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.
Continue with Pointer Tutorial

Chapter 10

Pointers to Functions
Up to this point we have been discussing pointers to data objects. C also
permits the declaration of pointers to functions. Pointers to functions have a
variety of uses and some of them will be discussed here.
Consider the following real problem. You want to write a function that is
capable of sorting virtually any collection of data that can be stored in an
array. This might be an array of strings, or integers, or floats, or even
structures. The sorting algorithm can be the same for all. For example, it could
be a simple bubble sort algorithm, or the more complex shell or quick sort
algorithm. We'll use a simple bubble sort for demonstration purposes.
Sedgewick [1] has described the bubble sort using C code by setting up a
function which when passed a pointer to the array would sort it. If we call that
function bubble(), a sort program is described by bubble_1.c, which follows:
/*-------------------- bubble_1.c --------------------*/

/* Program bubble_1.c from PTRTUT10.HTM 6/13/97 */

#include

int arr[10] = { 3,6,1,2,3,8,4,1,7,2};

void bubble(int a[], int N);

int main(void)
{
int i;
putchar('\n');
for (i = 0; i < 10; i++)
{
printf("%d ", arr[i]);
}
bubble(arr,10);
putchar('\n');

for (i = 0; i < 10; i++)
{
printf("%d ", arr[i]);
}
return 0;
}

void bubble(int a[], int N)
{
int i, j, t;
for (i = N-1; i >= 0; i--)
{
for (j = 1; j <= i; j++)
{
if (a[j-1] > a[j])
{
t = a[j-1];
a[j-1] = a[j];
a[j] = t;
}
}
}
}



/*---------------------- end bubble_1.c -----------------------*/


The bubble sort is one of the simpler sorts. The algorithm scans the array from
the second to the last element comparing each element with the one which
precedes it. If the one that precedes it is larger than the current element, the
two are swapped so the larger one is closer to the end of the array. On the
first pass, this results in the largest element ending up at the end of the
array. The array is now limited to all elements except the last and the process
repeated. This puts the next largest element at a point preceding the largest
element. The process is repeated for a number of times equal to the number of
elements minus 1. The end result is a sorted array.
Here our function is designed to sort an array of integers. Thus in line 1 we
are comparing integers and in lines 2 through 4 we are using temporary integer
storage to store integers. What we want to do now is see if we can convert this
code so we can use any data type, i.e. not be restricted to integers.
At the same time we don't want to have to analyze our algorithm and the code
associated with it each time we use it. We start by removing the comparison from
within the function bubble() so as to make it relatively easy to modify the
comparison function without having to re-write portions related to the actual
algorithm. This results in bubble_2.c:
/*---------------------- bubble_2.c -------------------------*/

/* Program bubble_2.c from PTRTUT10.HTM 6/13/97 */

/* Separating the comparison function */

#include

int arr[10] = { 3,6,1,2,3,8,4,1,7,2};

void bubble(int a[], int N);
int compare(int m, int n);

int main(void)
{
int i;
putchar('\n');
for (i = 0; i < 10; i++)
{
printf("%d ", arr[i]);
}
bubble(arr,10);
putchar('\n');

for (i = 0; i < 10; i++)
{
printf("%d ", arr[i]);
}
return 0;
}

void bubble(int a[], int N)

{
int i, j, t;
for (i = N-1; i >= 0; i--)
{
for (j = 1; j <= i; j++)
{
if (compare(a[j-1], a[j]))
{
t = a[j-1];
a[j-1] = a[j];
a[j] = t;
}
}
}
}

int compare(int m, int n)
{
return (m > n);
}
/*--------------------- end of bubble_2.c -----------------------*/

If our goal is to make our sort routine data type independent, one way of doing
this is to use pointers to type void to point to the data instead of using the
integer data type. As a start in that direction let's modify a few things in the
above so that pointers can be used. To begin with, we'll stick with pointers to
type integer.
/*----------------------- bubble_3.c -------------------------*/

/* Program bubble_3.c from PTRTUT10.HTM 6/13/97 */

#include

int arr[10] = { 3,6,1,2,3,8,4,1,7,2};

void bubble(int *p, int N);
int compare(int *m, int *n);

int main(void)
{
int i;
putchar('\n');

for (i = 0; i < 10; i++)
{
printf("%d ", arr[i]);
}
bubble(arr,10);
putchar('\n');

for (i = 0; i < 10; i++)
{
printf("%d ", arr[i]);
}
return 0;
}

void bubble(int *p, int N)
{
int i, j, t;
for (i = N-1; i >= 0; i--)
{
for (j = 1; j <= i; j++)
{
if (compare(&p[j-1], &p[j]))
{
t = p[j-1];
p[j-1] = p[j];
p[j] = t;
}
}
}
}

int compare(int *m, int *n)
{
return (*m > *n);
}

/*------------------ end of bubble3.c -------------------------*/


Note the changes. We are now passing a pointer to an integer (or array of
integers) to bubble(). And from within bubble we are passing pointers to the
elements of the array that we want to compare to our comparison function. And,
of course we are dereferencing these pointer in our compare() function in order
to make the actual comparison. Our next step will be to convert the pointers in
bubble() to pointers to type void so that that function will become more type
insensitive. This is shown in bubble_4.
/*------------------ bubble_4.c ----------------------------*/

/* Program bubble_4.c from PTRTUT10,HTM 6/13/97 */

#include

int arr[10] = { 3,6,1,2,3,8,4,1,7,2};

void bubble(int *p, int N);
int compare(void *m, void *n);

int main(void)
{
int i;
putchar('\n');

for (i = 0; i < 10; i++)
{
printf("%d ", arr[i]);
}
bubble(arr,10);
putchar('\n');

for (i = 0; i < 10; i++)
{
printf("%d ", arr[i]);
}
return 0;
}

void bubble(int *p, int N)
{
int i, j, t;
for (i = N-1; i >= 0; i--)
{
for (j = 1; j <= i; j++)
{
if (compare((void *)&p[j-1], (void *)&p[j]))
{
t = p[j-1];
p[j-1] = p[j];
p[j] = t;
}
}
}
}

int compare(void *m, void *n)
{
int *m1, *n1;
m1 = (int *)m;
n1 = (int *)n;
return (*m1 > *n1);
}

/*------------------ end of bubble_4.c ---------------------*/


Note that, in doing this, in compare() we had to introduce the casting of the
void pointer types passed to the actual type being sorted. But, as we'll see
later that's okay. And since what is being passed to bubble() is still a pointer
to an array of integers, we had to cast these pointers to void pointers when we
passed them as parameters in our call to compare().
We now address the problem of what we pass to bubble(). We want to make the
first parameter of that function a void pointer also. But, that means that
within bubble() we need to do something about the variable t, which is currently
an integer. Also, where we use t = p[j-1]; the type of p[j-1] needs to be known
in order to know how many bytes to copy to the variable t (or whatever we
replace t with).
Currently, in bubble_4.c, knowledge within bubble() as to the type of the data
being sorted (and hence the size of each individual element) is obtained from
the fact that the first parameter is a pointer to type integer. If we are going
to be able to use bubble() to sort any type of data, we need to make that
pointer a pointer to type void. But, in doing so we are going to lose
information concerning the size of individual elements within the array. So, in
bubble_5.c we will add a separate parameter to handle this size information.
These changes, from bubble4.c to bubble5.c are, perhaps, a bit more extensive
than those we have made in the past. So, compare the two modules carefully for
differences.
/*---------------------- bubble5.c ---------------------------*/

/* Program bubble_5.c from PTRTUT10.HTM 6/13/97 */



#include
#include

long arr[10] = { 3,6,1,2,3,8,4,1,7,2};

void bubble(void *p, size_t width, int N);
int compare(void *m, void *n);

int main(void)
{
int i;
putchar('\n');

for (i = 0; i < 10; i++)
{
printf("%d ", arr[i]);
}
bubble(arr, sizeof(long), 10);
putchar('\n');

for (i = 0; i < 10; i++)
{
printf("%ld ", arr[i]);
}

return 0;
}

void bubble(void *p, size_t width, int N)
{
int i, j;
unsigned char buf[4];
unsigned char *bp = p;

for (i = N-1; i >= 0; i--)
{
for (j = 1; j <= i; j++)
{
if (compare((void *)(bp + width*(j-1)),
(void *)(bp + j*width))) /* 1 */
{
/* t = p[j-1]; */
memcpy(buf, bp + width*(j-1), width);
/* p[j-1] = p[j]; */
memcpy(bp + width*(j-1), bp + j*width , width);
/* p[j] = t; */
memcpy(bp + j*width, buf, width);
}
}
}
}

int compare(void *m, void *n)
{
long *m1, *n1;
m1 = (long *)m;
n1 = (long *)n;
return (*m1 > *n1);
}

