Home | Gaming | Programming | Play Online | Contact | Keyword Query
Games++ Games & Game Programming

GAMES++
Games++ Home
Games++ Gaming
Games++ Programming
Beta Testing Games
Free Online Games
Hints & Cheats

BROWSER UTILITIES
E-mail This Page
Add to Favorites

SITE SEARCH

Web Games++

AFFILIATES
Cheat Codes
Trickster Wiki
Game Ratings
Gameboy Cheats
PlayStation Cheats
BlackBerry Games
Photoshop Tutorials
Illustrator Tutorials
ImageReady Tutorials

ADVERTISEMENT

ADVERTISEMENT

Part 1: What is a Pointer?

A Tutorial on Pointers and Arrays in C

By Ted Jensen

Version 0.1


This material is hereby placed in the public domain.


Table of Contents

Preface

This document is intended to introduce pointers to beginning programmers in the C programming language. Over several years of reading and contributing to various conferences on C including those on the FidoNet and UseNet, I have noted a large number of newcomers to C appear to have a difficult time in grasping the fundamentals of pointers. I therefore undertook the task of trying to explain them in plain language with lots of examples.

The first version of this document was placed in the public domain, as is this one. It was picked up by Bob Stout who included it as a file called PTR-HELP.TXT in his widely distributed collection of SNIPPETS. Since that release, I have added a significant amount of material and made some minor corrections in the original work.

Acknowledgements:

There are so many people who have unknowingly contributed to this work because of the questions they have posed in the FidoNet C Echo, or the UseNet Newsgroup comp.lang.c, or several other conferences in other networks, that it would be impossible to list them all. Special thanks go to Bob Stout who was kind enough to include the first version of this material in his SNIPPETS file.

About the Author:

Ted Jensen is a retired Electronics Engineer who worked as a hardware designer or manager of hardware designers in the field of magnetic recording. Programming has been a hobby of his off and on since 1968 when he learned how to keypunch cards for submission to be run on a mainframe. (The mainframe had 64K of magnetic core memory!).

Use of this Material:

Everything contained herein is hereby released to the Public Domain. Any person may copy or distribute this material in any manner they wish. The only thing I ask is that if this material is used as a teaching aid in a class, I would appreciate it if it were distributed in its entirety, i.e. including all chapters, the preface and the introduction. I would also appreciate it if under such circumstances the instructor of such a class would drop me a note at one of the addresses below informing me of this. I have written this with the hope that it will be useful to others and since I'm not asking any financial remuneration, the only way I know that I have at least partially reached that goal is via feedback from those how find this material useful.

By the way, you needn't be an instructor or teacher to contact me. I would appreciate a note from _anyone_ who finds the material useful, or who has constructive criticism to offer. I'm also willing to answer questions submitted by mail.

Ted Jensen

Introduction

If one is to be proficient in the writing of code in the C programming language, one must have a thorough working knowledge of how to use pointers. Unfortunately, C pointers appear to represent a stumbling block to newcomers, particularly those coming from other computer languages such as Fortran, Pascal or Basic.

To aid those newcomers in the understanding of pointers I have written the following material. To get the maximum benefit from this material, I feel it is important that the user be able to run the code in the various listings contained in the article. I have attempted, therefore, to keep all code ANSI compliant so that it will work with any ANSI compliant compiler. And I have tried to carefully block the code within the text so that with the help of an ASCII text editor one can copy a given block of code to a new file and compile it on their system. I recommend that readers do this as it will help in understanding the material.

Chapter 1: What is a pointer?

One of the things beginners in C find most difficult to understand is the concept of pointers. The purpose of this document is to provide an introduction to pointers and their use to these beginners.

I have found that often the main reason beginners have a problem with pointers is that they have a weak or minimal feeling for variables, (as they are used in C). Thus we start with a discussion of C variables in general.

A variable in a program is something with a name, the value of which can vary. The way the compiler and linker handles this is that it assigns a specific block of memory within the computer to hold the value of that variable. The size of that block depends on the range over which the variable is allowed to vary. For example, on PC's the size of an integer variable is 2 bytes, and that of a long integer is 4 bytes. In C the size of a variable type such as an integer need not be the same on all types of machines.

When we declare a variable we inform the compiler of two things, the name of the variable and the type of the variable. For example, we declare a variable of type integer with the name k by writing:

	int k;

On seeing the "int" part of this statement the compiler sets aside 2 bytes (on a PC) of memory to hold the value of the integer. It also sets up a symbol table. And in that table it adds the symbol k and the relative address in memory where those 2 bytes were set aside.

	Thus, later if we write:

	k = 2;

at run time we expect that the value 2 will be placed in that memory location reserved for the storage of the value of k. In C we refer to a variable such as the integer k as an "object".

In a sense there are two "values" associated with the object k, one being the value of the integer stored there (2 in the above example) and the other being the "value" of the memory location where it is stored, i.e. the address of k. Some texts refer to these two values with the nomenclature rvalue (right value, pronounced "are value") and lvalue (left value, pronounced "el value") respectively.

