Thread: Why are these pointers suddenly changed?

I created a series of linked lists and then printed them. Notice that I touch new->prev one time, and I verify it has been set via printf at the end of the for loop. However, when I then print all the nodes via printnode(), suddenly the ->prev pointers have all been changed to "0x11". What does that mean? How could this happen?? Where did the pointers go?

Code:

			A_previous=NULL; 

			for (i=0; i<n_areas; i++){
				new=(Area*)malloc(sizeof(Area*)); 
				if(i==0){head=new;} 
				new->name=i; 
				new->n_exits=0; 
				new->next=NULL; 
				new->prev=A_previous; 
				if(A_previous!=NULL){A_previous->next=new;} 
				A_previous=new; 
				printf("[[%p]]", new->prev);}

			printnode(head);

Code:

void printnode(Area *current) {
    while (current) {
        printf("\n<%d||[%p]>: <%p|%p>",
               current->name, current, current->prev, current->next);
        current = current->next;
    }
}

Output:

Code:

Reading: 10 areas...[[(nil)]][[0x804a130]][[0x806b008]][[0x806b018]][[0x806b028]][[0x806b038]][[0x806b048]][[0x806b058]][[0x806b068]][[0x806b078]]
<0||[0x804a130]>: <(nil)|0x806b008>
<1||[0x806b008]>: <0x11|0x806b018>
<2||[0x806b018]>: <0x11|0x806b028>
<3||[0x806b028]>: <0x11|0x806b038>
<4||[0x806b038]>: <0x11|0x806b048>
<5||[0x806b048]>: <0x11|0x806b058>
<6||[0x806b058]>: <0x11|0x806b068>
<7||[0x806b068]>: <0x11|0x806b078>
<8||[0x806b078]>: <0x11|0x806b088>
<9||[0x806b088]>: <0x806b078|(nil)>

-Adam Rinkleff

This is wrong:

Code:

new=(Area*)malloc(sizeof(Area*));

It should be:

Code:

new = (Area*)malloc(sizeof(Area));

Though I recommend:

Code:

new = malloc(sizeof(*new));

You may or may not find that this magically fixes your problem.

Originally Posted by Adam Rinkleff

I guess it knows the size of what new points to, because I declared new already as an Area*.

Yes.