/*--------------------- end of bubble5.c ---------------------*/


Note that I have changed the data type of the array from int to long to
illustrate the changes needed in the compare() function. Within bubble() I've
done away with the variable t (which we would have had to change from type int
to type long). I have added a buffer of size 4 unsigned characters, which is the
size needed to hold a long (this will change again in future modifications to
this code). The unsigned character pointer *bp is used to point to the base of
the array to be sorted, i.e. to the first element of that array.
We also had to modify what we passed to compare(), and how we do the swapping of
elements that the comparison indicates need swapping. Use of memcpy() and
pointer notation instead of array notation work towards this reduction in type
sensitivity.
Again, making a careful comparison of bubble5.c with bubble4.c can result in
improved understanding of what is happening and why.
We move now to bubble6.c where we use the same function bubble() that we used in
bubble5.c to sort strings instead of long integers. Of course we have to change
the comparison function since the means by which strings are compared is
different from that by which long integers are compared. And,in bubble6.c we
have deleted the lines within bubble() that were commented out in bubble5.c.
/*--------------------- bubble6.c ---------------------*/
/* Program bubble_6.c from PTRTUT10.HTM 6/13/97 */

#include
#include

#define MAX_BUF 256

char arr2[5][20] = { "Mickey Mouse",

"Donald Duck",

"Minnie Mouse",

"Goofy",

"Ted Jensen" };

void bubble(void *p, int width, int N);
int compare(void *m, void *n);

int main(void)
{
int i;
putchar('\n');

for (i = 0; i < 5; i++)
{
printf("%s\n", arr2[i]);
}
bubble(arr2, 20, 5);
putchar('\n\n');

for (i = 0; i < 5; i++)
{
printf("%s\n", arr2[i]);
}
return 0;
}

void bubble(void *p, int width, int N)
{
int i, j, k;
unsigned char buf[MAX_BUF];
unsigned char *bp = p;

for (i = N-1; i >= 0; i--)
{
for (j = 1; j <= i; j++)
{
k = compare((void *)(bp + width*(j-1)), (void *)(bp + j*width));
if (k > 0)
{
memcpy(buf, bp + width*(j-1), width);
memcpy(bp + width*(j-1), bp + j*width , width);
memcpy(bp + j*width, buf, width);
}
}
}
}

int compare(void *m, void *n)
{
char *m1 = m;
char *n1 = n;
return (strcmp(m1,n1));
}

/*------------------- end of bubble6.c ---------------------*/


But, the fact that bubble() was unchanged from that used in bubble5.c indicates
that that function is capable of sorting a wide variety of data types. What is
left to do is to pass to bubble() the name of the comparison function we want to
use so that it can be truly universal. Just as the name of an array is the
address of the first element of the array in the data segment, the name of a
function decays into the address of that function in the code segment. Thus we
need to use a pointer to a function. In this case the comparison function.
Pointers to functions must match the functions pointed to in the number and
types of the parameters and the type of the return value. In our case, we
declare our function pointer as:
int (*fptr)(const void *p1, const void *p2);

Note that were we to write:
int *fptr(const void *p1, const void *p2);

we would have a function prototype for a function which returned a pointer to
type int. That is because in C the parenthesis () operator have a higher
precedence than the pointer * operator. By putting the parenthesis around the
string (*fptr) we indicate that we are declaring a function pointer.
We now modify our declaration of bubble() by adding, as its 4th parameter, a
function pointer of the proper type. It's function prototype becomes:
void bubble(void *p, int width, int N,
int(*fptr)(const void *, const void *));

When we call the bubble(), we insert the name of the comparison function that we
want to use. bubble7.c illustrate how this approach permits the use of the same
bubble() function for sorting different types of data.
/*------------------- bubble7.c ------------------*/

/* Program bubble_7.c from PTRTUT10.HTM 6/10/97 */

#include
#include

#define MAX_BUF 256

long arr[10] = { 3,6,1,2,3,8,4,1,7,2};
char arr2[5][20] = { "Mickey Mouse",
"Donald Duck",
"Minnie Mouse",
"Goofy",
"Ted Jensen" };

void bubble(void *p, int width, int N,
int(*fptr)(const void *, const void *));
int compare_string(const void *m, const void *n);
int compare_long(const void *m, const void *n);

int main(void)
{
int i;
puts("\nBefore Sorting:\n");

for (i = 0; i < 10; i++) /* show the long ints */
{
printf("%ld ",arr[i]);
}
puts("\n");

for (i = 0; i < 5; i++) /* show the strings */
{
printf("%s\n", arr2[i]);
}
bubble(arr, 4, 10, compare_long); /* sort the longs */
bubble(arr2, 20, 5, compare_string); /* sort the strings */
puts("\n\nAfter Sorting:\n");

for (i = 0; i < 10; i++) /* show the sorted longs */
{
printf("%d ",arr[i]);
}
puts("\n");

for (i = 0; i < 5; i++) /* show the sorted strings */
{
printf("%s\n", arr2[i]);
}
return 0;
}

void bubble(void *p, int width, int N,
int(*fptr)(const void *, const void *))
{
int i, j, k;
unsigned char buf[MAX_BUF];
unsigned char *bp = p;

for (i = N-1; i >= 0; i--)
{
for (j = 1; j <= i; j++)
{
k = fptr((void *)(bp + width*(j-1)), (void *)(bp + j*width));
if (k > 0)
{
memcpy(buf, bp + width*(j-1), width);
memcpy(bp + width*(j-1), bp + j*width , width);
memcpy(bp + j*width, buf, width);
}
}
}
}

int compare_string(const void *m, const void *n)
{
char *m1 = (char *)m;
char *n1 = (char *)n;
return (strcmp(m1,n1));
}

int compare_long(const void *m, const void *n)
{
long *m1, *n1;
m1 = (long *)m;
n1 = (long *)n;
return (*m1 > *n1);
}

Chapter 8

Pointers to Arrays
Pointers, of course, can be "pointed at" any type of data object, including
arrays. While that was evident when we discussed program 3.1, it is important to
expand on how we do this when it comes to multi-dimensional arrays.
To review, in Chapter 2 we stated that given an array of integers we could point
an integer pointer at that array using:
int *ptr;
ptr = &my_array[0]; /* point our pointer at the first
integer in our array */

As we stated there, the type of the pointer variable must match the type of the
first element of the array.
In addition, we can use a pointer as a formal parameter of a function which is
designed to manipulate an array. e.g.
Given:
int array[3] = {'1', '5', '7'};
void a_func(int *p);

Some programmers might prefer to write the function prototype as:
void a_func(int p[]);

which would tend to inform others who might use this function that the function
is designed to manipulate the elements of an array. Of course, in either case,
what actually gets passed is the value of a pointer to the first element of the
array, independent of which notation is used in the function prototype or
definition. Note that if the array notation is used, there is no need to pass
the actual dimension of the array since we are not passing the whole array, only
the address to the first element.
We now turn to the problem of the 2 dimensional array. As stated in the last
chapter, C interprets a 2 dimensional array as an array of one dimensional
arrays. That being the case, the first element of a 2 dimensional array of
integers is a one dimensional array of integers. And a pointer to a two
dimensional array of integers must be a pointer to that data type. One way of
accomplishing this is through the use of the keyword "typedef". typedef assigns
a new name to a specified data type. For example:
typedef unsigned char byte;

causes the name byte to mean type unsigned char. Hence
byte b[10]; would be an array of unsigned characters.

