c pointers to functions

c pointers to functions

In pointers the way we can reference variables of type int, char and float etc using addresses, c pointers can also point to the function since c function have addresses.

If we know the function address, we can point to it using pointers and also the another way to invoke that function.

func_ptr is a pointer to a function which returns an function return type.

pointer to function syntax,

 (*func_ptr)(arg1,...);

Why need pointer to a function ?

Writing memory resident programs.

viruses program and vaccines to remove the virsuses.

COM / DCOM components development.

Connect events to function calls(VC++).

C program to get the address of a function

This c program is used to get the address of show function and displayed it.

To obtain the address of a function, we need to use the name of the function.

#include                                                               
                                                                                
void main()                                                                     
{                                                                               
  void show();                                                                  
  printf("\nMain method!");                                                     
  printf("\nAddress of show function: %u", show);                               
  show();                                                                       
}                                                                               
                                                                                
void show()                                                                     
{                                                                               
  printf("\nShow method!");                                                     
} 

Output:

$ cc pointer-to-function.c
$ ./a.out 

Main method!
Address of show function: 4195674
Show method!

C program to invoke function using pointer

#include                                                              
                                                                                
void main()                                                                     
{                                                                               
  void show();                                                                  
  void (*func_ptr)();                                                           
  printf("\nMain method!"); 
  // assign address of function 'show' to function pointer                                                    
  func_ptr = show;                                                              
  printf("\nAddress of show function: %u", func_ptr);                           
  // invokes show function                                                      
  (*func_ptr)();                                                                
}                                                                               
                                                                                
void show()                                                                     
{                                                                               
  printf("\nShow method!");                                                     
}   

Output:

$ cc pointer-to-function.c
$ ./a.out 

Main method!
Address of show function: 4195674
Show method!

C program to invoke function and gets return value using pointer

This c program invokes the show funation and returns integer value and also shows how to invoke the show function using pointer.

func_ptr is a pointer to invoke show function which returns an int data type.

#include                                                               
                                                                                
void main()                                                                     
{                                                                               
  int output;                                                                   
  int show();                                                                   
  int (*func_ptr)();                                                            
  printf("\nMain method!");                                                     
  func_ptr = show;                                                              
  printf("\nAddress of show function: %u", func_ptr);                           
  // invokes show function                                                      
  output = (*func_ptr)();                                                       
  printf("\nReturn value from show function: %d", output);                      
}                                                                               
                                                                                
int show()                                                                      
{                                                                               
  printf("\nShow method!");                                                     
  return 10;                                                                    
}    

Output:

$ cc pointer-to-function.c
$ ./a.out 

Main method!
Address of show function: 4195712
Show method!
Return value from show function: 10

C program to invoke function with arguments and gets return value using pointer

This c program invokes the show funation with int argument and returns integer value and also shows how to invoke the show function using pointer.

func_ptr is a pointer to invoke show function with integer argument which returns an int data type.

#include                                                               
                                                                                
void main()                                                                     
{                                                                               
  int output;                                                                   
  int show(int);                                                                
  int (*func_ptr)(int);                                                         
  printf("\nMain method!");                                                     
  func_ptr = show;                                                              
  printf("\nAddress of show function: %u", func_ptr);                           
  // invokes show function                                                      
  output = (*func_ptr)(20);                                                     
  printf("\nReturn value from show function: %d", output);                      
}                                                                               
                                                                                
int show(int val)                                                               
{                                                                               
  printf("\nShow method!");                                                     
  return val;                                                                   
}  

Output:

$ cc pointer-to-function.c
$ ./a.out 

Main method!
Address of show function: 4195712
Show method!
Return value from show function: 20