Tutorial on pointers and arrays in C/C++

A TUTORIAL ON POINTERS AND ARRAYS IN C

 

TABLE OF CONTENTS
INTRODUCTION
CHAPTER 1: What is a pointer? 

CHAPTER 2: Pointer types and Arrays 

CHAPTER 3: Pointers and Strings 

CHAPTER 4: More on Strings 

CHAPTER 5: Pointers and Structures 

CHAPTER 6: Some more on Strings, and Arrays of Strings 

CHAPTER 7: More on Multi-Dimensional Arrays 

CHAPTER 8: Pointers to Arrays 

CHAPTER 9: Pointers and Dynamic Allocation of Memory 

CHAPTER 10: Pointers to Functions 

INTRODUCTION
If you want to be proficient in the writing of code in the C programming language, you
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. I have also tried to carefully block the code within the text. That
way, with the help of an ASCII text editor, you can copy a given block of code to a new
file and compile it on your system. I recommend that readers do this as it will help in
understanding the material.


CHAPTER 1: What is a pointer?
One of those things beginners in C find difficult is the concept of pointers. The purpose
of this tutorial 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 of memory (on a
PC) to hold the value of the integer. It also sets up a symbol table. 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;
we expect that, at run time when this statement is executed, 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 is the value of the
integer stored there (2 in the above example) and the other the "value" of the memory
location, 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 II (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 is
made outside of any function ANSI compliant 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 guaranteed in such a way that it is guaranteed to not point to any C object or
function. A pointer initialized in this manner is called a "null" pointer.
The actual bit pattern used for a null pointer may or may not evaluate to zero since it
depends on the specific system on which the code is developed. To make the source code
compatible between various compilers on various systems, a macro is used to represent a
null pointer. That macro goes under the name NULL. Thus, setting the value of a pointer
using the NULL macro, as with an assignment statement such as ptr = NULL, guarantees
that the pointer has become a null pointer. Similarly, just as one can test for an integer
value of zero, as in if(k == 0), we can test for a null pointer using if (ptr == 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.

------------ Program 1.1 ---------------------------------
/* Program 1.1 from PTRTUT10.TXT 6/10/97 */

#include <stdio.h>
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, (void *)&j);
printf("k has the value %d and is stored at %p\n", k, (void *)&k);
printf("ptr has the value %p and is stored at %p\n", ptr, (void
*)&ptr);
printf("The value of the integer pointed to by ptr is %d\n", *ptr);
return 0;
}
Note: We have yet to discuss those aspects of C which require the use of the (void *)
expression used here. For now, include it in your test code. We'll explain the reason
behind this expression later.
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).

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