Note that in the typedef declaration, the word byte has replaced that which
would normally be the name of our unsigned char. That is, the rule for using
typedef is that the new name for the data type is the name used in the
definition of the data type. Thus in:
typedef int Array[10];

Array becomes a data type for an array of 10 integers. i.e. Array my_arr;
declares my_arr as an array of 10 integers and Array arr2d[5]; makes arr2d an
array of 5 arrays of 10 integers each.
Also note that Array *p1d; makes p1d a pointer to an array of 10 integers.
Because *p1d points to the same type as arr2d, assigning the address of the two
dimensional array arr2d to p1d, the pointer to a one dimensional array of 10
integers is acceptable. i.e. p1d = &arr2d[0]; or p1d = arr2d; are both correct.
Since the data type we use for our pointer is an array of 10 integers we would
expect that incrementing p1d by 1 would change its value by 10*sizeof(int),
which it does. That is, sizeof(*p1d) is 20. You can prove this to yourself by
writing and running a simple short program.
Now, while using typedef makes things clearer for the reader and easier on the
programmer, it is not really necessary. What we need is a way of declaring a
pointer like p1d without the need of the typedef keyword. It turns out that this
can be done and that
int (*p1d)[10];

is the proper declaration, i.e. p1d here is a pointer to an array of 10 integers
just as it was under the declaration using the Array type. Note that this is
different from
int *p1d[10];

which would make p1d the name of an array of 10 pointers to type int.
Continue with Pointer Tutorial
Back to Table of Contents

Chapter 7

More on Multi-Dimensional Arrays
In the previous chapter we noted that given
#define ROWS 5
#define COLS 10

int multi[ROWS][COLS];

we can access individual elements of the array multi using either:
multi[row][col]

or
*(*(multi + row) + col)

To understand more fully what is going on, let us replace
*(multi + row)

with X as in:
*(X + col)

Now, from this we see that X is like a pointer since the expression is
de-referenced and we know that col is an integer. Here the arithmetic being used
is of a special kind called "pointer arithmetic" is being used. That means that,
since we are talking about an integer array, the address pointed to by (i.e.
value of) X + col + 1 must be greater than the address X + col by and amount
equal to sizeof(int).
Since we know the memory layout for 2 dimensional arrays, we can determine that
in the expression multi + row as used above, multi + row + 1 must increase by
value an amount equal to that needed to "point to" the next row, which in this
case would be an amount equal to COLS * sizeof(int).
That says that if the expression *(*(multi + row) + col) is to be evaluated
correctly at run time, the compiler must generate code which takes into
consideration the value of COLS, i.e. the 2nd dimension. Because of the
equivalence of the two forms of expression, this is true whether we are using
the pointer expression as here or the array expression multi[row][col].
Thus, to evaluate either expression, a total of 5 values must be known:
The address of the first element of the array, which is returned by the
expression multi, i.e., the name of the array.
The size of the type of the elements of the array, in this case sizeof(int).
The 2nd dimension of the array
The specific index value for the first dimension, row in this case.
The specific index value for the second dimension, col in this case.
Given all of that, consider the problem of designing a function to manipulate
the element values of a previously declared array. For example, one which would
set all the elements of the array multi to the value 1.
void set_value(int m_array[][COLS])
{
int row, col;
for (row = 0; row < ROWS; row++)
{
for (col = 0; col < COLS; col++)
{
m_array[row][col] = 1;
}
}
}


And to call this function we would then use:
set_value(multi);

Now, within the function we have used the values #defined by ROWS and COLS that
set the limits on the for loops. But, these #defines are just constants as far
as the compiler is concerned, i.e. there is nothing to connect them to the array
size within the function. row and col are local variables, of course. The formal
parameter definition permits the compiler to determine the characteristics
associated with the pointer value that will be passed at run time. We really
don’t need the first dimension and, as will be seen later, there are occasions
where we would prefer not to define it within the parameter definition, out of
habit or consistency, I have not used it here. But, the second dimension must be
used as has been shown in the expression for the parameter. The reason is that
we need this in the evaluation of m_array[row][col] as has been described. While
the parameter defines the data type (int in this case) and the automatic
variables for row and column are defined in the for loops, only one value can be
passed using a single parameter. In this case, that is the value of multi as
noted in the call statement, i.e. the address of the first element, often
referred to as a pointer to the array. Thus, the only way we have of informing
the compiler of the 2nd dimension is by explicitly including it in the parameter
definition.
In fact, in general all dimensions of higher order than one are needed when
dealing with multi-dimensional arrays. That is if we are talking about 3
dimensional arrays, the 2nd and 3rd dimension must be specified in the parameter
definition.
Continue with Pointer Tutorial
Back to Table of Contents

Chapter 6

Some more on Strings, and Arrays of Strings
Well, let's go back to strings for a bit. In the following all assignments are
to be understood as being global, i.e. made outside of any function, including
main().
We pointed out in an earlier chapter that we could write:
char my_string[40] = "Ted";

which would allocate space for a 40 byte array and put the string in the first 4
bytes (three for the characters in the quotes and a 4th to handle the
terminating '\0').
Actually, if all we wanted to do was store the name "Ted" we could write:
char my_name[] = "Ted";

and the compiler would count the characters, leave room for the nul character
and store the total of the four characters in memory the location of which would
be returned by the array name, in this case my_name.
In some code, instead of the above, you might see:
char *my_name = "Ted";

which is an alternate approach. Is there a difference between these? The answer
is.. yes. Using the array notation 4 bytes of storage in the static memory block
are taken up, one for each character and one for the terminating nul character.
But, in the pointer notation the same 4 bytes required, plus N bytes to store
the pointer variable my_name (where N depends on the system but is usually a
minimum of 2 bytes and can be 4 or more).
In the array notation, my_name is short for &myname[0] which is the address of
the first element of the array. Since the location of the array is fixed during
run time, this is a constant (not a variable). In the pointer notation my_name
is a variable. As to which is the better method, that depends on what you are
going to do within the rest of the program.
Let's now go one step further and consider what happens if each of these
declarations are done within a function as opposed to globally outside the
bounds of any function.
void my_function_A(char *ptr)
{
char a[] = "ABCDE"
.
.
}


void my_function_B(char *ptr)
{
char *cp = "FGHIJ"
.
.
}

In the case of my_function_A, the content, or value(s), of the array a[] is
considered to be the data. The array is said to be initialized to the values
ABCDE. In the case of my_function_B, the value of the pointer cp is considered
to be the data. The pointer has been initialized to point to the string FGHIJ.
In both my_function_A and my_function_B the definitions are local variables and
thus the string ABCDE is stored on the stack, as is the value of the pointer cp.
The string FGHIJ can be stored anywhere. On my system it gets stored in the data
segment.
By the way, array initialization of automatic variables as I have done in
my_function_A was illegal in the older K&R C and only "came of age" in the newer
ANSI C. A fact that may be important when one is considering portability and
backwards compatibility.
As long as we are discussing the relationship/differences between pointers and
arrays, let's move on to multi-dimensional arrays. Consider, for example the
array:
char multi[5][10];

Just what does this mean? Well, let's consider it in the following light.
char multi[5][10];

Let's take the underlined part to be the "name" of an array. Then prepending the
char and appending the [10] we have an array of 10 characters. But, the name
multi[5] is itself an array indicating that there are 5 elements each being an
array of 10 characters. Hence we have an array of 5 arrays of 10 characters
each..
Assume we have filled this two dimensional array with data of some kind. In
memory, it might look as if it had been formed by initializing 5 separate arrays
using something like:
multi[0] = {'0','1','2','3','4','5','6','7','8','9'}
multi[1] = {'a','b','c','d','e','f','g','h','i','j'}
multi[2] = {'A','B','C','D','E','F','G','H','I','J'}
multi[3] = {'9','8','7','6','5','4','3','2','1','0'}
multi[4] = {'J','I','H','G','F','E','D','C','B','A'}



At the same time, individual elements might be addressable using syntax such as:

multi[0][3] = '3'
multi[1][7] = 'h'
multi[4][0] = 'J'

Since arrays are contiguous in memory, our actual memory block for the above
should look like:
0123456789abcdefghijABCDEFGHIJ9876543210JIHGFEDCBA
^
|_____ starting at the address &multi[0][0]


Note that I did not write multi[0] = "0123456789". Had I done so a terminating
'\0' would have been implied since whenever double quotes are used a '\0'
character is appended to the characters contained within those quotes. Had that
been the case I would have had to set aside room for 11 characters per row
instead of 10.
My goal in the above is to illustrate how memory is laid out for 2 dimensional
arrays. That is, this is a 2 dimensional array of characters, NOT an array of
"strings".
Now, the compiler knows how many columns are present in the array so it can
interpret multi + 1 as the address of the 'a' in the 2nd row above. That is, it
adds 10, the number of columns, to get this location. If we were dealing with
integers and an array with the same dimension the compiler would add
10*sizeof(int) which, on my machine, would be 20. Thus, the address of the 9 in
the 4th row above would be &multi[3][0] or *(multi + 3) in pointer notation. To
get to the content of the 2nd element in the 4th row we add 1 to this address
and dereference the result as in
*(*(multi + 3) + 1)

With a little thought we can see that:
*(*(multi + row) + col) and
multi[row][col] yield the same results.

