C [-Wincompatible-pointer-types] how to cast

Question

my code: https://godbolt.org/z/de7fbdjh7

code from source: https://stackoverflow.com/a/49072888/15603477
Almost exact the same.

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

typedef struct 
{
    int iValue;
    int kValue;
    char label[6];
} my_data;

int cmp_mydata_ivalue(my_data* item1 , my_data* item2 )
{
    if(item1->iValue < item2->iValue) return -1;
    if(item1->iValue > item2->iValue) return 1;
    return 0;
}

int main(void){
    my_data datalist[256] = {0}; 
    {
        int i;
        for(i = 0;i<20;i++){
            datalist[i].iValue = i+100;
            datalist[i].kValue = i+1000;
            sprintf(datalist[i].label,"%2.2d", i+10);
        }
    }
    printf("new line\n");
    {
        my_data srchitem = {105,1018,"13"}; 
        
        my_data *foundItem = (my_data*) bsearch(&srchitem, datalist,20, sizeof(my_data),cmp_mydata_ivalue);
        bsearch_results(&srchitem, foundItem);
    }
}

The same question asked many times. But I don't know how to cast it.
error code:

*callback1.c: In function ‘main’:
callback1.c:58:89: warning: passing argument 5 of ‘bsearch’ from incompatible pointer type [-Wincompatible-pointer-types]
   58 |         my_data *foundItem = (my_data*) bsearch(&srchitem, datalist,20, sizeof(my_data),cmp_mydata_ivalue);
      |                                                                                         ^~~~~~~~~~~~~~~~~
      |                                                                                         |
      |                                                                                         int (*)(my_data *, my_data *) {aka int (*)(struct <anonymous> *, struct <anonymous> *)}*

One way to try to use gcc option to supress the error. Another way is somewhere I need to cast. But now i don't know how to cast.

Tutorial I found so far: https://www.tutorialspoint.com/c_standard_library/c_function_bsearch.htm

Answer 1

The comparison function must have the type

int ( const void *, const void * )

See the declaration of the function bsearch

void *bsearch(const void *key, const void *base,
              size_t nmemb, size_t size,
              int (*compar)(const void *, const void *));

So you should declare and define your function like

int cmp_mydata_ivalue( const void *a , const void *b )
{
    const my_data *item1 = a;
    const my_data *item2 = b;
    if ( item1->iValue < item2->iValue) return -1;
    if(item1->iValue > item2->iValue) return 1;
    return 0;
}