How to pass a comparator to a C function? - c

Question

What is bool (*comparator)(void *, void *)?
I'm trying to use this function from a library, but I don't understand its signature.
The function seems to be expecting some kind of criteria to sort the list (as ASC or DESC in SQL, I think), as a higher order function.
void list_sort(t_list *self, bool (*comparator)(void *, void *)) {
int unsorted_elements = self->elements_count;
if(unsorted_elements < 2) {
return;
}
t_link_element *aux = NULL;
bool sorted = true;
do {
t_link_element *previous_element = self->head, *cursor = previous_element->next;
sorted = true;
int index = 0, last_changed = unsorted_elements;
while(index < unsorted_elements && cursor != NULL) {
if(!comparator(previous_element->data, cursor->data)) {
aux = cursor->data;
cursor->data = previous_element->data;
previous_element->data = aux;
last_changed = index;
sorted = false;
}
previous_element = cursor;
cursor = cursor->next;
index++;
}
unsorted_elements = last_changed;
} while(!sorted);
}
The last definition of the function was in commons/collections/list.h.
I can't figure how to use this function properly after many attempts.
#include <stdio.h>
#include <stdlib.h>
#include <commons/collections/list.h>
/*I added this function thanks to the help of the community,
*and now it works, the list is printed backwards now.*/
bool comparator(void * a, void * b) {
return (int*) a > (int*) b;
}
int main()
{
t_list *list = list_create();
int add[] = {4, 55, 9, 7, 17};
list_add(list, (void*) &add[0]);
list_add(list, (void*) &add[1]);
list_add(list, (void*) &add[2]);
list_add(list, (void*) &add[3]);
list_add(list, (void*) &add[4]);
int size = list_size(list);
int j = 0 ;
while( j++ < 2)
{
for ( int i = 0 ; i < size; i++)
{
int* element = (int*) list_get(list, i);
printf("Found %d\n", *element);
}
//I edited this line, now the second parameter is comparator
list_sort(list, comparator);
}
list_destroy(list);
}
The main() function prints
Found 4 Found 55 Found 9 Found 7 Found 17
Found 17 Found 7 Found 9 Found 55 Found 4
EDIT:
I created the function comparator that actually allows me to run the code
After printing the result I showed before, it prints the same list but backwards.
Thanks everyone who's been nice and helped me fixed this, I still don't know how to print the sorted list. Sorry if my question is useless and/or violates some guideline, I wouldn't ask it this way if I knew it did.

Answer 1

Looking at the code in your last edit, the function is plain wrong. (int*) a > (int*) b compares pointer addresses, not values. If you intend to return true if a is greater than b, then it should be:
bool is_greater (void * a, void * b) {
return *(int*) a > *(int*) b;
}
More readably written as:
bool is_greater (void* a, void* b)
{
const int* ia = a;
const int* ib = b;
return *ia > *ib;
}
The function pointer type used by that API has some code smell, the parameters should have been declared as const void* so that the code can be used on read-only data too, but I guess you can't change that part.

Answer 2

In C++ we would have a predicate - an operator which returns true if the lhs < rhs
So your C function looks like it is following that pattern.
bool myLessInt(void * lhs, void * rhs)
{
// assume input parameters lhs and rhs are pointers into the data.
int intLhs = *((int*)lhs);
int intRhs = *((int*)rhs);
if( intLhs < intRhs ) return true; // lhs was less than rhs
return false; // rhs == or is less than lhs
}
I would look for a function like the one above to solve your problem.