L7-2_ Dynamic Memory Allocation

7/29/2019 L7-2_ Dynamic Memory Allocation

1/16

Dynamic Memory Allocation

Spring Semester 2011 Programming and Data Structure 51


2/16

Basic Idea

Many a time we face situations where dataare dynamic in nature.

Amount of data cannot be predictedbeforehand.


um er o a a ems eeps c ang ng ur ngprogram execution.

Such situations can be handled more

easily and effectively using dynamicmemory management techniques.


3/16

Contd.

C language requires the number of elementsin an array to be specified at compile time.

Often leads to wastage or memory space orprogram failure.


- a ows s, owever Dynamic Memory Allocation

Memory space required can be specified at the

time of execution.

C supports allocating and freeing memorydynamically using library routines.


4/16

Memory Allocation Process in C

Local variables

Free memory

Global variables

Stack

Heap


Instructions

storage area


5/16

Contd.

The program instructions and the globalvariables are stored in a region known aspermanent storage area.

The local variables are stored in another


area ca e s ac . The memory space between these two

areas is available for dynamic allocation

during execution of the program. This free region is called the heap.

The size of the heap keeps changing.


6/16

Memory Allocation Functions

malloc

Allocates requested number of bytes and returns

a pointer to the first byte of the allocated space. calloc

Allocates space for an array of elements,


initializes them to zero and then returns a pointerto the memory.

free

Frees previously allocated space.

realloc

Modifies the size of previously allocated space.


7/16

Allocating a Block of Memory

A block of memory can be allocated usingthe functionmalloc.

Reserves a block of memory of specified sizeand returns a pointer of type void.


e re urn po n er can e ype-cas e o anypointer type.

General format:

ptr = (type *) malloc (byte_size);


8/16

Contd.

Examples

p = (int *) malloc(100 * sizeof(int));

A memory space equivalent to 100 times the sizeof an intb tes is reserved.


The address of the first byte of the allocatedmemory is assigned to the pointerp of type int.

p

400 bytes of space


9/16

Contd.

cptr = (char *) malloc (20);

Allocates 20 bytes of space for the pointer cptr oftype char.


sptr = (struct stud *) malloc

(10 * sizeof (struct stud));

Allocates space for a structure array of 10elements. sptr points to a structure element oftype struct stud.


10/16

Points to Note

malloc always allocates a block of

contiguous bytes.

The allocation can fail if sufficient contiguousmemory space is not available.


,ma oc .

if ((p = (int *) malloc(100 * sizeof(int))) == NULL)

{

printf (\n Memory cannot be allocated);

exit();

}


11/16

Releasing the Used Space

When we no longer need the data stored in ablock of memory, we may release the block for

future use.

How?


General syntax:

free (ptr);

whereptr is a pointer to a memory block whichhas been previously created usingmalloc.


12/16

Example 1: using 1-D array

printf("Input heights for %d

students \n",N);

for (i=0; i


13/16

Example 2: Bubble sort on array of structures

#include

typedef struct

{

int roll;

char dept_code[25];float cgpa;

} stud;

for (k=0; k


14/16

Some Points

class is a pointer to the starting address of the

allocated memory block.

We can therefore access the individual elementsof the structure array using array notation:

, . , .

As an alternative, we can also access usingpointersand the arrow notation:

Example: (class+k), (class+k)->roll, etc.



15/16

Altering the Size of a Block

Sometimes we need to alter the size of somepreviously allocated memory block.

More memory needed. Memory allocated is larger than necessary.


By using the realloc function.

If the original allocation is done as:

ptr = malloc (size);

then reallocation of space may be done as:

ptr = realloc (ptr, newsize);


16/16

Contd.

The new memory block may or may not begin atthe same place as the old one.

If it does not find space, it will create it in anentirely different region and move the contentsof the old block into the new block.


The function guarantees that the old dataremains intact.

If it is unable to allocate, it returnsNULL and frees

the original block.

L7-2_ Dynamic Memory Allocation

Documents

Transcript of L7-2_ Dynamic Memory Allocation