Implementing callback functions in C

Question

Hi,

I am a newbie to C.I am trying to implement callback function using function pointers.

I am getting an error

:test_callback.c:10: error: expected identifier or ‘(’ before ‘void’

when I try to compile the following program:

#include<stdio.h>

void (*callback) (void);

void callback_proc ()
{
  printf ("Inside callback funtion\n");
}

void register ((void (*callback) (void)))
{
  printf ("Inside registration \n");
  callback (); /* Calling an initial callback with function pointer */
}

int main ()
{
  callback = callback_proc;/* Assigning function to the function pointer */
  register (callback);/* Passing the function pointer */
  return 0;
}

---

What is this error?Can anyone help?

Answer 1

I would recommend to use a typedef

     #include<stdio.h>

     typedef void (*callback_t) (void);
     callback_t callback;

    void callback_proc(void)
    {
      printf ("Inside callback funtion\n");
    }

    void reg( callback_t _callback )
    {
      printf ("Inside registration \n");
      _callback();
    }

    int main ()
    {
      callback = callback_proc;
      reg(callback);

      return 0;
    }

EDIT: removed the register issue

Answer 2

You can't use 'register' as a function name as it's a C keyword.

Answer 3

1. register is a C keyword: Use another name for the function.

2. You have extra parantheses around the callback parameter. It should be:

   void funcName(void (*callback) (void))
   

Answer 4

2 problems:

• you can't use the name register as it's a keyword (not used often anymore, but it's still there)

• change the definition of the function from

  void wasRegister((void (*callback) (void)))
  

  to:

  void wasRegister(void (*callback) (void))
  

  (get rid of the parens around the parameter's declaration.

Also you might get a warning about callback_proc() not having a matching delaration to the callback variable (depending on how you compile the program - as C or C++), so you might want to change its declaration to:

void callback_proc (void)

to make it explicit that it takes no parameters.

Answer 5

Have a look at type safe callbacks from ccan. Its one thing to expose a typed function pointer for the world to use, its another to ensure sane casting.

Answer 6

#include<stdio.h>

typedef void (*callback_func) (void);

static callback_func the_callback = 0;

void process (void)
{
  printf ("Inside process function\n");
}

void callback_register (callback_func cb)
{
  the_callback = cb;
  printf ("Inside registration \n");
}

void callback(void)
{
    the_callback();
}

int main (void)
{
  callback_register(process); /* Passing the function pointer */
  callback();
  return 0;
}

Declaring the_callback static would make more sense if this code was modularized and then you would be forced to call callback_register in order to set it, and callback in order to call it - the_callback would not be accessible outside of the implementation (.c) only the function declarations would be in the header (.h).