I currently studying C and one of my practice is to create a linked list in C.
typedef struct deck
{
int index;
struct deck *next;
} decks;
My question is that why can't I create a new linked list as follows:(index, card after "=" is defined as variables)
decks* head = malloc(sizeof(decks));
decks* ptr = head;
for(int i = 0; i < 10; i++) {
ptr -> index = i;
ptr = ptr -> next;
ptr = malloc(sizeof(decks));
}
Appreciate so much!
Draw pictures. Make a box for each instance and an arrow from each non-null pointer. ALWAYS follow the rules listed below. Assuming all memory management is done with malloc and free there are NO exceptions to these rules.
malloc you should draw a new box. free you should remove (or just cross it over with an X or something) a box. free, remove all arrows from that box.Note: For a typical call to malloc, you would use both rule 1, 4 and 5. Using malloc without taking care of the return pointer means leaking memory.
Note2: Important! You should NOT remove an arrow TO the box you're deallocating when you use free. Only do that when you specifically assign a pointer to null. In the same way, do NOT remove a box when you assign a pointer to null.
Note3: Important! If you free a box, rule 3 says that you should remove all arrows from that box. Do NOT remove any boxes the arrows are pointing to.
Consider this:
ptr = ptr -> next;
ptr = malloc(sizeof(decks));
The first line has no effect whatsoever, since you are immediately reassigning ptr.
You need something like this:
ptr -> next = malloc(sizeof(decks));
ptr = ptr -> next;
I would recommend using sizeof on the variable rather than the type, like this: ptr->next = malloc(sizeof *(ptr->next)) This is a good habit in general. Take this code:
int *x;
x = malloc(sizeof(int));
Now consider if you want to change the type of the variable x. You would need to change the argument to sizeof in EVERY call to malloc. Using x = malloc(sizeof *x) completely removes this problem.
ptr->nextis uninitialized and certainly not pointing to an existing memory block. Otherwise you wouldn't need the firstmalloceither.