Question

How can i declare an array like this one:

int array[1000000];

as a static array, a stack array, and a heap-allocated array?

Answer 1

Your assignment appears to be looking for this:

// global variable; NOT on the stack. Exists in the data segment of the program
int globalvar[1000000];

void func()
{
    // local stack-variable. allocated on the stack on function entry
    //  unavailable outside this function scope.
    int stackvar[1000000];

    // allocated on the heap. the only stack space used in the
    //  space occupied by the pointer variable.
    int *heapvar = malloc(1000000 * sizeof(int));
    if (heapvar != NULL)
    {
        // use heap var

        // free heap var
        free(heapvar)
    }
}

Or perhaps this:

void func()
{
    // static variable; NOT on the stack. Exists in a program data segment (usually)
    static int staticvar[1000000];
    
    // local stack-variable. allocated on the stack on function entry
    //  unavailable outside this function scope.
    int stackvar[1000000];

    // allocated on the heap. the only stack space used in the
    //  space occupied by the pointer variable.
    int *heapvar = malloc(1000000 * sizeof(int));
    if (heapvar != NULL)
    {
        // use heap var

        // free heap var
        free(heapvar)
    }
}

For what it is worth, unless you have a four or eight megabyte reserved call-stack (or larger), the function above will likely croak on entry. For such large sizes it is customary to use the heap (malloc()/free()). But that is not what your assignment appears to be about (yet).