malloc() function in c.

Question

Can anyone please explain this?

struct node 
{ 
    int data; 
    struct node * link; 
} 
main() 
{
    struct node *p, *list, *temp; 
    list = p = temp = NULL; 
    ......................... 
    ......................... 
} 

addbeg() 
{ 
    int x;
    temp=malloc(sizeof(struct node));
    scanf("%d", &x); 
    temp->data=x;
    temp->link = list;
    list=temp;
}

This is a code for entering data in linkedlist through 'C' language.The code is not complete, but, I think its enough for the purpose. Please explain the coding basically these lines:

 temp=malloc(sizeof(struct node));

and

 temp->link = list;
 list=temp;.

Answer 1

malloc allocates space for a new node.

temp->link = list

and

list = temp

makes that new node the head of your list.

Answer 2

first line: Allocate memory for a single additional node of the list.

second line: Attach the current list as the continuation after this element.

third line: Make the current element the beginning of a list.

Answer 3

malloc() is used to allocate memory - in this case for a new node.

The addbeg() function defined in your code does the following:

1. Defines a temporary variable x.
2. Allocates space for a new node.
3. Inputs an integer (%d code to scanf) and stores it in x.
4. Stores the value that was saved to x in the data field of the newly allocated node.
5. Stores the old "head" of the list pointed to by the variable 'list' as a link in the newly allocated node.
6. Sets the new node to be the new head of the list stored in the variable 'list'.

It's a very basic implementation of a linked list (http://en.wikipedia.org/wiki/Linked_list) of integers.

Answer 4

The operator 'sizeof' is used to calculate the size of datatype. The operator calculated how much space should be allocate on the structure node. malloc - allocate memory for new structure. Remember, if You allocate memory using malloc method, You always must to free this memory using free method.

free(tmp);

If you do not call this method, you will have the memory leak. This is very important in C.

[operator sizeof][1]http://en.wikipedia.org/wiki/Sizeof

Answer 5

temp and list aren't visible in addbeg, since they're declared in main. They either need to be passed in to addbeg or declared outside of main (global, yuck).