Implementing callback functions in C - 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?

Answer 1

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))

Answer 2

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

Answer 3

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

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).