Function Pointer (Pointers to Functions)

helloworld.c

#include<stdio.h>
#include<stdlib.h>

void Hello();

void Hello()
{
    printf("Hello World!\n");         
}

int main()
{
    void (*Hello_ptr)() = Hello; 
    //giving the address of "Hello" funciton to the pointer
    
    (*Hello_ptr)();  //ugly way 
    Hello_ptr();     //elegent way to use function pointer
          
}           

 Example2.c

int my_func(int a,int b);
int your_func(int , int);

int main()
{
　　int (*funcPtr)(int,int); //declare a function pointer --> not pointing to anything now
　　funcPtr = my_func; //initializing function pointer

　　//use the function pointer
　　int x = (*funcPtr)(5,7);
　　//or
　　int y = funcPtr(5,7)；
}

math_pointers.c

 1 #include <stdio.h>
 2 #include <stdlib.h>
 3 
 4 int sum(int a, int b);
 5 int subtract(int a, int b);
 6 int mul(int a, int b);
 7 int divide(int a, int b);
 8 
 9 int (*p[4]) (int x, int y);
10 
11 int main(void)
12 {
13   char buf[80];
14   int result;
15   int i, j, op;
16 
17   p[0] = sum; /* address of sum() */
18   p[1] = subtract; /* address of subtract() */
19   p[2] = mul; /* address of mul() */
20   p[3] = divide; /* address of divide() */
21 
22   printf("Enter two numbers: ");
23   fgets(buf, 80, stdin);
24   sscanf(buf, "%d %d", &i, &j);
25   
26   printf("0: Add, 1: Subtract, 2: Multiply, 3: Div, 4: Quit\n");
27   printf("Enter number of operation: ");
28   fgets(buf, 80, stdin);
29   sscanf(buf, "%d", &op);
30 
31   while (op >= 0 && op <= 3)  {
32     result = (*p[op]) (i, j);
33     printf("%d\n\n", result);
34 
35     printf("0: Add, 1: Subtract, 2: Multiply, 3: Div, 4: Quit\n");
36     printf("Enter number of operation: ");
37     fgets(buf, 80, stdin);
38     scanf(buf, "%d", &op);
39   }
40 
41 
42   return 0;
43 }
44 
45 int sum(int a, int b)
46 {
47   return a + b;
48 }
49 
50 int subtract(int a, int b)
51 {
52   return a - b;
53 }
54 
55 int mul(int a, int b)
56 {
57   return a * b;
58 }
59 
60 int divide(int a, int b)
61 {
62   if (b)
63   {
64     return a / b;
65   }
66   else
67   {
68     return 0;
69   }
70 }

 qSort

 1 #include <stdio.h>
 2 #include <stdlib.h>
 3 
 4 /*
 5 int my_func(void *a, void *b)
 6 {
 7   int retval = 0;
 8 
 9   if (*a > *b)
10   {
11     retval = 1;
12   }
13   else if (*a < *b)
14   {
15     retval = -1;
16   }
17   else
18   {
19     retval = 0;
20   }
21 
22   return retval;
23 }
24 */
25 
26 int cmpfunc (const void * a, const void * b)
27 {
28    return ( *(int*)a - *(int*)b );
29 }
30 
31 int main()
32 {
33   int i;
34   /* int (*myfuncPtr)(void *,void *);
35    * myfuncPtr = my_func;
36    */
37 
38   int array[10] = { 34, 33, 54,234,7645, 22,1, 645,345, 237 };
39 
40   /* int X = myfuncPtr(37, 24);
41    * int X = (*myfuncPtr)(37, 24);
42    */
43   (*HelloPtr)();
44 
45   for (i = 0; i < 10; i++)
46   {
47     printf("%d\n", array[i]);
48   }
49     
50   qsort(array, 10, sizeof(int), cmpfunc);
51 
52   for (i = 0; i < 10; i++)
53   {
54     printf("%d\n", array[i]);
55   }
56 }