NetOsPgm: function pointer and state event machine

points need to be remembered in the function pointer

Declaration: as like normal pointer variable, we have to initialize the function pointer but return type and arguments should be like declared function

ex: void (*fp)();

Initialization: while initialization have to give & operator of the defined function

ex: fp = &function_a;

Invocation : have to invocate the function pointer with necessary argument if the defined function expects the parameter

ex: (*fp)();

function definition:

void function_a(void) { printf("a \n"); }

A function’s name can also be used to get functions’ address. For example, in the below program, we have removed address operator ‘&’ in assignment. We have also changed function call by removing *, the program still works.

#include <stdio.h>

// A normal function with an int parameter

// and void return type

void fun(int a)

{

    printf("Value of a is %d\n", a);

}

int main()

{ 

    void (*fun_ptr)(int) = fun;  // & removed

    fun_ptr(10);  // * removed

    return 0;

}

Functions that Return an ArrayIn C, we can return a pointer to an array, as in the following program:
#include <stdio.h>
int * build_array();
int main() {
  int *a;
  a = build_array(); /* get first 5 even numbers */
  for (k = 0; k < 5; k++)
    printf("%d\n", a[k]);
  return 0;}
int * build_array() {
  static int Tab[5]={1,2,3,4,5};
   return (Tab);}

Array of Function Pointers

An array of function pointers can play a switch or an if statement role for making a decision, as in the next program:

#include <stdio.h>
int sum(int num1, int num2);
int sub(int num1, int num2);
int mult(int num1, int num2);
int div(int num1, int num2);

int main() 
{  int x, y, choice, result;
  int (*ope[4])(int, int);
  ope[0] = sum;
  ope[1] = sub;
  ope[2] = mult;
  ope[3] = div;
  printf("Enter two integer numbers: ");
  scanf("%d%d", &x, &y);
  printf("Enter 0 to sum, 1 to subtract, 2 to multiply, or 3 to divide: ");
  scanf("%d", &choice);
  result = ope[choice](x, y);
  printf("%d", result);
return 0;}

int sum(int x, int y) {return(x + y);}
int sub(int x, int y) {return(x - y);}
int mult(int x, int y) {return(x * y);}
int div(int x, int y) {if (y != 0) return (x / y); else  return 0;}

Enter two integer numbers: 13 48
Enter 0 to sum, 1 to subtract, 2 to multiply, or 3 to divide: 2
624

https://www.guru99.com/c-function-pointers.html



root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer# cat func1.c 


#include<stdio.h>

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

int main()
{
 //function pointer declaration
 int (*addptr)(int,int);
 //assinging address to function pointer
 addptr=&add;
 //calling via function pointer
 printf("\n%d\n",addptr(10,20));
 return 0;
}
root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer# gcc func1.c 
root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer# ./a.out 

30
root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer#




root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer# cat func2.c 
#include<stdio.h>
void function_a(void);
void function_b(void);
void main() {

     void (*fp)();

    fp = &function_a;

    (*fp)();  //prints "a"

    fp = &function_b;

    (*fp)();  //prints "b"

}

void function_a(void) {
      printf("a \n");
}

void function_b(void) {
     printf("b\n");
}



root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer# gcc func2.c 
root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer# ./a.out 
a 
b
root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer#




root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer# cat calculator_using_fptr.c 
#include<stdio.h>
float Plus    (float a, float b) { return a+b; }
float Minus   (float a, float b) { return a-b; }
float Multiply(float a, float b) { return a*b; }
float Divide  (float a, float b) { return a/b; }

// Solution with a switch-statement -  specifies which operation to execute
void Switch(float a, float b, char opCode)
{
   float result;

   // execute operation
   switch(opCode)
   {
      case '+' : result = Plus     (a, b); break;
      case '-' : result = Minus    (a, b); break;
      case '*' : result = Multiply (a, b); break;
      case '/' : result = Divide   (a, b); break;
   }
   printf("\n result=%f",result);
}

// Solution with a function pointer
// a function which takes two floats and returns a float. The function pointer
// "specifies" which operation shall be executed.
void Switch_With_Function_Pointer(float a, float b, float (*pt2Func)(float, float))
{
   float result = pt2Func(a, b);    // call using function pointer
   printf("\n result=%f",result);
}

// Execute example code
int main()
{
   Switch(2, 5, /* '+' specifies function 'Plus' to be executed */ '+');
   Switch_With_Function_Pointer(2, 5, /* pointer to function 'Minus' */ &Minus);
   return 0;
}
root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer#


root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer# gcc calculator_using_fptr.c 
root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer# ./a.out 

 result=7.000000
 result=-3.000000root@selvakumar-Lenovo-B460e:/C_pgms/function_pointer#

State machine implementation u can go through using following link:

https://stackoverflow.com/questions/10758811/c-syntax-for-functions-returning-function-pointers

State machine implementation u can go through using following link:

http://www.geeksforgeeks.org/function-pointer-in-c/

NetOsPgm

Saturday, August 30, 2014

function pointer and state event machine

points need to be remembered in the function pointer

Functions that Return an Array

Array of Function Pointers

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