Deferencing double pointer in C

Question

I'm trying to write a compare function for qsort and is having trouble with de-referencing double pointers.

I defined a structure

typedef struct {
    float x;
    float y;
} point;

and I made an array of pointers to point:

point* ptarr[2]={pt1,pt3} //pt1 and pt3 have type point*

My compare function is defined as follows:

int compare(const void * a, const void * b){
    return *(point**)a->x-*(point**)b->x;  //This is wrong
}

Since a and b are pointers to the values in the arrays, in my case they are pointers to pointers which point to struct point. Therefore I cast them into double pointer (point**), and then de-referenced it once, and tried to access the values inside struct. Compiler gave me a "request for member 'x' in something not a structure or union" error

I'm really confused about this. Can someone help me with it? thanks

point* ptarr[2]={pt1,pt3} //pt1 and pt3 have type point Does that actually work ? — cnicutar
– cnicutar, Commented Feb 5, 2013 at 6:57

Some programmer dude · Accepted Answer · 2013-02-05 06:56:14Z

1

If you check a reference table on operator precedence you will see that the -> access operator has higher priority than typecasting. This means that the expression (point**)a->x actually typecasts the x member not the a structure pointer. You want e.g. (*(point**)a)->x.

answered Feb 5, 2013 at 6:56

Some programmer dude

411k36 gold badges420 silver badges655 bronze badges

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

Comments

Dietrich Epp · Accepted Answer · 2013-02-05 06:55:42Z

1

It's just a syntax problem, really.

return *(point**)a->x-*(point**)b->x;        // This is wrong

return (*(point**)a)->x - (*(point**)b)->x;  // This is right

The reason you get an error because *x->y is the same as *(x->y), but you want (*x)->y.

Also you have a bug in your code:

point* ptarr[2]={pt1,pt3} // pt1 and pt3 have type point

Do you see the bug? Either pt1 and pt3 should have type point * (in which case the comment is wrong), or pt1 and pt3 should be replaced by &pt1 and &pt3. Errors in the comments should be treated as bugs, so it is a bug either way.

answered Feb 5, 2013 at 6:55

Dietrich Epp

216k39 gold badges366 silver badges426 bronze badges

Collectives™ on Stack Overflow

Deferencing double pointer 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