Go4Expert - Understanding C Function Pointers

Function pointers are pointers that can hold the address of a particular type of functions. These pointers, once initialized can be used to call the function whose address they are pointing to.

Declaring Function Pointer

A function pointer can be declared in following way :

<return type of the function being pointed to> (*<name of func ptr>)(<expected arguments by the function being pointed to>)

For example, if we have a function 'int func(char)', then a function pointer to this function would be dclared as:

int(*func_ptr)(char)

So, here 'func_ptr' is the name of the function pointer which can point to a funtion that expects a 'char' as argument and returns an 'int'

Initializing & Using Function Pointer

Continuing with the above example, the func_ptr can be initialized as follows :

Code:

func_ptr = func; // the name of any function represents the address of function in memory.

func_ptr = &func; // This is also valid and same as the above line.

and can be used as :

Code:

int ret = fun_ptr('a'); // Since now func_pre holds the address of 'func', This line calls the function 'func' with argument 'a'

Example

Code:

#include<stdio.h>

#include<string.h>



int func(char c)

{

        printf("\n [%c] \n",c);

        return 0;

}



int main(void)

{

        int(*func_ptr)(char); // Declare a function pointer

        func_ptr = func; // store the address of 'func' in 'func_ptr'

        return func_ptr('a'); // Calling 'func' through 'func_ptr'

}

Where Function Pointers Are Used?

Function pointers are used mainly in callback mechanisms. A callback mechanism usually comes into picture when an application is using some library. Some functions defined in library expect a function pointer from application using which the library functions can send some error or status message to the application to keep the application informed of whats going inside.

For example, I have created an example to simulate the above stated scenario. Here the function 'divide()' can be thought of as a library function(though I have defined it in the same file for simplicity). The application registers the function 'func' as a callback to receive the status messages from the function 'divide()'.

Code:

#include<stdio.h>

#include<string.h>



int func(char *s)

{

        printf("Callback recieved : %s",s);

        return 0;

}



float divide(unsigned int a, unsigned int b, int(*status_callback)(char*))

{

        status_callback("\n Recieved values...validating\n");

        if(b == 0)

        {

                status_callback("\n b = 0 detected, aborting....\n");

                return -1;

        }

        status_callback("\n Validation complete, performing division\n");

        float ret = a/b;

        status_callback("\n Division complete, returning value\n");

        return ret;

}





int main(void)

{

        int(*func_ptr)(char*);

        unsigned int a = 6, b = 3;

        printf("\n This is a prgram to find a/b\n");



        func_ptr = func;



        float ret = divide(a,b,func_ptr);

        printf("\n divide returned [%f]\n",ret);



        return 0;



}

The output of the above program is as :

Code:

 This is a prgram to find a/b

Callback recieved : 

 Recieved values...validating

Callback recieved : 

 Validation complete, performing division

Callback recieved : 

 Division complete, returning value



 divide returned [2.000000]

Secret Tip for Function Pointer

We can make a particular function pointer a type during declaration using 'typedef'

typedef int(*func_ptr)(char);

So the above line makes 'func_ptr' as a type (just like we have int, char etc as types) and we can declare variables of this type

func_ptr ptr1;

So the above lines creates a ptr1 of type func_ptr, hence ptr1 becomes a function pointer of type func_ptr.