function pointers in a struct

Question

I was trying to figure out function pointers. My code looks like this I have a file.h where I have a struct that hast 2 members

typedef struct _node_ {
    char* string;
    int (*compare)(int a, int b);
} node

in that same file, I have a prototype for a function called:

void init_with_function_pointer(node* list, int (*comp)(int x, int y));

then in my file.c, I define the function :

void init_with_function_pointer(node* list, int (*comp)(int x, int y)){
    node_init(list);
    list->compare = comp;
}

and in my main.c

int main(){
    node tree;
    init_with_function_pointer(&tree, /* what should I pass here */)
}

And that function that I need to point to should be defined in file.c

but I can't get it to work, if I define the function in main and a pass it, then it works, but if I try to use extern for that same function which I define in file.c the compiler tells me comp is not defined.

and this is my function that i would like to point to:

extern int comp(int x,int y) {
  if (x < y) {
    return -1;
  } else if(x == y) {
    return 0;
  } else {
    return 1;
  }
}

Can someone help me?

Answer 1

You need a prototype for the comp function in file.h:

int comp(int, int);

Then in main() you write:

init_with_function_pointer(&tree, comp);