cmpfunc in qsort() function in c

Question

Can someone explain me cmpfunc which is used in the qsort function? What are a and b in this function and what are they pointing to?

int cmpfunc(const void *a, const void *b)
{
    return(*(int*)a - *(int*)b);
}

@Idos Nitpicking: "*They are the two elements ...*" They are *not* the two elements themselves to be compared, but referring to, pointing to the elements to be compared. — alk, Jan 02 '16 at 11:07

haccks · Accepted Answer · 2016-01-02T11:01:24.363

4

a and b in cmpfunc are pointers to const void type. cmpfunc can accept pointer to elements of array of any data type.
void * pointer can't be dereferenced, therefore a cast int * is needed before dereferencing.

edited Jan 02 '16 at 11:01

answered Jan 02 '16 at 10:53

haccks

104,019
25
176
264

score 3 · Answer 2 · answered Jan 02 '16 at 11:05

3

In this inputs are *void and you need to comaper integers in your case. So you will need to convert types. That's why there are

     *(int *) a

it can be float type

     *(float *) a

and so on other type...

you can find this implementation :

 int cmpfunc(const void *a, const void *b)
 {
  if(*(int *)a  <  *(int *)b) return -1;
  if(*(int *)a  == *(int *)b) return 0;
  if(*(int *)a  >  *(int *)b) return 1; 
}

answered Jan 02 '16 at 11:05

Vinod

300
3
18

2

You meant `void *` not `*void`, do you? – alk Jan 02 '16 at 11:08
no I want just say void pointer, who can take address of any data type – Vinod Jan 02 '16 at 11:17
2

`void *` is the type of a `void`-pointer, the type of a pointer to `void`, what `*void` is not. In fact the latter is not valid C. – alk Jan 02 '16 at 11:20
so first they are referencing a pointer to the element and then dereferencing it. – user007 Jan 05 '16 at 07:01

cmpfunc in qsort() function in c

2 Answers2

Linked