Use function pointer initialized in other source file

Question

I have function pointer, that is declared as follows in file 1.h

// file : 1.h
typedef void (*my_func_ptr_T)(int , int);

In file 1.c I create a global instance and initialize it

// file 1.c
my_func_ptr_T func;

void callback (my_func_ptr_T _ptr) {
    func = _ptr;
}

Now, how can i use this function ptr 'func' in another file say 2.c

// file 2.c 
void do_callback() {
    //  i want to do this
    func(1,2);
}

Answer 1

Check the below changes

// file : 1.h
typedef void (*my_func_ptr_T)(int , int);
extern my_func_ptr_T func;

//file 1.c
#include "1.h"

//func is visible


// file 2.c
#include "1.h"

//func is visible

Answer 2

Changing 2.c to below will help. Leave 1.h and 1.c unchanged.

#include "1.h"
extern my_func_ptr_T func;

void do_callback() {
    func(1,2);
}

//sample sum function
void sum( int a, int b ) {
    printf ("SUM : %d\n", a+b);
}

//main
int main() {
    func = sum;
    do_callback( );
}

It is important to understand that, keyword extern doesn't define the variable. It is just a variable declaration . No memory is allocated for an extern variable declaration. The func variable is actually defined in 1.c and that is where memory is allocated. You are free to use extern declarations in as many files ( for eg 3.c ) as you need.

Note: Careful when adding extern to header file as in accepted answer. That makes variable visible, in which ever source file the header in included and so a potential risk of name collisions or unintentional modifications.

Answer 3

This is an example about using function pointers...

 #include <stdio.h>

 void prev(int x)
 {
     printf( "%d\n", x-1 );
 }

 void next(int x)
 {
     printf( "%d\n", x+1 );
 }

 typedef void (*my_func_ptr_T)(int);

 int main()
 {
     my_func_ptr_T foo;

     foo = &prev;

     foo( 5 );

     return 0;
 }

You can run the code in this DEMO

Answer 4

You make the variable func known in file 2.c:

// file 2.c
extern my_func_ptr_T func;

void do_callback() {
//  i want to do this
    func(1,2);
}

By declaring func as extern you tell the compiler that this particular variable will live in a file / different compilation unit (eg 1.o compiled from 1.c, where a variable of the same name and type is declared without the extern keyword), and it does not need to create it when compiling 2.c into 2.o.