比较内置qsort函数中的函数

Question

I am trying to use the inbuilt qsort function in C to sort a structure

typedef struct abc{
  long long int fir;
  long long int sec;
}abc;

In the compare function I used the below code so that if the variable "fir" is same between any two elements in the array of structure abc then the sorting condition will depend on the variable "sec".

long long int cmp(const abc*  e1, const abc* e2)
{
if(e1->fir==e2->fir)
       return e1->sec-e2->sec;
else
    return e1->fir-e2->fir;
} 

The code doesn't seem to work properly. What is the correct method to do it?

Answer 1

long long int cmp(const abc*  e1, const abc* e2)

does not have the correct signature for a qsort comparison function, so your program has undefined behavior. You should set your compiler warning flags high, and it will warn you of such things.

Your actual test is fine as long as the subtraction can't overflow, which is probably not a problem for long long. Edit The subtraction is not fine here, precisely because the return value of cmp must be an int . The result of the subtraction long long , so if you have large values the result is too big to fit into an int and qsort is sure to produce the wrong order.

More generally, though, this is more accurate:

int cmp(const void* v1, const void* v2)
{
    const struct abc* p1 = v1;
    const struct abc* p2 = v2;

    if (p1->fir < p2->fir)
        return -1;
    else if (p1->fir > p2->fir)
        return 1;
    else
        return p1->sec < p2->sec? -1 : p1->sec > p2->sec? 1 : 0;       
}

Answer 2

The built-in qsort you're using probably expects a "lower-than" (operator<) predicate, which is the normal terminology for C++. And yours returns true in both cases as long as the arguments aren't equal.

You have to return true IFF (if and only if) e1<e2 .