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Unit 14: Pointers

Learning Objectives

After this unit, students should:

  • understand what the address-of operator, &, represents and what is its type
  • be able to use the & operator to access the address of a variable
  • understand what the dereference operator, *, represents
  • be aware that there is a pointer type associated with each type in C and that when * is used in the variable declaration it has a different meaning to the dereference operator
  • be able to define pointer type variables and use the * operator, including:
    • declaration of a pointer type variable
    • assigning an address (of the appropriate type) to a pointer type variable
    • accessing/assigning the value that a pointer variable points at
    • assigning a new value by dereferencing a pointer variable
  • understand why we cannot only change the address referenced by a variable
  • understand the units being stored by a pointer and thus, how arithmetic operations would affect pointers
  • be aware of pointers to pointers (and more), and the possibility of multiple levels of dereferencing
  • be aware of the `NULL pointer, understand what it represents, and be able to use it

Memory Address

Every memory location has an address. Unlike many higher-level languages, such as Java, Python, and JavaScript, C allows us direct access to memory addresses. This empowers programmers to do wonderful things that cannot be done in other languages. But, it is also dangerous at the same time -- using it improperly can lead to bugs that are hard to track down and debug.

The Address-of Operator

C has an operator called "address-of", denoted by &. This operator returns, well, the address of a variable.

Let say that we have a variable 

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long c;

The expression &c has the type "address of long" and evaluates to a number that corresponds to the memory address of c. Note that "address of long" and long are two different types to C. In general, the expression &x has the type "address of \(T\)" where \(T\) is the type of variable x.

Suppose we want to print out the address of c. We cannot use cs1010_println_long(&c), since the type does not match. We can, however, typecast the address of a variable into long and print it out to examine its value. Consider this:

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#include "cs1010.h"

void add(long sum, long a, long b) {
  sum = a + b;
  cs1010_println_long((long)&sum);
}

int main()
{
  long x = 1;
  long sum;
  add(sum, x, 10);
  cs1010_println_long((long)&sum);
}

Running the program above prints something like this: 

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140723025685528
140723025685552

Your results will most likely be different since the OS allocates different regions of the memory to this program every time it is run.

pointers

The Dereference Operator

The dereference operator is the reversed of address-of and is denoted by *. I call it "location-of-address". We use this operator in two places:

  • to declare an "address" variable, and
  • to reference the location of an address.

We can declare a variable that is an address type. We need to tell C the type of the variable this address is referencing. For instance,

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double *addr;

declares a variable addr that is an address to a variable of type double. The way to read this is that *addr, or location-of-address addr is of type double, so addr is an address of a location containing a double.

Common Bug

It is possible to write as 

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double* addr;
too, but this is not recommended. Suppose you want to declare two addresses, you might write, 
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double* from_addr, to_addr;
thinking that both from_addr and to_addr are of type double*. But C treats to_addr as a double, not an address of a double! In any case, if you follow CS1010 style, you shouldn't be declaring two variables in one line.

Another name for a variable of type address is pointer. We can visualize a pointer as pointing to some location in the memory.

Changing the Value via Pointer

Suppose we declare a pointer to a double variable (or, for short, a double pointer): 

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double *addr;

We can use *addr just like a normal double variable: 

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*addr = 1.0;

The line above means that, we take the address stored in addr, go to the location at that address, and store the value 1.0 in the location.

This is where things can get dangerous. You could be changing the value in a memory location that you do not mean to. If you are lucky, your program crashes with a segmentation fault error1. We say that your program has segfault. If you are unlucky, your program runs normally but produces incorrect output occasionally.

pointers

So, always make sure that your pointer is pointing to the right location before dereferencing and writing to the location.

In the code above, if we write: 

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double *addr;
*addr = 1.0;

back-to-back, the program will almost certainly segfault, because the pointer variable addr is not initialized, so it is pointing to the location of whatever address happens to be in the memory at that time.

