Dynamic Allocation and Linked Lists

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Dynamic Allocation and Linked Lists
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Dynamic memory allocation in C

• C uses the functions malloc() and free() to
  implement dynamic allocation.
• malloc is defined in ltstdlib.hgt header files.
• malloc returns a void pointer.
• the pointer returned from malloc requires a type
  cast to make it usable. (void just a memory
  address)

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Null Pointers

• When a memory allocation function is called,
  theres always a possibility that it wont be
  able to locate a block of memory large enough to
  satisfy the requres.
• A null pointer will be returned.
• Test the return value
• p malloc(10000)
• if(p NULL)

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Example

• struct rec
• char LastName41
• char FirstName41
• struct rec r
• r (struct rec ) malloc(sizeof(struct rec))

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Freeing Memory

• Memory allocated with malloc must be released
  after you are done with them.
• this memory is released by calling the function
  free().
• free expects as an argument the pointer returned
  by malloc.
• free( r )

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Allocate Memory for Arrays
int p // one dimension int q // tow
dimension int i p (int ) malloc( sizeof(int)
n) q (int ) malloc( sizeof(int)
n) for(i0 iltn i) qi (int )
malloc( sizeof(int) n) // free(p) free(q)
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The Dangling Pointer Problem

• char p malloc(4)
• .
• free(p)
• .
• strcpy(p, abc) // WRONG

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Linked List

• A linked list is a basic data structure.
• For easier understanding divide the linked list
  into two parts.
• Nodes make up linked lists.
• Nodes are structures made up of data and a
  pointer to another node.
• Usually the pointer is called next.

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Nodes

• struct node
• struct rec r
• struct node next

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LIST

• The list will contain a pointer to the start of
  the list.
• For our purposes all data will be added to the
  start of the list.
• Initialize the start pointer to NULL.

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Creating a New Node

• Allocate space for a new node.
• Set the next pointer to the value of NULL
• Set the data value for the node.

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Adding Nodes to the List

• If the start node is null then the start node
  becomes the new node.
• If start is not null then start becomes the new
  nodes next and the start becomes the new node.

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Deleting a node
Q
R
P
I want to delete Q
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Deleting a node
Q
R
P
P-gtnext Q-gtnext Q-gtnext NULL Free(Q)
I want to delete Q