The following program illustrates this using integer arrays instead of character
arrays.
------------------- program 6.1 ----------------------

/* Program 6.1 from PTRTUT10.HTM 6/13/97*/

#include
#define ROWS 5
#define COLS 10

int multi[ROWS][COLS];

int main(void)
{
int row, col;
for (row = 0; row < ROWS; row++)
{
for (col = 0; col < COLS; col++)
{
multi[row][col] = row*col;
}
}

for (row = 0; row < ROWS; row++)
{
for (col = 0; col < COLS; col++)
{
printf("\n%d ",multi[row][col]);
printf("%d ",*(*(multi + row) + col));
}
}

return 0;
}
----------------- end of program 6.1 ---------------------

Because of the double de-referencing required in the pointer version, the name
of a 2 dimensional array is often said to be equivalent to a pointer to a
pointer. With a three dimensional array we would be dealing with an array of
arrays of arrays and some might say its name would be equivalent to a pointer to
a pointer to a pointer. However, here we have initially set aside the block of
memory for the array by defining it using array notation. Hence, we are dealing
with a constant, not a variable. That is we are talking about a fixed address
not a variable pointer. The dereferencing function used above permits us to
access any element in the array of arrays without the need of changing the value
of that address (the address of multi[0][0] as given by the symbol multi).
Continue with Pointer Tutorial
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Chapter 5

Pointers and Structures
As you may know, we can declare the form of a block of data containing different
data types by means of a structure declaration. For example, a personnel file
might contain structures which look something like:
struct tag {
char lname[20]; /* last name */
char fname[20]; /* first name */
int age; /* age */
float rate; /* e.g. 12.75 per hour */
};

Let's say we have a bunch of these structures in a disk file and we want to read
each one out and print out the first and last name of each one so that we can
have a list of the people in our files. The remaining information will not be
printed out. We will want to do this printing with a function call and pass to
that function a pointer to the structure at hand. For demonstration purposes I
will use only one structure for now. But realize the goal is the writing of the
function, not the reading of the file which, presumably, we know how to do.
For review, recall that we can access structure members with the dot operator as
in:
--------------- program 5.1 ------------------

/* Program 5.1 from PTRTUT10.HTM 6/13/97 */


#include
#include

struct tag {
char lname[20]; /* last name */
char fname[20]; /* first name */
int age; /* age */
float rate; /* e.g. 12.75 per hour */
};

struct tag my_struct; /* declare the structure my_struct */

int main(void)
{
strcpy(my_struct.lname,"Jensen");
strcpy(my_struct.fname,"Ted");
printf("\n%s ",my_struct.fname);
printf("%s\n",my_struct.lname);
return 0;
}

-------------- end of program 5.1 --------------

Now, this particular structure is rather small compared to many used in C
programs. To the above we might want to add:
date_of_hire; (data types not shown)
date_of_last_raise;
last_percent_increase;
emergency_phone;
medical_plan;
Social_S_Nbr;
etc.....

If we have a large number of employees, what we want to do is manipulate the
data in these structures by means of functions. For example we might want a
function print out the name of the employee listed in any structure passed to
it. However, in the original C (Kernighan & Ritchie, 1st Edition) it was not
possible to pass a structure, only a pointer to a structure could be passed. In
ANSI C, it is now permissible to pass the complete structure. But, since our
goal here is to learn more about pointers, we won't pursue that.
Anyway, if we pass the whole structure it means that we must copy the contents
of the structure from the calling function to the called function. In systems
using stacks, this is done by pushing the contents of the structure on the
stack. With large structures this could prove to be a problem. However, passing
a pointer uses a minimum amount of stack space.
In any case, since this is a discussion of pointers, we will discuss how we go
about passing a pointer to a structure and then using it within the function.
Consider the case described, i.e. we want a function that will accept as a
parameter a pointer to a structure and from within that function we want to
access members of the structure. For example we want to print out the name of
the employee in our example structure.
Okay, so we know that our pointer is going to point to a structure declared
using struct tag. We declare such a pointer with the declaration:
struct tag *st_ptr;

and we point it to our example structure with:
st_ptr = &my_struct;

Now, we can access a given member by de-referencing the pointer. But, how do we
de-reference the pointer to a structure? Well, consider the fact that we might
want to use the pointer to set the age of the employee. We would write:
(*st_ptr).age = 63;

Look at this carefully. It says, replace that within the parenthesis with that
which st_ptr points to, which is the structure my_struct. Thus, this breaks down
to the same as my_struct.age.
However, this is a fairly often used expression and the designers of C have
created an alternate syntax with the same meaning which is:
st_ptr->age = 63;

With that in mind, look at the following program:
------------ program 5.2 ---------------------

/* Program 5.2 from PTRTUT10.HTM 6/13/97 */

#include
#include

struct tag{ /* the structure type */
char lname[20]; /* last name */
char fname[20]; /* first name */
int age; /* age */
float rate; /* e.g. 12.75 per hour */
};

struct tag my_struct; /* define the structure */
void show_name(struct tag *p); /* function prototype */

int main(void)
{
struct tag *st_ptr; /* a pointer to a structure */
st_ptr = &my_struct; /* point the pointer to my_struct */
strcpy(my_struct.lname,"Jensen");
strcpy(my_struct.fname,"Ted");
printf("\n%s ",my_struct.fname);
printf("%s\n",my_struct.lname);
my_struct.age = 63;
show_name(st_ptr); /* pass the pointer */
return 0;
}

void show_name(struct tag *p)
{
printf("\n%s ", p->fname); /* p points to a structure */
printf("%s ", p->lname);
printf("%d\n", p->age);
}

-------------------- end of program 5.2 ----------------

Again, this is a lot of information to absorb at one time. The reader should
compile and run the various code snippets and using a debugger monitor things
like my_struct and p while single stepping through the main and following the
code down into the function to see what is happening.

Continue with Pointer Tutorial
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Chapter 4

More on Strings
Well, we have progressed quite a way in a short time! Let's back up a little and
look at what was done in Chapter 3 on copying of strings but in a different
light. Consider the following function:
char *my_strcpy(char dest[], char source[])
{
int i = 0;
while (source[i] != '\0')
{
dest[i] = source[i];
i++;
}
dest[i] = '\0';
return dest;
}

Recall that strings are arrays of characters. Here we have chosen to use array
notation instead of pointer notation to do the actual copying. The results are
the same, i.e. the string gets copied using this notation just as accurately as
it did before. This raises some interesting points which we will discuss.
Since parameters are passed by value, in both the passing of a character pointer
or the name of the array as above, what actually gets passed is the address of
the first element of each array. Thus, the numerical value of the parameter
passed is the same whether we use a character pointer or an array name as a
parameter. This would tend to imply that somehow source[i] is the same as
*(p+i).
In fact, this is true, i.e wherever one writes a[i] it can be replaced with *(a
+ i) without any problems. In fact, the compiler will create the same code in
either case. Thus we see that pointer arithmetic is the same thing as array
indexing. Either syntax produces the same result.
This is NOT saying that pointers and arrays are the same thing, they are not. We
are only saying that to identify a given element of an array we have the choice
of two syntaxes, one using array indexing and the other using pointer
arithmetic, which yield identical results.
Now, looking at this last expression, part of it.. (a + i), is a simple addition
using the + operator and the rules of C state that such an expression is
commutative. That is (a + i) is identical to (i + a). Thus we could write *(i +
a) just as easily as *(a + i).
But *(i + a) could have come from i[a] ! From all of this comes the curious
truth that if:
char a[20];
int i;

writing
a[3] = 'x';

is the same as writing
3[a] = 'x';

Try it! Set up an array of characters, integers or longs, etc. and assigned the
3rd or 4th element a value using the conventional approach and then print out
that value to be sure you have that working. Then reverse the array notation as
I have done above. A good compiler will not balk and the results will be
identical. A curiosity... nothing more!
Now, looking at our function above, when we write:
dest[i] = source[i];

due to the fact that array indexing and pointer arithmetic yield identical
results, we can write this as:
*(dest + i) = *(source + i);

