Thread: update head pointer in linked list

Hi, I have a slight confusion about the function for updating head pointer of a linked list.

Here is the code:

Code:

struct node *deleteFirst(struct node *head)
{
    if(head != NULL)
    {
       // store the old value of head pointer
       struct node *temp = head;
 
       // Change head pointer to point to next node
       head = head->next;
 
       // delete memory allocated for the previous head node
       free(temp);
    }
 
    return head;
}

Here, I want to know, as head is a formal parameter of the function, it will be stored on the stack when the function is called and automatically destroyed when the function returns. So returning this head pointer from the function should result in some sort of error/ incorrect result instead of the head pointer being updated. Correct me where I am wrong. Waiting for the replies.

The object “head” will be destroyed when the function exits, yes. That's OK, though, because you're not returning the address of “head”. You're returning the value of head: you're making a copy.

This would be like if I showed you a piece of paper with the number 5 on it, which you copied down onto your own piece of paper. When I destroy my piece of paper, you don't care, because you already copied what was on it.

The result will still be correct. Yes head will be destroyed when the function returns but it will not destroy (or free) what head points to. Just remember head is just a variable that holds a location (pointer) to a value. When you destroy head you will just loose the location but the value still lives on that location. I don't know if I explained it right its a hard topic. Try searching for Pointers and Memory tutorials for better understanding.

Originally Posted by gaurav_13191

Hi, I have a slight confusion about the function for updating head pointer of a linked list.

Here is the code:

Code:

struct node *deleteFirst(struct node *head)
{
    if(head != NULL)
    {
       // store the old value of head pointer
       struct node *temp = head;
 
       // Change head pointer to point to next node
       head = head->next;
 
       // delete memory allocated for the previous head node
       free(temp);
    }
 
    return head;
}

Here, I want to know, as head is a formal parameter of the function, it will be stored on the stack when the function is called and automatically destroyed when the function returns. So returning this head pointer from the function should result in some sort of error/ incorrect result instead of the head pointer being updated. Correct me where I am wrong. Waiting for the replies.

In your example, temp is destroyed when the function exits... head is not.

@commontater You mean that formal parameters are not destroyed when the function returns? Please clarify.

@others Thanks for your replies. What I understood is that though head will be destroyed it will return the address of the location where data exists. Correct me if I am getting this wrong.

Originally Posted by gaurav_13191

@commontater You mean that formal parameters are not destroyed when the function returns? Please clarify.

The value returned is not destroyed... it is held in a CPU register.

@others Thanks for your replies. What I understood is that though head will be destroyed it will return the address of the location where data exists. Correct me if I am getting this wrong.

Nope... you got it right.

@commontater Ok.. I got it .. thanks!