C中这两个指针赋值有什么区别？

Question

I have been doing a lot of research and also playing around with code, but I'm still confused with this: Why is case 1 erroring?

Case 1:

int arr[] = {0,4,7};
int *p = arr[0]; // pointer points to the first element in the array

printf("%d", ++*p);
// This results in Segmentation fault

Case 2:

int arr[] = {0,4,7};
int *p = arr; // pointer points to the first element in the array

printf("%d", ++*p);
// This prints 1

Answer 1

In case 1: arr[0] is an int value. int *p = arr[0]; is an error.

Answer 2

What is the difference between these two pointer assignments in C?

Firstly, one thing common with both is that neither is an assignment.

 int *p = arr[0]; // pointer points to the first element in the array

The comment here is wrong. arr is an array of int . arr[0] is the first element of that array. It is an int . int *p = arr[0] is an attempt at initialising a pointer with the value that is stored in the array. However, the type doesn't match since the value is not a pointer. int is not implicitly convertible to int* and therefore the program is ill-formed.

A standard conforming language implementation is required to diagnose this issue for you. This is what my compiler says:

 error: invalid conversion from 'int' to 'int*'

---

Why is case 1 erroring?

If the program even compiles, you are initialising a pointer with the value 0. This likely means that it is a null pointer and that it doesn't point to any object. ++*p attempts to indirect through this null pointer and access the non-existing integer.

 int *p = arr; // pointer points to the first element in the array

Comment here is correct.

Answer 3

I guess it is throwing an error because you did not use '&' before assigning the value to the pointer. As it is a pointer so you can not assign a value to it. You can only use it to store addresses. I coded it in C++ and here is the correct form.I guess it helps.

#include <iostream>

using namespace std;

int main(){
  int arr[] = {0,4,7};
  int *p = &arr[0]; //you are missing '&' before array[0]

  printf("%d", ++*p);
  system("pause");
  // This prints 1
  }