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);
}