The C99 keyword restrict is an indication to the compiler that different object pointer types and function parameter arrays do not point to overlapping regions of memory. 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++.