C语言 C 双指针

C double pointer

提问by Jono

struct counter{
    long long counter;
}

struct instruction{
    struct counter *counter
    int repetitions;
    void (*work_fn)(long long *);
};

int ncounter; //number of counters
struct counter *counter; //counter array

int nthreads; //number of threads
int *ninstructions; //number of instructions

struct instruction **instructions; 

How does this actually works ? I am having trouble with **pointers

这实际上是如何工作的？我在使用**指针时遇到问题

回答by Seth Carnegie

A **is just a pointer to a pointer. So where an instruction*contains the address of an instructionstruct, an instruction**contains the address of an instruction*that contains the address of an instructionobject.

A**只是一个指向指针的指针。因此，其中 aninstruction*包含instruction结构instruction**的地址，而 aninstruction*包含包含instruction对象地址的 an 的地址。

To access the instructionpointed to by the pointer pointed to by an instruction**, you just use two asterisks instead of one, like (**p).repetitionsor something similar.

要访问instructionan 指向的指针所指向的instruction**，您只需使用两个星号而不是一个，例如(**p).repetitions或类似的东西。

You can visualize it like this:

你可以像这样想象它：

instruction*  ----> instruction
instruction** ----> instruction* ----> instruction

Remember, however, that simply declaring struct instruction** instructions;doesn't actually create an instructionstruct. It just creates a pointer that holds a garbage value. You'll have to initialize it:

但是请记住，简单的声明struct instruction** instructions;实际上并不会创建instruction结构体。它只是创建一个包含垃圾值的指针。你必须初始化它：

struct instruction inst;
// set members of inst...
*instructions = &inst;

...

(*instructions)->repetitions++; // or whatever

However, it looks like you're using an instruction**to point to an array of instruction*s. To initialize the array, you need a forloop:

但是，看起来您正在使用 aninstruction**指向一个instruction*s数组。要初始化数组，您需要一个for循环：

instructions = malloc(sizeof(struct instruction*) * num_of_arrays);
for (i = 0; i < num_of_arrays; ++i)
    instructions[i] = malloc(sizeof(struct instruction) * size_of_each_subarray);

And then you can access an element like instructions[i]->datamember.

然后你可以访问像instructions[i]->datamember.

回答by littleadv

struct instruction **instructions; // How does this actually works ? I am having trouble with ** pointers

struct instruction **instructions; // How does this actually works ? I am having trouble with ** pointers

I'm not sure what the real issue is, but I'll try to answer the question.

我不确定真正的问题是什么，但我会尽力回答这个问题。

Double pointer is a pointer to pointer. It can be sued as array of pointers for example (if you allocate memory accordingly). For example:

双指针是指向指针的指针。例如，它可以用作指针数组（如果您相应地分配内存）。例如：

instructions = malloc(5*sizeof(struct instruction*));
for (int i = 0; i < 5; i++)
    instructions[i] = malloc(sizeof(struct instruction));

And you got yourself nice array of 5 pointers to struct instruction. Use it like this:

你得到了 5 个指向struct instruction. 像这样使用它：

instructions[0]->repetitions = 0;

回答by Noufal Ibrahim

instructionsis a pointer to a pointer to struct instruction.

instructions是一个指向 的指针struct instruction。

This means that *instructionswill give you a pointer to a struct instruction. This kind of construct is often used to create a dynamic array of pointers to some compound type.

这意味着*instructions会给你一个指向 a 的指针struct instruction。这种构造通常用于创建指向某种复合类型的指针的动态数组。