How *restrict / *__restrict__ works in C / C++?

Question

Here is some code I wrote (using GCC's __restrict__ extension to C++):

#include <iostream>

using namespace std;

int main(void) {
    int i = 7;
    int *__restrict__ a = &i;
    *a = 5;
    int *b = &i, *c = &i;
    *b = 8;
    *c = 9;

    cout << **&a << endl; // *a - which prints 9 in this case

    return 0;
}

Or, the C version (in case the C++ version is not clear due to the use of an extension which every popular C++ compiler supports), using GCC:

#include <stdio.h>

int main(void) {
    int i = 7;
    int *restrict a = &i;
    *a = 5;
    int *b = &i, *c = &i;
    *b = 8;
    *c = 9;

    printf("%d \n", **&a); // *a - which prints 9 in this case

    return 0;
}

From what I've read, if I do *a = 5, it changes the value of the memory he, a, is pointing to; after that, the memory to which he is pointing to should not be modified by anyone else except a, which means that these programs are wrong because b and c modify it after that. Or, even if b modifies i first, after that only a should have access to that memory (i). Am I getting it correctly?

P.S: Restrict in this program doesn't change anything. With or without restrict, the compiler will produce the same assembly code. I wrote this program just to clarify things, it is not a good example of restrict usage. A good example of restrict usage you can see here: http://cellperformance.beyond3d.com/articles/2006/05/demystifying-the-restrict-keyword.html

Answer 1

No.

Statements

*b = 8;
*c = 9;

will cause undefined behavior.

From documentation:

A pointer is the address of a location in memory. More than one pointer can access the same chunk of memory and modify it during the course of a program. The restrict type qualifier is an indication to the compiler that, if the memory addressed by the restrict-qualified pointer is modified, no other pointer will access that same memory. The compiler may choose to optimize code involving restrict-qualified pointers in a way that might otherwise result in incorrect behavior. It is the responsibility of the programmer to ensure that restrict-qualified pointers are used as they were intended to be used. Otherwise, undefined behavior may result.