Question

I have a question regarding restricted pointer assignments. See the comments in code for specific questions. Overall, I’m just wondering what’s legal with restrict (I’ve read the standard, but still have questions 🙁

int* Q = malloc(sizeof(int)*100);

{
    int* restrict R = Q;

    for(int j = 0; j < rand()%50; j++)
    {
        R[j] = rand();
    }

    Q = R; // The standard says assigning restricted child pointers to their parent is illegal.
           // If Q was a restricted pointer, is it correct to assume that this would be ILLEGAL?
           //
           // Since Q is unrestricted, is this a legal assignment?
           //
           // I guess I'm just wondering: 
           // What's the appropriate way to carry the value of R out of the block so
           // the code can continue where the above loop left off? 
}

{
    int* S = Q;   // unrestricted child pointers, continuing where R left off above
    int* T = Q+1; // S and T alias with these assignments

    for(int j = 0; j < 50; j++)
    {
        S[j] = T[j];
    }
}

Thanks for your help!

Answer 1

Since the object being modified (the array allocated in first line) isn’t modified through an lvalue expression except involving the restricted pointer, R in that block where R is declared, I think that the code in your example is well-defined.

If Q were a restricted pointer, the example would be undefined.