Passing variables using call back function pointers

Question

I would like to pass variable 'c' from the main to (an arbitrary) function A using callback function B. As indicated, I can pass the variable 'b' to function A within function b, but I cannot find a syntax to pass from main.

#include<stdio.h>

void A(int a)
{
    printf("I am in function A and here's a: %d\n", a);
}

// callback function
void B(void (*ptr)(int b) )
{
//    int b = 5;
    int b;
    printf("I am in function B and b = %d\n", b);
    (*ptr) (b); // callback to A
}

int main()
{   
    int c = 6;
    void (*ptr)(int c) = &A;

    // calling function B and passing
    // address of the function A as argument
    B(ptr);

   return 0;
}

Is there a syntax to pass a variable from main to an arbitrary function using a call back?

viraptor · Accepted Answer · 2018-02-19 02:39:14Z

2

What may be confusing you is the name of the parameter in function declarations. They're not necessary and it's better to drop them.

These two lines are the same:

void (*ptr)(int c) = &A;
void (*ptr)(int) = &A;

the c in that expression had nothing to do with the variable c. Additionally you can use:

typedef void (*callback_t)(int);

and use the following for declaration instead.

callback_t ptr = &A;

Then it's more obvious you need two parameters for B.

void B(callback_t ptr, int param) {
    ptr(param);
}

int main() {
    ...
    B(ptr, c);
}

answered Feb 19, 2018 at 2:39

viraptor

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Comments

Pablo · Accepted Answer · 2018-02-19 02:34:13Z

1

Calling the function pointer with (*ptr) (b) is OK, but I prefer to call it ptr(b), for me that's more readable. That's just my preference.

If you want to pass the variable to the callback from main, then change B to accept a value for the callback and use that variable when calling the callback with the function pointer, like this:

void B(void (*ptr)(int b), int var_for_callback)
{
    printf("I am in function B and var_for_callback is %d\n", var_for_callback);
    ptr(var_for_callback);
}


int main()
{   
    int c = 6;
    void (*ptr)(int c) = &A;

    // calling function B and passing
    // address of the function A as argument
    B(ptr, c);

   return 0;
}

Now you pass the value of c to B and B passes that to the callback passed in ptr.

edited Feb 19, 2018 at 2:34

answered Feb 19, 2018 at 2:20

Pablo

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1 Comment

Pablo Over a year ago

I know they are equally fine, I only think ptr(b) is more readable, I wanted to show that. But you are right, it is pointless because it's correct anyway. I'll remove that part as it may confuse more than it helps. Thanks for the feedback.

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