error: ISO C forbids nested functions – What’s wrong?

You missed a closing braces for for loop in the function arraycpy

void arraycpy(int *dest, int *src, int n)
{
    int i;
    int *src_p, *dst_p;

    src_p = (int*)src;
    dst_p = (int*)dest;

    for (i = 0; i < n; i++) {
        *(dst_p+i) = *(src_p+i);
    }
 // ^^^
}

So, the next braces is being taken as the braces for the for loop, and thus when you define the next function, it is being defined inside the next function arraycpy