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

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

int sum(int a, int b);
int subtract(int a, int b);
int mul(int a, int b);
int divide(int a, int b);

int (*p[4]) (int x, int y);

int main(void)
{
  char buf[80];
  int result;
  int i, j, op;

  p[0] = sum; /* address of sum() */
  p[1] = subtract; /* address of subtract() */
  p[2] = mul; /* address of mul() */
  p[3] = divide; /* address of divide() */

  printf("Enter two numbers: ");
  fgets(buf, 80, stdin);
  sscanf(buf, "%d %d", &i, &j);
  
  printf("0: Add, 1: Subtract, 2: Multiply, 3: Div, 4: Quit\n");
  printf("Enter number of operation: ");
  fgets(buf, 80, stdin);
  sscanf(buf, "%d", &op);

  while (op >= 0 && op <= 3)  {
    result = (*p[op]) (i, j);
    printf("%d\n\n", result);

    printf("0: Add, 1: Subtract, 2: Multiply, 3: Div, 4: Quit\n");
    printf("Enter number of operation: ");
    fgets(buf, 80, stdin);
    scanf(buf, "%d", &op);
  }


  return 0;
}

int sum(int a, int b)
{
  return a + b;
}

int subtract(int a, int b)
{
  return a - b;
}

int mul(int a, int b)
{
  return a * b;
}

int divide(int a, int b)
{
  if (b)
  {
    return a / b;
  }
  else
  {
    return 0;
  }
}

 qSort

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

/*
int my_func(void *a, void *b)
{
  int retval = 0;

  if (*a > *b)
  {
    retval = 1;
  }
  else if (*a < *b)
  {
    retval = -1;
  }
  else
  {
    retval = 0;
  }

  return retval;
}
*/

int cmpfunc (const void * a, const void * b)
{
   return ( *(int*)a - *(int*)b );
}

int main()
{
  int i;
  /* int (*myfuncPtr)(void *,void *);
   * myfuncPtr = my_func;
   */

  int array[10] = { 34, 33, 54,234,7645, 22,1, 645,345, 237 };

  /* int X = myfuncPtr(37, 24);
   * int X = (*myfuncPtr)(37, 24);
   */
  (*HelloPtr)();

  for (i = 0; i < 10; i++)
  {
    printf("%d\n", array[i]);
  }
    
  qsort(array, 10, sizeof(int), cmpfunc);

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