Function pointer inactive with struct in C

Question

I have the following code:

#include <stdio.h>
#include <stdlib.h>

struct Book {
    char title[50];
    char author[50];
    char subject[100];
    int numPages;
    int numBooks;

    int (*p) (int *, int *);
};

int sum (int *a, int *b) {
    return *a + *b;
}

int main() {
    struct Book var;
    var.numPages = 7;
    var.numBooks = 9;
    int allPages = (*var.p) (&var.numPages, &var.numBooks);

    printf("%d\n", allPages);
    return (EXIT_SUCCESS);
}

I trying to use function in struct but my program have no result, no warning although I used -Wall, -Wextra. I'm a newbie. Hope everybody help.

You didn't set `p` to a valid function! You need `var.p = sum`. I'm surprised you didn't get an access violation. The behavior is undefined. The fact that it compiles without warnings doesn't catch something like that since it's a run time error. — lurker, Dec 11 '15 at 17:05

score 2 · Accepted Answer · answered Dec 11 '15 at 17:04

2

var.p is not initialized (meaning it almost certainly doesn't refer to a valid function), creating undefined behavior. Use var.p = sum; to initialize it before the function call.

answered Dec 11 '15 at 17:04

owacoder

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Ah ha. Thanks everyone. My faukt. I forgot it. – Dung Nguyen Dec 11 '15 at 17:32

score 0 · Answer 2 · edited May 23 '17 at 12:23

In your code you should first assign a function to a pointer and then by using a pointer call a function. for this your code should be

 int main()

  {
   struct Book var;
   var.numPages = 7;
   var.numBooks = 9;
   var.p=sum;

    int allPages =var.p(&var.numPages, &var.numBooks);

     printf("%d\n", allPages);
     return (EXIT_SUCCESS);
  }

for further information you can refer

C - function inside struct

Function pointer inactive with struct in C

2 Answers2