Function Pointer

What is Function Pointers?

Function pointer is same as pointer to variables except that function pointers will points to the address of function to be called. Every function and its associated variables will be stored in some memory location.  When we assign function address to function pointer it will store the starting address of function.  Function pointers can be used in task schedulers and state machines.  Function pointers provide efficient and easily manageable programming techniques.

We can implement efficient task schedulers to call a function from task table.  We will discuss state machine implementation using function pointer in next blogs. Main steps to  use function pointers is definition of function pointer, assigning address to function pointer and calling a function using function pointer.

Function pointer declaration

Function pointer declaration mainly contains three parameters:

• Return time
• Function pointer name
• Parameter list

Any function pointer name should be preceded by “*” and enclosed in parentheses.

Return_type (*functionpointer_name) (parameter list)

Return type can be void, int, char, float etc, function pointer_name will be the name of the function pointer and parameter list will be list of parameters function passes.

Example of function pointer:

void (*funptr1)(void); //function pointer declaration for the          //function that takes no thing and return        //nothing.

int  (*funptr2)(int, float); // this function pointer takes two              //values integer and floats and                 //return integer value

int (*funptr2)( int*);  // this function pointer takes pointers to a     // variables and return integer value

We can also pass the one function pointer to another function pointer.

int (*funptr2)(int, char, void  (*funptr1)(void));    

//this function pointer accept the variables and a function pointe //and return integer value.

Assigning Function address to Function Pointers

Assigning function address to function pointers is same as assigning variable address to pointer variable.

        Function_pointer_name = &Function_name;

“&” operator will give the address of function pointer. Example of function pointers are as follows:

funptr1=&Fuction1;                         

funptr2=&Function2;

Before using function pointer it should keep mind that function pointer should be initialized. If function pointer is not initialized or contain a garbage value then program start execution from address pointed by function pointer. If address is out of range then whole system can be crash. You can check function pointer to be initialized by comparing it with null variable or address of function. If function pointer contains null value it means it does not set. So don’t call function pointer.

if(funptr1!=NULL)

{

     //call function

}

else

{

    //do no thing

}

Calling a function using function pointer

It requires three parameter to call a function using pointers, result_value if function pointer is returning something, function pointer name preceded by “*” mark and parameter list.

Result_value  (*function_pointer_name)(parameter list);

If function does not returning anything we can omit the result i.e.:

 (*function_pointer_name)(Parameter list);

For examples:

(*funptr1); //call the void Function1 pointed by function pointer               //funptr1

value = (*funptr2)(4, 1.5);// call void Function2 pointed by                //function pointer funptr2 and return the        //integer value in value variable.

Continued..............