But, this takes 2 additions for each value taken on by i. Additions, generally
speaking, take more time than incrementations (such as those done using the ++
operator as in i++). This may not be true in modern optimizing compilers, but
one can never be sure. Thus, the pointer version may be a bit faster than the
array version.
Another way to speed up the pointer version would be to change:
while (*source != '\0')

to simply
while (*source)

since the value within the parenthesis will go to zero (FALSE) at the same time
in either case.
At this point you might want to experiment a bit with writing some of your own
programs using pointers. Manipulating strings is a good place to experiment. You
might want to write your own versions of such standard functions as:
strlen();
strcat();
strchr();

and any others you might have on your system.
We will come back to strings and their manipulation through pointers in a future
chapter. For now, let's move on and discuss structures for a bit.
Continue with Pointer Tutorial
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CHAPTER 3

Pointers and Strings
The study of strings is useful to further tie in the relationship between
pointers and arrays. It also makes it easy to illustrate how some of the
standard C string functions can be implemented. Finally it illustrates how and
when pointers can and should be passed to functions.
In C, strings are arrays of characters. This is not necessarily true in other
languages. In BASIC, Pascal, Fortran and various other languages, a string has
its own data type. But in C it does not. In C a string is an array of characters
terminated with a binary zero character (written as '\0'). To start off our
discussion we will write some code which, while preferred for illustrative
purposes, you would probably never write in an actual program. Consider, for
example:
char my_string[40];

my_string[0] = 'T';
my_string[1] = 'e';
my_string[2] = 'd':
my_string[3] = '\0';


While one would never build a string like this, the end result is a string in
that it is an array of characters terminated with a nul character. By
definition, in C, a string is an array of characters terminated with the nul
character. Be aware that "nul" is not the same as "NULL". The nul refers to a
zero as defined by the escape sequence '\0'. That is it occupies one byte of
memory. NULL, on the other hand, is the name of the macro used to initialize
null pointers. NULL is #defined in a header file in your C compiler, nul may not
be #defined at all.
Since writing the above code would be very time consuming, C permits two
alternate ways of achieving the same thing. First, one might write:
char my_string[40] = {'T', 'e', 'd', '\0',};

But this also takes more typing than is convenient. So, C permits:
char my_string[40] = "Ted";

When the double quotes are used, instead of the single quotes as was done in the
previous examples, the nul character ( '\0' ) is automatically appended to the
end of the string.
In all of the above cases, the same thing happens. The compiler sets aside an
contiguous block of memory 40 bytes long to hold characters and initialized it
such that the first 4 characters are Ted\0.
Now, consider the following program:
------------------program 3.1-------------------------------------

/* Program 3.1 from PTRTUT10.HTM 6/13/97 */

#include

char strA[80] = "A string to be used for demonstration purposes";
char strB[80];

int main(void)
{

char *pA; /* a pointer to type character */
char *pB; /* another pointer to type character */
puts(strA); /* show string A */
pA = strA; /* point pA at string A */
puts(pA); /* show what pA is pointing to */
pB = strB; /* point pB at string B */
putchar('\n'); /* move down one line on the screen */
while(*pA != '\0') /* line A (see text) */
{
*pB++ = *pA++; /* line B (see text) */
}
*pB = '\0'; /* line C (see text) */
puts(strB); /* show strB on screen */
return 0;
}

--------- end program 3.1 -------------------------------------



In the above we start out by defining two character arrays of 80 characters
each. Since these are globally defined, they are initialized to all '\0's first.
Then, strA has the first 42 characters initialized to the string in quotes.
Now, moving into the code, we declare two character pointers and show the string
on the screen. We then "point" the pointer pA at strA. That is, by means of the
assignment statement we copy the address of strA[0] into our variable pA. We now
use puts() to show that which is pointed to by pA on the screen. Consider here
that the function prototype for puts() is:
int puts(const char *s);

For the moment, ignore the const. The parameter passed to puts() is a pointer,
that is the value of a pointer (since all parameters in C are passed by value),
and the value of a pointer is the address to which it points, or, simply, an
address. Thus when we write puts(strA); as we have seen, we are passing the
address of strA[0].
Similarly, when we write puts(pA); we are passing the same address, since we
have set pA = strA;
Given that, follow the code down to the while() statement on line A. Line A
states:
While the character pointed to by pA (i.e. *pA) is not a nul character (i.e. the
terminating '\0'), do the following:
Line B states: copy the character pointed to by pA to the space pointed to by
pB, then increment pA so it points to the next character and pB so it points to
the next space.
When we have copied the last character, pA now points to the terminating nul
character and the loop ends. However, we have not copied the nul character. And,
by definition a string in C must be nul terminated. So, we add the nul character
with line C.
It is very educational to run this program with your debugger while watching
strA, strB, pA and pB and single stepping through the program. It is even more
educational if instead of simply defining strB[] as has been done above,
initialize it also with something like:
strB[80] = "12345678901234567890123456789012345678901234567890"

where the number of digits used is greater than the length of strA and then
repeat the single stepping procedure while watching the above variables. Give
these things a try!
Getting back to the prototype for puts() for a moment, the "const" used as a
parameter modifier informs the user that the function will not modify the string
pointed to by s, i.e. it will treat that string as a constant.
Of course, what the above program illustrates is a simple way of copying a
string. After playing with the above until you have a good understanding of what
is happening, we can proceed to creating our own replacement for the standard
strcpy() that comes with C. It might look like:
char *my_strcpy(char *destination, char *source)
{
char *p = destination;
while (*source != '\0')
{
*p++ = *source++;
}
*p = '\0';
return destination;
}

In this case, I have followed the practice used in the standard routine of
returning a pointer to the destination.
Again, the function is designed to accept the values of two character pointers,
i.e. addresses, and thus in the previous program we could write:
int main(void)
{
my_strcpy(strB, strA);
puts(strB);
}

I have deviated slightly from the form used in standard C which would have the
prototype:
char *my_strcpy(char *destination, const char *source);

Here the "const" modifier is used to assure the user that the function will not
modify the contents pointed to by the source pointer. You can prove this by
modifying the function above, and its prototype, to include the "const" modifier
as shown. Then, within the function you can add a statement which attempts to
change the contents of that which is pointed to by source, such as:
*source = 'X';

which would normally change the first character of the string to an X. The const
modifier should cause your compiler to catch this as an error. Try it and see.
Now, let's consider some of the things the above examples have shown us. First
off, consider the fact that *ptr++ is to be interpreted as returning the value
pointed to by ptr and then incrementing the pointer value. This has to do with
the precedence of the operators. Were we to write (*ptr)++ we would increment,
not the pointer, but that which the pointer points to! i.e. if used on the first
character of the above example string the 'T' would be incremented to a 'U'. You
can write some simple example code to illustrate this.
Recall again that a string is nothing more than an array of characters, with the
last character being a '\0'. What we have done above is deal with copying an
array. It happens to be an array of characters but the technique could be
applied to an array of integers, doubles, etc. In those cases, however, we would
not be dealing with strings and hence the end of the array would not be marked
with a special value like the nul character. We could implement a version that
relied on a special value to identify the end. For example, we could copy an
array of positive integers by marking the end with a negative integer. On the
other hand, it is more usual that when we write a function to copy an array of
items other than strings we pass the function the number of items to be copied
as well as the address of the array, e.g. something like the following prototype
might indicate:
void int_copy(int *ptrA, int *ptrB, int nbr);

where nbr is the number of integers to be copied. You might want to play with
this idea and create an array of integers and see if you can write the function
int_copy() and make it work.
This permits using functions to manipulate large arrays. For example, if we have
an array of 5000 integers that we want to manipulate with a function, we need
only pass to that function the address of the array (and any auxiliary
information such as nbr above, depending on what we are doing). The array itself
does not get passed, i.e. the whole array is not copied and put on the stack
before calling the function, only its address is sent.
This is different from passing, say an integer, to a function. When we pass an
integer we make a copy of the integer, i.e. get its value and put it on the
stack. Within the function any manipulation of the value passed can in no way
effect the original integer. But, with arrays and pointers we can pass the
address of the variable and hence manipulate the values of the original
variables.
Continue with Pointer Tutorial
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Chapter 2

Pointer types and Arrays
Okay, let's move on. Let us consider why we need to identify the type of
variable that a pointer points to, as in:
int *ptr;

One reason for doing this is so that later, once ptr "points to" something, if
we write:
*ptr = 2;

the compiler will know how many bytes to copy into that memory location pointed
to by ptr. If ptr was declared as pointing to an integer, 2 bytes would be
copied, if a long, 4 bytes would be copied. Similarly for floats and doubles the
appropriate number will be copied. But, defining the type that the pointer
points to permits a number of other interesting ways a compiler can interpret
code. For example, consider a block in memory consisting if ten integers in a
row. That is, 20 bytes of memory are set aside to hold 10 integers.
Now, let's say we point our integer pointer ptr at the first of these integers.
Furthermore lets say that integer is located at memory location 100 (decimal).
What happens when we write:
ptr + 1;


