Function pointer question in C

Question

#include <stdlib.h>

int int_sorter( const void *first_arg, const void *second_arg )
{
    int first = *(int*)first_arg;
    int second = *(int*)second_arg;
    if ( first < second )
    {
        return -1;
    }
    else if ( first == second )
    {
        return 0;
    }
    else
    {
        return 1;
    }
}

int main()
{
    int array[10];
    int i;
    /* fill array */
    for ( i = 0; i < 10; ++i )
    {
        array[ i ] = 10 - i;
    }
    qsort( array, 10 , sizeof( int ), int_sorter );
    for ( i = 0; i < 10; ++i )
    {
        printf ( "%d\n" ,array[ i ] );
    }

}

I don't understand this line :

int first = *(int*)first_arg;

could anyone help me? explain it? thank you very much!!!

Is this a casting? explicit cast, from void* to int? but why we need ampersand outside ()?

"Why do we need an ampersand outside ()?"?

user541686
– user541686

2011-04-05 04:03:26 +00:00
Commented Apr 5, 2011 at 4:03 — user541686
– user541686, Commented Apr 5, 2011 at 4:03

Christopher Armstrong · Accepted Answer · 2011-04-05 04:01:04Z

9

You are first casting the void pointer to an int pointer: (int*)first_arg

And then dereferencing the pointer: *(int*)first_arg to get the integer it points to. This is then assigned to an integer variable.

answered Apr 5, 2011 at 4:01

Christopher Armstrong

7,9812 gold badges28 silver badges28 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Keith · Accepted Answer · 2011-04-05 04:11:44Z

3

int first = *(int*)first_arg;

is the same as:

int* ptrToFirst = (int*)first_arg;

This is an explicit cast from const void* to int.

int first = *ptrToFirst;

Here the * is the dereferencing operator. This is pronounced 'star'.

This is an 'ampersand': '&'.

Why are we doing this? qsort() needs to have the callback determine the ordering. By passing the values indirectly as const void pointers, the arguments to the callback have fixed size even though the values could be any size. That is why the values need cast back in the callback.

answered Apr 5, 2011 at 4:11

Keith

6,84422 silver badges24 bronze badges

Collectives™ on Stack Overflow

Function pointer question in C

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related