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.
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:
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'
Continuing with the above example, the func_ptr can be initialized as follows :
and can be used as :
Example
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()'.
The output of the above program is as :
We can make a particular function pointer a type during declaration using 'typedef'
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
So the above lines creates a ptr1 of type func_ptr, hence ptr1 becomes a function pointer of type func_ptr.
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.
Code:
int ret = fun_ptr('a'); // Since now func_pre holds the address of 'func', This line calls the function 'func' with argument 'a'
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;
}
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.
lionaneesh
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All the best and Hoping for more of them!
