Implementing callback functions in C

Question

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 function\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?

Just a tip: typedefs help make things a little more readable: typedef void (*CallbackFunc)(void);. Then your function signature is void registerFunc(CallbackFunc callback) and your declarations are CallbackFunc my_cb = &callback_proc — detly
– detly, Commented Apr 29, 2010 at 15:28
Oh, one more thing: void callback_proc () is NOT THE SAME as void callback_proc (void) in C. — detly
– detly, Commented Apr 29, 2010 at 15:28

interjay · Accepted Answer · 2010-04-29 15:16:31Z

19

register is a C keyword: Use another name for the function.
You have extra parantheses around the callback parameter. It should be:
```
void funcName(void (*callback) (void))
```

answered Apr 29, 2010 at 15:16

interjay

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1 Comment

Luca Matteis Over a year ago

Yep, the extra parens was hard to catch. +1

Patrik · Accepted Answer · 2015-11-26 23:33:57Z

3

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 function\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

edited Nov 26, 2015 at 23:33

Patrik

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answered Apr 29, 2010 at 15:14

stacker

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3 Comments

Adrien Plisson Over a year ago

where is the typedef ? that's a very good recommendation, but your example is missing it.

user50049 Over a year ago

I fail to see the typedef? What you've done is defined a function pointer, which is not type safe.

stacker Over a year ago

Sorry didn't realize that I had a problem with copy and paste

cjg · Accepted Answer · 2010-04-29 15:15:22Z

2

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

answered Apr 29, 2010 at 15:15

cjg

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Comments

Michael Burr · Accepted Answer · 2010-04-29 15:17:57Z

0

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.

answered Apr 29, 2010 at 15:17

Michael Burr

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Comments

user50049 · Accepted Answer · 2010-04-29 15:29:23Z

0

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.

answered Apr 29, 2010 at 15:29

user50049

2 Comments

bummi Over a year ago

Hi Tim, the link in your answer is dead.

Bhargav Rao Over a year ago

@bummi, if Tim ever decides to delete this answer, He'll get a message This post was deleted by Tim Post♦. To know why your answer was deleted please use meta :D

James Morris · Accepted Answer · 2010-04-29 15:38:33Z

0

#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).

edited Apr 29, 2010 at 15:38

answered Apr 29, 2010 at 15:32

James Morris

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