In some languages, the lvalue is the value permitted on the left side of the assignment operator '=' (i.e. the address where the result of evaluation of the right side ends up). The rvalue is that which is on the right side of the assignment statement, the '2' above. Rvalues cannot be used on the left side of the assignment statement. Thus: 2 = k; is illegal.

Actually, the above definition of "lvalue" is somewhat modified for C. According to K&R-2 (page 197): [1]

    "An _object_ is a named region of storage; an _lvalue_ is an
     expression referring to an object."

However, at this point, the definition originally cited above is sufficient. As we become more familiar with pointers we will go into more detail on this.

    Okay, now consider:

    int j, k;
    k = 2;
    j = 7;    <-- line 1
    k = j;    <-- line 2

In the above, the compiler interprets the j in line 1 as the address of the variable j (its lvalue) and creates code to copy the value 7 to that address. In line 2, however, the j is interpreted as its rvalue (since it is on the right hand side of the assignment operator '='). That is, here the j refers to the value _stored_ at the memory location set aside for j, in this case 7. So, the 7 is copied to the address designated by the lvalue of k.

In all of these examples, we are using 2 byte integers so all copying of rvalues from one storage location to the other is done by copying 2 bytes. Had we been using long integers, we would be copying 4 bytes.

Now, let's say that we have a reason for wanting a variable designed to hold an lvalue (an address). The size required to hold such a value depends on the system. On older desk top computers with 64K of memory total, the address of any point in memory can be contained in 2 bytes. Computers with more memory would require more bytes to hold an address. Some computers, such as the IBM PC might require special handling to hold a segment and offset under certain circumstances. The actual size required is not too important so long as we have a way of informing the compiler that what we want to store is an address.

Such a variable is called a "pointer variable" (for reasons which hopefully will become clearer a little later). In C when we define a pointer variable we do so by preceding its name with an asterisk. In C we also give our pointer a type which, in this case, refers to the type of data stored at the address we will be storing in our pointer. For example, consider the variable declaration:

    int *ptr;

ptr is the _name_ of our variable (just as 'k' was the name of our integer variable). The '*' informs the compiler that we want a pointer variable, i.e. to set aside however many bytes is required to store an address in memory. The "int" says that we intend to use our pointer variable to store the address of an integer. Such a pointer is said to "point to" an integer. However, note that when we wrote "int k;" we did not give k a value. If this definition was made outside of any function many compilers will initialize it to zero. Similarly, ptr has no value, that is we haven't stored an address in it in the above declaration. In this case, again if the declaration is outside of any function, it is initialized to a value #defined by your compiler as NULL. It is called a NULL pointer. While in most cases NULL is #defined as zero, it need not be. That is, different compilers handle this differently. Also while zero is an integer, NULL need not be. However, the value that NULL actually has internally is of little consequence to the programmer since at the source code level NULL == 0 is guaranteed to evaluate to true regardless of the internal value of NULL.

But, back to using our new variable ptr. Suppose now that we want to store in ptr the address of our integer variable k. To do this we use the unary '&' operator and write:

    ptr = &k;

What the '&' operator does is retrieve the lvalue (address) of k, even though k is on the right hand side of the assignment operator '=', and copies that to the contents of our pointer ptr. Now, ptr is said to "point to" k. Bear with us now, there is only one more operator we need to discuss.

The "dereferencing operator" is the asterisk and it is used as follows:

    *ptr = 7;

will copy 7 to the address pointed to by ptr. Thus if ptr "points to" (contains the address of) k, the above statement will set the value of k to 7. That is, when we use the '*' this way we are referring to the value of that which ptr is pointing to, not the value of the pointer itself.

    Similarly, we could write:

    printf("%d\n",*ptr);

to print to the screen the integer value stored at the address pointed to by "ptr".

One way to see how all this stuff fits together would be to run the following program and then review the code and the output carefully.

#include 

int j, k;
int *ptr;


int main(void)
{
   j = 1;
   k = 2;
   ptr = &k;
   printf("\n");
   printf("j has the value %d and is stored at %p\n",j,&j);
   printf("k has the value %d and is stored at %p\n",k,&k);
   printf("ptr has the value %p and is stored at %p\n",ptr,&ptr);
   printf("The value of the integer pointed to by ptr is %d\n",
           *ptr);
   return 0;
}

To review:

  • A variable is declared by giving it a type and a name (e.g. int k;)
  • A pointer variable is declared by giving it a type and a name (e.g. int *ptr) where the asterisk tells the compiler that the variable named ptr is a pointer variable and the type tells the compiler what type the pointer is to point to (integer in this case).
  • Once a variable is declared, we can get its address by preceding its name with the unary '&' operator, as in &k.
  • We can "dereference" a pointer, i.e. refer to the value of that which it points to, by using the unary '*' operator as in *ptr.
  • An "lvalue" of a variable is the value of its address, i.e. where it is stored in memory. The "rvalue" of a variable is the value stored in that variable (at that address).

References in Chapter 1:

  1. "The C Programming Language" 2nd Edition
    B. Kernighan and D. Ritchie
    Prentice Hall
    ISBN 0-13-110362-8

Copyright © 1998-2007, Games++ All rights reserved. | Privacy Policy