We should point addr to a value location first, like this: 

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double c;
double *addr;
addr = &c;
*addr = 1.0;

pointers

Of course, the above could be simply written as: 

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double c = 1.0;

I am just doing it the complicated way (which you should avoid unless you have good reasons to do so) to demonstrate the concept of pointers.

Basic Rules About Using Pointers

  1. When we use pointers, it must point to the variable of the same type as that declared by the pointer. For instance,

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    double pi = 3.1415926;
long radius = 5;
double *addr;
addr = π // ok
addr = &radius; // not ok

    Line 5 above would lead to a compilation error since we try to point a double pointer to a long.

  2. We cannot change the address of a variable. For instance

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    long x = 1;
long y = 2;
&x = &y;

    We try to set the address of x to be the address of y. This is not allowed since the allocation of variables in the memory is determined by the OS, a process we have no control over.

  3. We can perform arithmetic operations on pointers, but not in the way you expect.

    Suppose we have a pointer: 

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    long x;
long *ptr;
x = 1;
ptr = &x;
ptr += 1;

    Suppose that x is stored in memory address 1000, after Line 4, ptr would have the value of 1000. After the line ptr += 1, using normal arithmetic operation, we would think that ptr will have a value of 1001. However, the semantic for arithmetic operation is different for pointers. The + operation for ptr causes the ptr variable to move forward by the size of the variable pointed to by the pointer. In this example, ptr points to long, assuming that long is 8 bytes, after ptr += 1, ptr will have the value of 1008.

    We can only do addition and subtraction for pointers.

Pointer of Pointer (of Pointer..)

A pointer variable is also stored in the memory, so it has an address too.

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long x;
long *ptr;
ptr = &x;

For instance, in the above, ptr would have a memory location allocated on the stack too, and so it has an address, and we can have a variable ptrptr referring to the address of ptr. What would the type of this variable be? Since ptr is an address of long, ptrptr is an address of an address of long, and can be written as:

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long x;
long *ptr;
long **ptrptr;
ptr = &x;
ptrptr = &ptr;

This dereference can go on since ptrptr is also a variable and has been allocated in some memory location on the stack. We rarely need to dereference more than twice in practice, but if the situation arises, such multiple layers of dereferencing are possible.

The NULL Pointer

NULL is a special value that is used to indicate that a pointer is pointing to nothing. In C, NULL is actually 0 (i.e., pointing to memory location 0).

We use NULL to indicate that the pointer is invalid, typically to mean that we have not initialized the pointer or to flag an error condition.

Billion Dollar Mistakes

Sir Tony Hoare (the same one whom we met when we talked about Assertion) also invented the null pointer. He called it his billion-dollar mistake. Quoting from him: "I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years." As you start to use pointers in CS1010, you will see why it is a pain.

Problem Set 14

Problem 14.1

Sketch the content of the memory while tracing through the following code. What would be printed?

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long *ptr1;
long *ptr2;
long x;
long y;

ptr1 = &x;
ptr2 = &y;

*ptr1 = 123;
*ptr2 = -1;

cs1010_println_long(x);
cs1010_println_long(y);
cs1010_println_long(*ptr1);
cs1010_println_long(*ptr2);

ptr1 = ptr2;
*ptr1 = 1946;

cs1010_println_long(x);
cs1010_println_long(y);
cs1010_println_long(*ptr1);
cs1010_println_long(*ptr2);

y = 10;

cs1010_println_long(x);
cs1010_println_long(y);
cs1010_println_long(*ptr1);
cs1010_println_long(*ptr2);

Problem 14.2

What is wrong with both programs below?

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double *addr_of(double x)
{
  return &x;
}

int main()
{
  double c = 0.0;
  double *ptr;

  ptr = addr_of(c);
  *ptr = 10;
}

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double *triple_of(double x)
{
  double triple = 3 * x;
  return &triple;
}

int main()
{
  double *ptr;

  ptr = triple_of(10);
  cs1010_println_double(*ptr);
}

  1. I leave it to the later OS classes CG2271 / CS2106 to explain the term "segmentation" and "fault". Interested students can always google and read on Wikipedia. ↩