C6: Pointers

Question 1

What is a pointer?

Answer 1

Pointers contain address of variable that
has specific value (indirect reference).
Ex: int *myPtr;
type of pointer is int *

Question 2

How do you declare a pointer?

Answer 2

Pointer is declared by putting ‘*’ in front of variable identifier

Question 3

What are the pointer operators?

Answer 3

& and .
‘’ can declare a pointer and is used to dereference a pointer, i.e. returns ‘synonym’ (object) its pointer operand points to.
‘&’ returns memory address of its operand and also defines a pointer.
For example:
int *countPtr2;
countPtr2 = &count;
printf(“pointer2 %i\n”, *countPtr2);

Question 4

What is manual memory management?

Answer 4

Manual memory management is used to manage memory on the heap (not on stack or in data or bss segments.)

Question 5

Where are Pointers stored when using malloc?

Answer 5

on the heap

Question 6

If you don’t free up memory allocated to the heap, what can happen?

Answer 6

Dangling references, memory leak.

Question 7

What is memory leak?

Answer 7

In computer science, a memory leak is a type of resource leak that occurs when a computer program incorrectly manages memory allocations[1] in such a way that memory which is no longer needed is not released. A memory leak may also happen when an object is stored in memory but cannot be accessed by the running code.[2]

Question 8

What is a null pointer?

Answer 8

A pointer that points to nothing.

int * ptr =0; or int * ptr = NULL;

Question 9

What would a programmer want to use a null pointer?

Answer 9

It can be used to indicate that a pointer hasn’t been initialized or that a function wasn’t successful, for ex. if it should return a pointer but it returns the null pointer. Note: an uninitialized pointer is not automatically a null pointer and usually points to a random address.

Question 10

What is a null pointer constant?

Answer 10

It is a constant integer 0 (?)
GNU: The null pointer constant is guaranteed not to point to any real object. You can assign it to any pointer variable since it has type void *. The preferred way to write a null pointer constant is with NULL.

Question 11

Express the array items[0] and items[3] as pointers.

Answer 11

*(items+0) and (items+3) where + is pointer arithmetic adding n sizeof(arraytype) to int pointer

Question 12

Is it possible to call by value with an array (passing an array to a function without the array being altered in the calling function rather only locally in the function)?

Answer 12

No, because arrays decay into pointers, so this becomes call by reference.
int * foldArray(int a[3][3], int * result){

}
int input[3][3]= {{1, 2, 3}, {2, 2, 3}, {0, 0, 1}};
int result[3];
foldArray(input, result); 
for(int i =0; i<3; i++){
	printf("result[%i]: %i\n", i, r 
         result[i]);
}
=> result is changed in main!

Question 13

What is a constant pointer?

Answer 13

A constant pointer has following declaration:
int * const x; (BUT NOT int const * x;) and it means that it is not possible to change the value of the pointer. i.e. x = &v; is not possible after definition (like x = &w;)

Question 14

What is constant data (for a pointer)?

Answer 14

A pointer has constant data with the following declaration: int const * x; the integer value is constant and cannot be changed. For example *x = 10; produces an error.

Question 15

What is a function pointer?

Answer 15

A function pointer is a pointer that points to a function; i.e. the pointer stores as value the address in memory of a function.
ret_type (*var_name) (params) = &func_name;

Question 16

Given a function is_bigger:
bool is_bigger(int a, int b){
  return a>b;
}
how do you declare and define a function pointer pointing to is_bigger?

Answer 16

int main(void){
  bool (*compare)(int, int);
  compare = &is_bigger;
  bool b = (*compare)(3,4); 
  //since is_bigger returns a 
  //boolean return type
  printf(b ? "true": "false");
}

Question 17

Given the following function definition 
int retSum(int n1, int n2){
return n1 + n2;}
create  a typedef for a function pointer with the name ptrToFunct and instantiate the function pointer

Answer 17

int main(void){
typedef (*ptrToFunct)(int, int);
//from now on ptrToFunct is a type
ptrToFunct pFunct = &retSum;
int result = (*pFunct) (1,2);
//returns 3

Question 18

What are function pointers used for?

Answer 18

Callback functions
 User starts a system operation, function
returns immediately
 System informs user upon completion by
calling a user-defined function
 Pointer to this function is given in first
system call
43
Function Pointer Use Cases
Jump tables: array of function pointers
 For example for menu-driven systems
 Menu choice is numeric and taken as
subscript into array to call wanted
operation:
(*table[menu_choice])();

Question 19

How do you pass a function as a parameter?

Answer 19

#include 
void printNumber(int nbr)  
{
    printf("%d\n", nbr);
}
void myFunction(void (*f)(int))  
{
    for(int i = 0; i < 5; i++) 
    {
        (*f)(i);
    }
}
int main(void)  
{
    myFunction(printNumber);
    return (0);
}