Because the compiler "knows" this is a pointer (i.e. its value is an address)
and that it points to an integer (its current address, 100, is the address of an
integer), it adds 2 to ptr instead of 1, so the pointer "points to" the next
integer, at memory location 102. Similarly, were the ptr declared as a pointer
to a long, it would add 4 to it instead of 1. The same goes for other data types
such as floats, doubles, or even user defined data types such as structures.
This is obviously not the same kind of "addition" that we normally think of. In
C it is referred to as addition using "pointer arithmetic", a term which we will
come back to later.
Similarly, since ++ptr and ptr++ are both equivalent to ptr + 1 (though the
point in the program when ptr is incremented may be different), incrementing a
pointer using the unary ++ operator, either pre- or post-, increments the
address it stores by the amount sizeof(type) where "type" is the type of the
object pointed to. (i.e. 2 for an integer, 4 for a long, etc.).
Since a block of 10 integers located contiguously in memory is, by definition,
an array of integers, this brings up an interesting relationship between arrays
and pointers.
Consider the following:
int my_array[] = {1,23,17,4,-5,100};

Here we have an array containing 6 integers. We refer to each of these integers
by means of a subscript to my_array, i.e. using my_array[0] through my_array[5].
But, we could alternatively access them via a pointer as follows:
int *ptr;
ptr = &my_array[0]; /* point our pointer at the first
integer in our array */

And then we could print out our array either using the array notation or by
dereferencing our pointer. The following code illustrates this:
----------- Program 2.1 -----------------------------------

/* Program 2.1 from PTRTUT10.HTM 6/13/97 */

#include

int my_array[] = {1,23,17,4,-5,100};
int *ptr;

int main(void)
{
int i;
ptr = &my_array[0]; /* point our pointer to the first
element of the array */
printf("\n\n");
for (i = 0; i < 6; i++)
{
printf("my_array[%d] = %d ",i,my_array[i]); /*<-- A */
printf("ptr + %d = %d\n",i, *(ptr + i)); /*<-- B */
}
return 0;
}

Compile and run the above program and carefully note lines A and B and that the
program prints out the same values in either case. Also observe how we
dereferenced our pointer in line B, i.e. we first added i to it and then
dereferenced the new pointer. Change line B to read:
printf("ptr + %d = %d\n",i, *ptr++);

and run it again... then change it to:
printf("ptr + %d = %d\n",i, *(++ptr));

and try once more. Each time try and predict the outcome and carefully look at
the actual outcome.
In C, the standard states that wherever we might use &var_name[0] we can replace
that with var_name, thus in our code where we wrote:
ptr = &my_array[0];

we can write:
ptr = my_array;

to achieve the same result.
This leads many texts to state that the name of an array is a pointer. I prefer
to mentally think "the name of the array is the address of first element in the
array". Many beginners (including myself when I was learning) have a tendency to
become confused by thinking of it as a pointer. For example, while we can write
ptr = my_array;

we cannot write
my_array = ptr;

The reason is that while ptr is a variable, my_array is a constant. That is, the
location at which the first element of my_array will be stored cannot be changed
once my_array[] has been declared.
Earlier when discussing the term "lvalue" I cited K&R-2 where it stated:
"An object is a named region of storage; an lvalue is an expression referring
to an object".
This raises an interesting problem. Since my_array is a named region of storage,
why is my_array in the above assignment statement not an lvalue? To resolve this
problem, some refer to my_array as an "unmodifiable lvalue".
Modify the example program above by changing
ptr = &my_array[0];

to
ptr = my_array;

and run it again to verify the results are identical.
Now, let's delve a little further into the difference between the names ptr and
my_array as used above. Some writers will refer to an array's name as a constant
pointer. What do we mean by that? Well, to understand the term "constant" in
this sense, let's go back to our definition of the term "variable". When we
declare a variable we set aside a spot in memory to hold the value of the
appropriate type. Once that is done the name of the variable can be interpreted
in one of two ways. When used on the left side of the assignment operator, the
compiler interprets it as the memory location to which to move that value
resulting from evaluation of the right side of the assignment operator. But,
when used on the right side of the assignment operator, the name of a variable
is interpreted to mean the contents stored at that memory address set aside to
hold the value of that variable.
With that in mind, let's now consider the simplest of constants, as in:
int i, k;
i = 2;

Here, while i is a variable and then occupies space in the data portion of
memory, 2 is a constant and, as such, instead of setting aside memory in the
data segment, it is imbedded directly in the code segment of memory. That is,
while writing something like k = i; tells the compiler to create code which at
run time will look at memory location &i to determine the value to be moved to
k, code created by i = 2; simply puts the 2 in the code and there is no
referencing of the data segment. That is, both k and i are objects, but 2 is not
an object.
Similarly, in the above, since my_array is a constant, once the compiler
establishes where the array itself is to be stored, it "knows" the address of
my_array[0] and on seeing:
ptr = my_array;

it simply uses this address as a constant in the code segment and there is no
referencing of the data segment beyond that.
This might be a good place explain further the use of the (void *) expression
used in Program 1.1 of Chapter 1. As we have seen we can have pointers of
various types. So far we have discussed pointers to integers and pointers to
characters. In coming chapters we will be learning about pointers to structures
and even pointer to pointers.
Also we have learned that on different systems the size of a pointer can vary.
As it turns out it is also possible that the size of a pointer can vary
depending on the data type of the object to which it points. Thus, as with
integers where you can run into trouble attempting to assign a long integer to a
variable of type short integer, you can run into trouble attempting to assign
the values of pointers of various types to pointer variables of other types.
To minimize this problem, C provides for a pointer of type void. We can declare
such a pointer by writing:
void *vptr;

A void pointer is sort of a generic pointer. For example, while C will not
permit the comparison of a pointer to type integer with a pointer to type
character, for example, either of these can be compared to a void pointer. Of
course, as with other variables, casts can be used to convert from one type of
pointer to another under the proper circumstances. In Program 1.1. of Chapter 1
I cast the pointers to integers into void pointers to make them compatible with
the %p conversion specification. In later chapters other casts will be made for
reasons defined therein.
Well, that's a lot of technical stuff to digest and I don't expect a beginner to
understand all of it on first reading. With time and experimentation you will
want to come back and re-read the first 2 chapters. But for now, let's move on
to the relationship between pointers, character arrays, and strings.
Continue with Pointer Tutorial
Back to Table of Contents

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

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

Continue with Pointer Tutorial
Back to Table of Contents

Database

What are the different types of joins?

Explain normalization with examples.

What cursor type do you use to retrieve multiple recordsets?

Diffrence between a "where" clause and a "having" clause

What is the difference between "procedure" and "function"?

How will you copy the structure of a table without copying the data?

How to find out the database name from SQL*PLUS command prompt?

Tadeoffs with having indexes

Talk about "Exception Handling" in PL/SQL?

What is the diference between "NULL in C" and "NULL in Oracle?"

What is Pro*C? What is OCI?

Give some examples of Analytical functions.

What is the difference between "translate" and "replace"?

What is DYNAMIC SQL method 4?

How to remove duplicate records from a table?

What is the use of ANALYZing the tables?

How to run SQL script from a Unix Shell?

What is a "transaction"? Why are they necessary?

Explain Normalizationa dn Denormalization with examples.

When do you get contraint violtaion? What are the types of constraints?

How to convert RAW datatype into TEXT?

Difference - Primary Key and Aggregate Key

How functional dependency is related to database table design?

What is a "trigger"?

Why can a "group by" or "order by" clause be expensive to process?

What are "HINTS"? What is "index covering" of a query?

What is a VIEW? How to get script for a view?

What are the Large object types suported by Oracle?

What is SQL*Loader?

Difference between "VARCHAR" and "VARCHAR2" datatypes.

What is the difference among "dropping a table", "truncating a table" and "deleting all records" from a table.

Difference between "ORACLE" and "MICROSOFT ACCESS" databases.

How to create a database link ?

