C-指向指针的索引 [英] C - index on pointer to pointer

问题描述

如果我有

struct X {
  int a;
  struct X* next;
}

struct X** ptr;

ptr初始化为结构X的地址，每个结构X的 接下来指向另一个结构X.使用

ptr is initialized with the address of a struct X, and each struct X 's next points to another struct X. What will one get by using

ptr[i] // where i is unsigned int

以下是我问题的来源 :(它是来自selinux的代码的链接):

Here is the source of my question : (it's link to code from selinux) : code

这些类型在以下类型中定义:类型

The types are defined in : types

推荐答案

您将获得垃圾.为了使p[i]中的指针算法起作用，您需要在内存中(在数组中)一个接一个地放置对象(在这种情况下，指向结构的指针).

You will get garbage. For the pointer arithmetic in p[i] to work you need to have the objects (in this case the pointers-to-struct) one after the other in the memory (in an array).

根据您的情况，您需要遵循next指针i次.

In your case you need to follow the next pointer i times.

struct X *p = *ptr;
for (j = 0; j < i; ++j) p = p->next;
/* now p points to the i-th struct */

如果结构实际上在数组中，则等效于

If the structs are in fact in an array, this is equivalent to

struct X *p = (*ptr)[i];

但是在那种情况下，next将毫无用处.

But in that case, the next would be useless.

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