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13-03-03 P.P.Chakrabarti, IIT Kharagpur1

Dynamic Allocation

P. P. Chakrabarti

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Fun with scope rules:

int data=0, test=1;

main()

{ int i=2, j=3, test=2;

printf("M1: i = %d, j = %d\n", i,j);

printf("M2: test = %d, data = %d\n", test, data);

{

int data = 1, i =5;

printf("B1: i = %d, j = %d\n", i,j);

printf("B2: test = %d, data = %d\n", test, data);

}

printf("M3: i = %d, j = %d\n", i,j);

printf("M4: test = %d, data = %d\n", test, data);

}

[ppchak]$ ./a.out

M1: i = 2, j = 3

M2: test = 2, data = 0

B1: i = 5, j = 3

B2: test = 2, data = 1M3: i = 2, j = 3

M4: test = 2, data = 0

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Spot the difference:

int data=0, test=1;

main()

{ int i=2, j=3, test=2;

printf("M1: i = %d, j = %d\n", i,j);

printf("M2: test = %d, data = %d\n", test, data);

{

int data = 1; i =5;

printf("B1: i = %d, j = %d\n", i,j);

printf("B2: test = %d, data = %d\n", test, data);

}

printf("M3: i = %d, j = %d\n", i,j);

printf("M4: test = %d, data = %d\n", test, data);

}

[ppchak]$ ./a.out

M1: i = 2, j = 3

M2: test = 2, data = 0

B1: i = 5, j = 3

B2: test = 2, data = 1M3: i = 5, j = 3

M4: test = 2, data = 0

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More on scope:

int data=0, test=1;

main()

{ int i=2, j=3, test=2;

printf("M1: i = %d, j = %d\n", i,j);

printf("M2: test = %d, data = %d\n", test, data);

fun();

}

fun()

{ int data =5;

int i=6, j =7;

printf("F1: i = %d, j = %d\n", i,j);

printf("F2: test = %d, data = %d\n", test, data);

}

[ppchak]$ ./a.out

M1: i = 2, j = 3

M2: test = 2, data = 0

F1: i = 6, j = 7F2: test = 1, data = 5

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Problems with static allocation

Scenario 1: Read an integer n, read n integers and sortthem.

[Q: How do we declare the array size?]

Scenario 2: Make the set ADT with the insert, delete,

member, union functions with no idea of how manyelements may exist at a given point in time.

[Q: Can we use an array at all for this?]

[ How to prevent space wastage in these scenarios?]

To solve these, C provides scope for dynamically

allocating and releasing memory using library

functions

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Memory Allocation Process in C

Local variables

Free memory

Global variables

InstructionsPermanentstorage area

Stack

Heap

Used for dynamic

allocation

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Memory Allocation Functions

?malloc: Allocates requested number of bytes andreturns 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 pointer tothe memory.

? free : Frees previously allocated space.

? realloc: Modifies the size of previously allocatedspace.

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malloc( )main()

{int *A, n, i;

scanf("%d", &n);

A = (int *) malloc(n*sizeof(int));

for (i=0; i

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malloc( )-ing recordsmain()

{ typedef struct{

char name[20];

int roll;

float SGPA[8], CGPA;

} person;

person *student;int i,j,n;

scanf("%d", &n);

student = (person *)malloc(n*sizeof(person));

for (i=0; i

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Arrays of pointers

#define N 20

#define M 10main()

{ char word[N], *w[M];

int i, n;

scanf("%d",&n);for (i=0; i

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w

0

1

2

3

9

How it will look like

\0rakludneT

\0varuoS

\0nahK

\0aidnI

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Dynamic Arrays of pointers

#define N 20

main(){

char word[N], **w; /* **w is a pointer to a pointer array */

int i, n;

scanf("%d",&n);

w = (char **) malloc (n * sizeof(char *));for (i=0; i

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w

0

1

2

3

4

How this will look like

\0aidnI

\0aknaLirS

\0ailartsuA

\0ayneK

N \0dnalaeZwe