Using bsearch() function with array of structs in C

Question

I have an array of structs and I would like to find a key that is an integer in the array of structs. My structure looks like this:

typedef struct{
    int x;
    int y;
    int area;
    int occurence;
}pair;

And I would like to use bsearch() to find that specific value in the array that is the area. I have an array of pairs sorted by qsort() in ascending order. My code looks like this:

qsort(pairs, nPairs, sizeof(pair), cmpArea);

int * tmp = NULL, area = someValue; 
tmp = (int *)bsearch(&area, pairs, nPairs, sizeof(pair), cmpArea);

if (tmp!=NULL)
   //do something
else
   //do something

//pairs is the name of the array, nPairs is number of elements

I tried to look at the values using a debugger, and bsearch() always returns NULL not sure why. I doubt that my compare function is the problem, but it looks like this :

int cmpArea(const void *a, const void *b){

    pair * pairA = (pair*)a;
    pair * pairB = (pair*)b;

    return (pairA->area - pairB->area);
} 

I tried different comparing functions, but none works. I´m opened to your ideas. Thank you.

Answer 1

In your code, you are pointing to a key value and not an actual member object.

The call to bsearch should be more like this:

pair key_obj;
pair *result;
key_obj.area = someValue;
result = bsearch(&key_obj, pairs, nPairs, sizeof(pair), cmpArea);

It was pointed out to me that the first arg could point to an int * if the comparator was written to handle that. In that case we would have:

int key_obj = someValue;
pair *result;

result = bsearch(&key_obj, pairs, nPairs, sizeof(pair), cmpArea);

int cmpArea(const void *a, const void *b){

    int *key = (int *)a;
    pair * pairB = (pair*)b;

    return (*key - pairB->area);
}