C language - pointers to functions

[impakt]

Hello all,
I have a problem with understanding how pointers to functions work. I have some books at home, but there is poor explanation on this. Is there an article/book/link that explains how pointers to function work, but explained for dummies? Do you know something like this?
Thank you!

 

[impakt]

Hello all,
I have a problem with understanding how pointers to functions work. I have some books at home, but there is poor explanation on this. Is there an article/book/link that explains how pointers to function work, but explained for dummies? Do you know something like this?
Thank you!

Nobody?

 

[btbass]

They are not that difficult. A function pointer just contains the address of the entry point of the function.
If you have a function like:

int myfunction(int x);

you declare a function pointer that matches the arguments:

int (*myfunctionpointer)(int x);
The brackets define it as a function pointer.

Then you can assign the entry address of your function to it.

myfunctionpointer = myfunction; /* one way */
myfunctionpointer = &myfunction; /* Another way */

Then you call it like you would call the function:

answer = myfunctionpointer(argument_x);

There are many places were they help in writing elegant afficient code.
A simple example is in a menu system where the selection is an index into an array of functionpointers.

Here is a small example of a switch statement implemented as an index into an array of state function pointers. The return from the called function is the state switch.

/*--- State Machine ---*/

enum STATE{IDLE_S = 0, MASSAGE_S, LUMBAR_S, BOLSTER_S, ERROR_S};

/*--- State machine functions. ---*/

enum STATE idle(void);
enum STATE massage(void);
enum STATE lumbar(void);	
enum STATE bolster(void);
enum STATE error_state(void);
	
/*--- Program entry point ---*/

void main(void)
  {
  enum STATE (*const Pump_State[])(void) = {idle, massage, lumbar, bolster, error_state}; 
  enum STATE State = IDLE_S;
  
  for(;;)
    {    
    State = Pump_State[State]();  /* execute state machine */    
    }
  }
    
/*--- End of File ---*/

 

[impakt]

Hello btbass,
For this example do you have also the classic implementation of the switch statement? This would help me get a better view of this implementation.
Thank you!

 

[btbass]

The same example as a switch statement would be:

/*--- State Machine ---*/ 

enum STATE{IDLE_S = 0, MASSAGE_S, LUMBAR_S, BOLSTER_S, ERROR_S}; 

/*--- State machine functions. ---*/ 

enum STATE idle(void); 
enum STATE massage(void); 
enum STATE lumbar(void);    
enum STATE bolster(void); 
enum STATE error_state(void); 
    
/*--- Program entry point ---*/ 

void main(void) 
  { 
  enum STATE State = IDLE_S; 
  
  for(;;) 
    {    
    switch(State)
        {
        case IDLE_S: State = idle();
                     break;
 
        case MASSAGE_S: State = massage();
                        break;

        case LUMBAR_S: State = lumbar();
                       break;

        case BOLSTER_S: State = bolster();
                        break;

        case ERROR_S: State = error_state();
                      break;

        default: State =  ERROR_S;
        } 
    } 
  } 
    
/*--- End of File ---*/

This is a trivial example. Once you start using them you will find they can provide a nice clean solution for a lot of programming problems you might come up against.