Pointers in C

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Bhatnagar Achindra

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Aug 31, 2005, 3:39:03 AM8/31/05
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Pointer Basics

Section 1 -- Pointer Rules
One of the nice things about pointers is that the rules which govern how they work are pretty simple. The rules can be layered together to get complex results, but the individual rules remain simple.

1) Pointers and Pointees

A pointer stores a reference to something. Unfortunately there is no fixed term for the thing that the pointer points to, and across different computer languages there is a wide variety of things that pointers point to. We use the term pointee for the thing that the pointer points to, and we stick to the basic properties of the pointer/pointee relationship which are true in all languages. The term "reference" means pretty much the same thing as "pointer" -- "reference" implies a more high-level discussion, while "pointer" implies the traditional compiled language implementation of pointers as addresses. For the basic pointer/pointee rules covered here, the terms are effectively equivalent.

Pointer pointing to pointee
The above drawing shows a pointer named x pointing to a pointee which is storing the value 42. A pointer is usually drawn as a box, and the reference it stores is drawn as an arrow starting in the box and leading to its pointee.

Allocating a pointer and allocating a pointee for it to point to are two separate steps. You can think of the pointer/pointee structure as operating at two levels. Both the levels must be set up for things to work. The most common error is concentrating on writing code which manipulates the pointer level, but forgetting to set up the pointee level. Sometimes pointer operations that do not touch the pointees are called "shallow" while operations on the pointees are called "deep".

2) Dereferencing

The dereference operation starts at the pointer and follows its arrow over to access its pointee. The goal may be to look at the pointee state or to change the pointee state.

The dereference operation on a pointer only works if the pointer has a pointee -- the pointee must be allocated and the pointer must be set to point to it. The most common error in pointer code is forgetting to set up the pointee. The most common runtime crash because of that error in the code is a failed dereference operation. In Java the incorrect dereference will be flagged politely by the runtime system. In compiled languages such as C, C++, and Pascal, the incorrect dereference will sometimes crash, and other times corrupt memory in some subtle, random way. Pointer bugs in compiled languages can be difficult to track down for this reason.

3) Pointer Assignment

Pointer assignment between two pointers makes them point to the same pointee. So the assignment y = x; makes y point to the same pointee as x. Pointer assignment does not touch the pointees. It just changes one pointer to have the same reference as another pointer. After pointer assignment, the two pointers are said to be "sharing" the pointee.

Section 2 -- Code Example

1. Allocate two pointers x and y. Allocating the pointers does not allocate any pointees.
2. Allocate a pointee and set x to point to it. Each language has its own syntax for this. What matters is that memory is dynamically allocated for one pointee, and x is set to point to that pointee.
3. Dereference x to store 42 in its pointee. This is a basic example of the dereference operation. Start at x, follow the arrow over to access its pointee.
4. Try to dereference y to store 13 in its pointee. This crashes because y does not have a pointee -- it was never assigned one.
5. Assign y = x; so that y points to x's pointee. Now x and y point to the same pointee -- they are "sharing".
6. Try to dereference y to store 13 in its pointee. This time it works, because the previous assignment gave y a pointee.

C Version

The pointers x and y are allocated as local variables. The type int* means "pointer which points to ints". The pointers do not automatically get pointees. The pointee for x is dynamically allocated separately with the standard library function malloc(). The syntax *x dereferences x to access its pointee.

void main() {   
int* x; // Allocate the pointers x and y
int* y; // (but not the pointees)

x = malloc(sizeof(int)); // Allocate an int pointee,
// and set x to point to it


*x = 42; // Dereference x to store 42 in its pointee

*y = 13; // CRASH -- y does not have a pointee yet

y = x; // Pointer assignment sets y to point to x's pointee


*y = 13; // Dereference y to store 13 in its (shared) pointee
}

Another way to play with pointers in C (or C++) is using the ampersand (&) operator to compute a pointer to local memory in the stack. However, pointees dynamically allocated in the heap are the most common.

C++ Version

The only difference in this version from the C version above is that the standard operator new is used instead of malloc().
void main() {   
int* x; // Allocate the pointers x and y
int* y; // (but not the pointees)

x = new int; // Allocate an int pointee,
// and set x to point to it


*x = 42; // Dereference x to store 42 in its pointee

*y = 13; // CRASH -- y does not have a pointee yet

y = x; // Pointer assignment sets y to point to x's pointee


*y = 13; // Dereference y to store 13 in its (shared) pointee
}

Section 3 -- Study Questions

These study questions cover review basic features of pointers. Two of the questions make heavy use of memory drawings. Memory drawings are an excellent way to think through pointer problems.

Question 1

At the end of the above code, y is set to have a pointee and then dereferenced it store the number 13 into its pointee. After this happens, what is the value of x's pointee?

Answer: The value of x's pointee is 13 because it is also y's pointee. This is what sharing is all about -- multiple pointers pointing to one pointee.

Question 2

Consider the following drawing...

Using the language of your choice, write some code that creates the above pointer structure.

Answer: The basic steps are...

  1. Allocate two pointers.
  2. Allocate two pointees and set the pointers to point to them.
  3. Store the numbers 1 and 2 into the pointees.
  4. Assign the first pointer to point to the second pointee. This "loses" the reference to the first pointee which is unusual, but that's what the question calls for.
C Code
{
int* x;
int* y;

x = malloc(sizeof(int));
y = malloc(sizeof(int));

*x = 1;
*y = 2;

x = y;
}

Question 3

Suppose you have a pointee type called "Node" which contains two things: an int, and a pointer to another Node (the declaration for such a Node type is given below). With such a pointee type, you could arrange three Node pointees in a structure where they were pointing to each other like this...

The pointer named x points to the first Node pointee. The first Node contains a pointer to the second, the second contains a pointer to the third, and the third contains a pointer back to the first. This structure can be build using only the rules of pointee allocation, dereferencing, and assignment that we have seen. Using the declaration below, each Node contains an integer named value and a pointer to another Node named next.

C Code
struct Node {
int value;
struct Node* next;
};

Write the code to build the structure in the above drawing. For convenience, you may use temporary pointers in addition to x. The only new syntax required is that in C, the operator -> dereferences a pointer to access a field in the pointee -- so ->value accesses the field named value in x's pointee.

Answer The basic steps are...

  1. Allocate three pointers: x for the first Node, and temporary pointers y and z for the other two Nodes.
  2. Allocate three Node pointees and store references to them in the three pointers.
  3. Dereference each pointer to store the appropriate number into the value field in its pointee.
  4. Dereference each pointer to access the .next field in its pointee, and use pointer assignment to set the .next field to point to the appropriate Node.
C Code
{
// Allocate the pointers
struct Node* x;
struct Node* y;
struct Node* z;

// Allocate the pointees
x = malloc(sizeof(Node));
y = malloc(sizeof(Node));
z = malloc(sizeof(Node));


// Put the numbers in the pointees
x->value = 1;
y->value = 2;
z->value = 3;

// Put the pointers in the pointees
x->next = y;
y->next = z;
z->next = x;
}

The Node structure introduced here is actually a real data type used to build the "linked list" data structure. Linked lists are a realistic applied use of pointers and are an excellent area to develop your pointer skills. 



Hope, with this, every one builds a good understanding of Pointers in C!!!


--
Bhatnagar Achindra

Towards Invincibility !
www.mcp-achindra.it.tt


"All men seek one goal: success or happiness. The only way to achieve true success is to express yourself completely in service to society. First, have a definite, clear, practical ideal—a goal, an objective. Second, have the necessary means to achieve your ends—wisdom, money, material and methods. Third, adjust all your means to that end,"

-Aristotle
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