Pointers to Functions
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 ——————–*/
#include <stdio.h>
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
the actual algorithm. This results in bubble_2.c:
/*———————- bubble_2.c ————————-*/
/* Separating the comparison function */
#include <stdio.h>
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 ————————-*/
#include <stdio.h>
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 —————————-*/
#include <stdio.h>
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 buffer() 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 —————————*/
#include <stdio.h>
#include <string.h>
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(”%d “, 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 ———————*/
#include <stdio.h>
#include <string.h>
#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 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 ——————*/
#include <stdio.h>
#include <string.h>
#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);
}
/*—————– end of bubble7.c —————–*/
