Question

Im trying a simple example of the usage of a generic comparison-function in c but when initializing it I get the warning message that the types are incompatible when initializing them. Feels as I am missing something basic here…

#include <stdio.h>
#include <string.h>

int
compare(int (*comp)(void *first, void *second), void *arr, int size)
{
    int i, j, dups=0, arrsize=sizeof(arr)/size;
    for(i=0; i<arrsize; i++)
    {
        for(j=0; j<arrsize; j++)
        {
            if(j!=i)
                dups+=(*comp)(&arr[i], &arr[j]);
        }  
    }
     return dups;
}

int
compareints(int *first, int *second)
{
    if(*first==*second)
        return 1;
    return 0;
}

int main(int argc, const char * argv[])
{
    int (*comp)(void *, void *)=&compareints; //Here is where I get the warning...
    int arr[10]={22, 39, 78, 22, 99, 12, 82, 10, 11, 28};
    printf("The arr has dups: %d\n", compare(comp, arr, sizeof(int)));
    return 0;
}

EDIT:
the program was not functioning properly and it had to do with the handling of the array. The modifications for functioning prog are below:

int
compare(int (*comp)(void *first, void *second), void **arr, int size)
    ....
    ....
    ....
    dups+=(*comp)(arr[i], arr[j]);

int 
compareints(....)
    if((*((int *)first))==(*((int *)second)))
       return 1;


int 
main(int argc, const char * argv[])
{
     int (*comp)(void *, void *)=&compareints;
     int arr[10]={22, 39, 78, 22, 99, 12, 82, 10, 11, 28};
     void **arr2=malloc(sizeof(int *)*10);
     for(int i=0; i<10; i++)
     {
         arr2[i]=&arr[i];
     }
     printf("The arr has dups: %d\n", compare(comp, arr2, 10));
     return 0;
}

Answer 1

Unfortunately you can’t mix void * and int * like this when they’re part of a function signature. Even though int * can be converted to void *, int (*)(int *, int *) can’t be converted to int (*)(void *, void *). The signatures have to match exactly.

int
compareints(void *first, void *second)
{
    if (*((int *) first) == *((int *) second))
        return 1;
    return 0;
}