Here are few precautionary steps to follow in your efforts to avoid viruses:

􀂾 If students, or you, bring diskettes from outside (home) implement a “SCAN FIRST” policy. This
means that all diskettes or CD-ROMS (even those purchased from a store or distributor) MUST be
scanned for viruses prior to running applications from them or installation of a program.
• Insert the diskette/CD ROM/Zip disk into the appropriate drive but DO NOT open any files. Go
to your AVG Anti-Virus icon on the task bar or shortcut on your desktop and right click:
􀂃 Select “Run AVG Anti-Virus”. The AVG Anti-Virus System window opens (see picture
on page 16). If using the FREE version this window automatically opens.
􀂃 If using AVG Anti-Virus Professional Edition, double click on Program from the menu
bar, select and click Switch to Basic.
􀂃 Double click on Removable Media Test button.
􀂃 Select the device you wish to test (i.e. 3.5 floppy drive, zip drive, CD-ROM drive)
􀂃 Click on the Run button. The program scans the designated media for viruses. If a virus
is found on the media being scanned, a warning notice will be displayed on the screen
and then the program will prompt you to choose what you want the anti-virus program to
do. Use the help menu if you are unsure.
􀂾 Be selective about what email messages you decide to read. If your district provides email service
via a district server, by opening an infected email message you could infect the server. Check with
your district’s technology department regarding this issue.
􀂾 DO NOT download an attachment that is an executable file (.exe as the file extension) and be very
cautious of .zip files as well. You may not want to open email messages that have been forwarded
since viruses are known to attach and send themselves through a user’s address book.
􀂾 Update your anti-virus software DAILY and set your anti-virus program to scan the system DAILY.
Make sure that the computer will be ON, it cannot scan when the system is off.
􀂾 Update the operating system directly from the Microsoft web site (www.microsoft.com). You can
set your computer to do this automatically but again, the system must be on. Note: Microsoft does
not send email updates

How can I protect my computer from a virus?

As the saying goes “the best defense is a good offense,” install and update an anti-virus program
IMMEDIATELY. We recommend using AVG Anti-Virus from Grisoft but there are other anti-virus
programs used so contact your district’s technology department if you do not have such a program
installed on your classroom computer. You can download a FREE version of AVG by going to the
URL: www.grisoft.com. Grisoft does not provide technical support for users of the free version.
Before downloading any software, close all running/open programs. If downloading from the Grisoft web site, follow the
instructions as given on the site:
• Grisoft emails the Registration Number for each program therefore you must have an email account to install AVG.
• Once you select the download button, the AVG License Agreement is displayed. Read through the agreement and
the click “Yes, I Agree” button.
• Next the Registration page opens. Fill in the Questionnaire Form and then click the “Continue” button at the
bottom of the page.
• You will be prompted to choose the path where the program is going to be stored. The default window option is to
“Save this program to disk”, click “OK”.
• The “Save As” window will open. In the “Save In” box click the down arrow and select the desktop (if this is not
the default on your computer). Click the “Save” button. Download will begin.
• Once download is completed AVG will have sent you an email with the program’s registration number. Check
your email after downloading the program. Open the email sent from Grisoft, highlight and copy the
Registration Number from the email message. You must insert this number during setup.
• Once you have obtained the registration number click on the “Open” button at the bottom of the download
window, OR locate the icon on your desktop and double-click it, the AVG-6.0 Anti-Virus
System Setup window opens. Click on the “Setup” button and follow the instructions from
the Setup screen, pasting the Registration Number (that you copied from the email) in the
appropriate dialog box.
• Once installed, delete the .exe icon from your desktop. Right click on the icon and select “delete” from the menu.
It will tell you that you will be unable to run the program if you delete that file, ignore this and click “Yes” or
“Continue.” You already have the program installed therefore you no longer need this file.
Follow the steps below to configure the program after installation is complete:
First Phase of Setup
1) Once the program is installed you MUST configure your virus
protection.
2) Double click on the shortcut icon on the desktop to open the
program and choose the Control Center. In the “Resident
Shield” tab window, check all three boxes in the “Check
Viruses” box. Also, check “Confirmation” and “Ask what to
do next” options. Click the “Apply” button.
3) Click on the “Email Scanner” tab and check all boxes. Click the “Apply” button.
4) Click on the “Update Manager” tab. This is crucial to the configuration
because this is where you will set AVG to download updates to the
program. If you do not update the program on a regular basis it is
pointless to have it installed.
a. Click the box next to “Allow scheduled update”. Set “Update to
start at:” The program cannot update when the computer is off so
set it to update at a time when you first turn your computer on, enter
the time (use the up/down arrows to the right of the box to select a
time).
b. The next option is “Update if the database is older than:”. We advise updating every 5 days. The
system must be restarted after update is completed.
c. The next option is “If not successful than repeat in:” set this at 1 to 5
days.
d. Click the “Apply” button.
5) Click on the “Scheduler” tab. Click to check the box “Enable scheduled
tests.” Click the “Apply” button. Click the “OK” button. The window will
close.
6) If you need to know what version of AVG you installed and/or the serial
number, click on the “Information tab.” This also is where you will find the
date of the last virus update performed.
Second Phase of Setup
It is imperative that you run an AVG “Complete Test” on your system because it will intercept viruses entering through
email but it will NOT intercept a virus obtained through browsing a web site until it runs the “Complete Test.”
7) Right click on the AVG icon in the system tray on the task bar, select and click Run
AVG Anti-Virus or double click on the AVG shortcut icon on the desktop.
8) Click on the “Scheduler” button. Set the time that you want AVG to
automatically run the complete test. Click in the box to check
“Enable scheduled tests” and then click the “OK” button.
􀂾 Enter a time when the computer will not be in use (when at
lunch, recess or on duty). The computer MUST be ON for
AVG, or any other virus protection program, to scan for viruses!
Final Step of Installation:
9) Click on the “Virus Database” button. The AVG Center window will open (see #4).
10) Click on the “Update Now!” button.
11) Once completed retrieving updates, a window opens displaying all of the new update files, you must restart
your computer in order to install and update the AVG Anti-Virus program. (Make sure that you do not have
any open files before you restart your computer, if so you must save all work and exit the programs first.)
12) Uninstall, properly, any other virus protection program if going to use AVG.
Below is a list of common methods of virus infection and steps to take to avoid them. While the
numbers of viruses continues to grow in leaps and bounds, a few smart moves to protect the system can
help keep the problems at bay.
• Garbage In, Garbage Out: This is one of those old computer clichés that is so true in the
computer world and elsewhere. The only way to get a virus infection is by allowing infected
software into the PC or through email and Internet sites. Viruses cannot spontaneously generate
on a PC.
• More Connections Means More Risk: The more ways you interface your PC to others, the
more chances there are of a virus making its way onto your system. A standalone PC with a
stable software base has much less chance of becoming infected by a virus than a PC shared by
multiple users that is connected to a large network.
• Piracy Has Its Price: While infections from store-bought software happen, they are extremely
rare. On the other hand, software that is shared from PC to PC, or worse is obtained from illegal
sources, has a much higher chance of being infected.
• Use Backups: If you have the ability and the discipline to maintain multiple backups of your
system over a period of time, this is a useful "last ditch" defense against virus infection. It doesn't
really prevent viruses from striking your system, but it can save you in the event that you are
unlucky and suffer data loss due to viruses. (Note that you need to have a reasonably long

Region 4 Teaching, Learning Technology Center, Revised: 10/2003
Rima Duhon
retention period in your backup cycle for this to work. If you just backup your entire disk onto
the same backup tape once every week, then you only have one week at most to catch any given
virus before you end up copying it onto the backup tape as well.)
• Control Access to Your PC: You should be careful about who uses your system. Generally
speaking, a PC in an open area used by dozens of people, such as in the classroom, will develop
viruses far more often than one on an individual's desk. The reasons are obvious.

How do viruses infect your computer and how are they spread?

The fact of the matter is that you can get a virus by reading email messages and by downloading an
attachment infected with the virus. You must be careful of files attached to email messages that must be
downloaded. If the attached file is an executable program (has the extension “.exe”), a Word document
or any other file that is capable of being infected with a true virus, it may have a virus in it.
REMEMBER, the email itself can carry a real virus therefore the best practice (if you do not have a
virus protection program) may be NOT to open emails from someone that you do not know or that have
been forwarded (“FWD” in the subject line) since many viruses attach to a user’s email address book
and forward themselves to others, unknowingly to the user.

