This enables the compiler to perform optimizations that might otherwise be prevented because of possible aliasing.

In the following example, pointer a does not, and must not, point to the same region of memory as pointer b:

void copy_array(int n, int *restrict a, int *restrict b)
{
    while (n-- > 0)
        *a++ = *b++;
}
void test(void)
{
    extern int array[100];
    copy_array(50, array + 50, array);    // valid
    copy_array(50, array + 1, array);     // undefined behavior
}

Pointers qualified with restrict can however point to different arrays, or to different regions within an array.

It is your responsibility to ensure that restrict-qualified pointers do not point to overlapping regions of memory.

__restrict, permitted in C90 and C++, is a synonym for restrict.

--restrict enables restrict to be used in C90 and C++.