOOK_DETAIL *detail;

    struct __PHONE_BOOK_ENTRY *pNext;

} entry;

int main()
{
    int i;

    entry  **pHead;
    entry  *e; 
    pHead = (entry **) malloc(sizeof(entry *)*TABLE_SIZE + sizeof(entry)* TABLE_SIZE);

    for(i=0;i<TABLE_SIZE;i++){
        pHead[i]=  (pHead+ TABLE_SIZE) + sizeof(entry)*i;
        printf("i=%d , phead[i]=%p &phead[i]=%p, sizeof (entry)=%d sizeof(e)=%d \n",i, pHead[i],&pHead[i],sizeof(entry),sizeof(e));
        //pHead[i]->pNext=NULL;
    }
    return 0;
}

输出：

i=0 , phead[i]=6b597010 &phead[i]=6b497010, sizeof (entry)=32 sizeof(e)=8
i=1 , phead[i]=6b597110 &phead[i]=6b497018, sizeof (entry)=32 sizeof(e)=8
i=2 , phead[i]=6b597210 &phead[i]=6b497020, sizeof (entry)=32 sizeof(e)=8
i=3 , phead[i]=6b597310 &phead[i]=6b497028, sizeof (entry)=32 sizeof(e)=8
i=4 , phead[i]=6b597410 &phead[i]=6b497030, sizeof (entry)=32 sizeof(e)=8
i=5 , phead[i]=6b597510 &phead[i]=6b497038, sizeof (entry)=32 sizeof(e)=8
i=6 , phead[i]=6b597610 &phead[i]=6b497040, sizeof (entry)=32 sizeof(e)=8
i=7 , phead[i]=6b597710 &phead[i]=6b497048, sizeof (entry)=32 sizeof(e)=8
i=8 , phead[i]=6b597810 &phead[i]=6b497050, sizeof (entry)=32 sizeof(e)=8
i=9 , phead[i]=6b597910 &phead[i]=6b497058, sizeof (entry)=32 sizeof(e)=8
                      ....

当i = 16369时程序崩溃。这似乎是内存访问违规。

我的CPU是64位，因此指针大小为8个字节。

然而，虽然我期望条目的大小是32byte，你可以看到phead [i]的增量是256.任何人都可以解释一下吗？

谢谢！

Answer 1

我期望输入的大小是32byte，你可以看到phead [i]的增量是256.
任何人都可以解释一下吗？

意外的错误指针数学。

//                                 + this adds by the size of *pHead 
// pHead[i] = (pHead + TABLE_SIZE) + (sizeof(entry) * i);

在完全启用编译器警告的情况下，OP可能已收到以下信息，暗示了问题。

warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]

提示：节省时间，启用所有编译器警告。

还存在对齐/大小问题，因为后半部分entry[]数组可能无法根据需要对齐。 @Ian Abbott

避免问题的一个简单方法是创建所需内存的struct并分配给它。

int main(void) {
  //  This is the memory layout that OP seeks
  typedef struct {
    entry *pHead[TABLE_SIZE];
    // Compiler will determine if padding needed between the 2 members.
    // OP's code reasonably assumed zero padding - yet best not to assume it.
    entry e[TABLE_SIZE];
  } all;

  all *a = malloc(sizeof *a);  // Simple!
  if (a == NULL) {
    return (EXIT_FAILURE);
  }

  entry **pHead = a->pHead;
  entry *e = a->e;

  for (size_t i = 0; i < TABLE_SIZE; i++) {
    pHead[i] = &e[i];
    printf(   // format corrected
        "i=%zu, phead[i]=%p &phead[i]=%p, sizeof (entry)=%zu sizeof(e)=%zu \n",
        i, (void *) pHead[i], (void *) &pHead[i], sizeof entry, sizeof e);
  }
  free(pHead); // Note pHead and `a` point to the same memory.
}

输出

i=0, phead[i]=0x600061fc8 &phead[i]=0x600061f90, sizeof (entry)=32 sizeof(e)=8 
i=1, phead[i]=0x600061fe8 &phead[i]=0x600061f98, sizeof (entry)=32 sizeof(e)=8 
i=2, phead[i]=0x600062008 &phead[i]=0x600061fa0, sizeof (entry)=32 sizeof(e)=8
...

Malloc 1 d指针数组错误

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