VIRUSES

Viruses are unwanted programs that spread from computer to computer, much the way real viruses do in
humans and animals. They are sometimes very dangerous and can in fact wipe out your entire hard disk
if you are unlucky--and if you don't protect yourself. Taking steps to protect your computer from viruses
is an essential part of any data problem prevention routine.
What is a virus? A computer virus is a program that attaches to other pieces of code, so that when the
user tries to run the original they also unintentionally run the virus code as well; the virus code is
designed to replicate itself and "infect" other programs, possibly in a modified form, and may also
exhibit other behavior as well. So, in order to be a virus, the program must have the ability to do all of
the following:
• Run without the user wanting it to and/or create effects that the programmer wants but that the
user did not want or request.
• Have the ability to "infect" or modify other files or disk structures.
• Replicate itself so it can spread to other files or systems.
Note one thing that is not on this list: a virus does not necessarily have to trash your hard drive or exhibit
other malicious behavior, in order to be a virus. While many viruses do damage files and disk structures,
many are just nuisances or exhibit "prank" behavior such as playing music on the PC speaker or putting
funny phrases on the screen when the system is booted. However, the risk of damage from viruses is
substantial. Many can cause serious data loss; sometimes the virus writer doesn't even intend some of
the effects that the virus produces (viruses can have bugs!). Damage can also occur from program files
being altered when the virus infects them. Often it is not possible to repair the damage, even when the
virus is removed.
There are many different types of viruses. In addition to the classical virus, there are other virus-like
programs that are similar to viruses in terms of how they work and what they do, but differ from them in
one or more respect:
• Worms: A worm is a program that is self-contained and when run, has the ability to spread itself to other systems.
In essence, a worm is a virus that doesn't infect other programs. Instead, it acts independently, seeking to spread to
other computers connected to its current host. They tend to spread over network connections. They can have other
undesirable effects when run.
Note: The acronym "WORM" is also used as a short form for "write once, read many", a storage technology that is
used by devices such as CD-R drives. The concepts are totally unrelated.
• Trojan Horses: A trojan horse is any program that, once run, does something that the user doesn't want or request.
The program doesn't necessarily infect other files or spread to other systems. It is the generic term to refer to any
software that is intentionally coded to do something other than what it is supposed to. Some people think of viruses
as a special form of trojan horse: one that can infect other files (thus turning them into trojan horses) and duplicate
itself. Trojan horses are sometimes just called "trojans" for short.
• Bugs: A bug is an error in a program. It is included here even though it really isn't in the same class as viruses and
trojans, because it is similar to a trojan horse in that it causes behavior other than what the user wanted. The
difference of course is that with a bug, the aberrant behavior is unintentional! With a trojan horse the author is doing
it on purpose.
• Droppers: A dropper is a program designed to install or deliver a virus or trojan horse onto a target system. The
dropper is specially designed to avoid detection by standard virus detection programs, because the virus is specially
encrypted so that the dropper itself doesn't appear to the virus scanners like a regular infected program file would. In
some ways, a dropper is like a "virus egg", waiting to be hatched. They are uncommon.
• Virus Impostors (Joke Programs): Some oh-so-clever programmers have devised cute programs that mimic the
effects of true viruses when they are run but are not considered viruses themselves, or even trojan horses, because
here the user of the file knows that the program is going to do something strange. These are often installed by
someone as a practical joke on his/her coworkers or friends for a good laugh.

I was inserting text into my document and it was erasing the text as I typed rather than inserting.

Your “Insert” key has been toggled off. Push the “Insert” key on the keyboard. Go to “Edit” on the
Menu Bar and select “Undo” until you get back to the point where you were trying to insert text and try
again.
• If working in Microsoft Word, you can “Undo” unlimited times but other programs only “Undo”
the last action performed.
• Another method to toggle “insert” on, double click on the letters “OVR” or “OUS” on the
bottom of the program window.
􀂾 If the letters “OVR” or “OUS” are greyed out, insert is toggled on.
􀂾 If the letters “OVR” or “OUS” are black, insert is toggled off.

Somebody has changed my desktop, the mouse pointer and the sounds are different too. How do I go back to the original settings?

Children will play around and change the settings on your computer; many of them have a greater
knowledge of technology than those of us teaching them. This, like #15 above, is not difficult to change
but it does require several steps in the process to make the necessary changes.
• See #15 to change the desktop back to the default setting.
• To Reset the Mouse Pointer:
􀂾 Go to Start on the task bar.
􀂾 Select Settings then Control Panel.
􀂾 The Control Panel window will open. Double click on the Mouse
icon.
􀂾 The Mouse Properties window will open. Click on the Pointers
tab.
􀂾 In the Scheme box, click the downward arrow and select None.
􀂾 You will have to select each individual category and click the Use Default button for each
one until all settings have been returned to the default setting.
􀂾 Once the task is completed, click the Apply” button and then click the OK button.
• To Reset the Mouse Speed or Cursor Trails:
􀂾 In the Mouse Properties window select the Motion tab.
􀂾 Here you can reset the speed of the cursor movement on
the screen.
􀂾 If the cursor trails the box next to Enable will be
checked. Deselect the box by clicking in it
• To Reset the Sounds:
􀂾 Go to Start on the task bar. Select Settings then Control
Panel.
􀂾 Double click on the Sounds icon.
􀂾 The Sounds Properties window will
open. Click on the down pointing arrow in the last
Schemes section of the window. Select Windows
Default, click the Apply button, and then click the OK
button.
􀂾 Sounds will return to the original default settings.

Somebody has changed my desktop, the mouse pointer and the sounds are different too. How do I go back to the original settings?

Children will play around and change the settings on your computer; many of them have a greater
knowledge of technology than those of us teaching them. This, like #15 above, is not difficult to change
but it does require several steps in the process to make the necessary changes.
• See #15 to change the desktop back to the default setting.
• To Reset the Mouse Pointer:
􀂾 Go to Start on the task bar.
􀂾 Select Settings then Control Panel.
􀂾 The Control Panel window will open. Double click on the Mouse
icon.
􀂾 The Mouse Properties window will open. Click on the Pointers
tab.
􀂾 In the Scheme box, click the downward arrow and select None.

One of my students has put a picture from the Internet on my desktop. How do I get it back to the regular desktop?

This is a common problem for classroom teachers but it is easily remedied so do not panic.
• Go to Start on the task bar.
• Select Settings, Control Panel.
• The Control Panel window will open. Double click on the Display icon.
• The Display Properties window will open (Background Tab should be active in the window).
• Select None in the Wallpaper window, Click the Apply button
• Click the OK button.
• You may want to adopt a “No Download” policy to avoid this
in the future. This could cause a virus problem on your system
(Nimbda can spread from Internet web sites).
Another method of accessing the Display Properties:
• Right click on the desktop, select Properties from the pop-up
menu.
• Follow the steps above once the Display Properties window opens

When I try to start a program from its shortcut I get a "Missing Program" error.

A "shortcut" is a path from the icon to the actual location of the program files for that program. If this
path is absent or invalid, you will get this error. This is usually caused by having deleted or moved the
program. If this is the case, delete the shortcut.
• If you moved the program, delete the old shortcut and create a new shortcut to the new location.
• The program may have been deleted without being properly uninstalled. Go to the
Control Panel and properly delete the program through the Add/Remove
Programs option.
• The Add/Remove Programs Properties window will open. In the second part of
the window you will see a list of the programs installed on your hard drive.
Scroll down the list until you find the program whose shortcut you were trying to
use. Highlight the program and then click the “Add/Remove” Button.
• Another common reason is that your shortcut is trying to access the program from the CD-ROM
drive but the needed program is not in the drive. Insert the program CD into the drive and click
the Retry button.
• A second hard drive or another drive has been added which
kicked the CD-ROM letter up to a new letter. In this case,
open My Computer and write down the letter assigned to the
CD-ROM drive. On the desktop locate the icon for the
shortcut, right click on the shortcut icon and select Properties
from the pop-up window. In the Target box delete the old
drive letter and enter the new drive letter. Click the OK
button.

I was running a program and got a full blue screen with a Fatal Exception Error.

This is the so-called BSOD or the "Blue Screen of Death". It is a full screen Fatal Exception Error and
the only way out is to reboot. This usually points to duplicate or incompatible DLL files being called
into memory. This can also occur if you were working from a diskette or CD-ROM and removed it
before the system completed reading from it.
• Hit ENTER and try to get back to the Desktop, save your work under a different file name and
reboot.
• If you cannot get back to the Desktop reboot the system by pressing Ctrl + Alt + Delete
simultaneously or pushing the “reset” button, not the power switch, on the desktop/tower case.
In the future try to notice patterns or particular combinations of programs that cause the error. Look for
software patches at the vendor's web sites. It could also be a problem with your memory or its settings.
You might want to try setting your memory settings to default. Other than that, there is not a whole lot to
be done; shut down or reboot