Calloc and free

 #include <stdio.h>

#include <stdlib.h> 

int main() {

    int *ptr;

    int n = 5;

    printf("Enter number of values:\n");

    scanf("%d",&n);

    ptr = (int *)calloc(n, sizeof(int));

    if (ptr == NULL) {

        printf("Memory allocation failed!\n");

        return 1; 

    }

    printf("\n");

    for (int i = 0; i < n; i++) {

        ptr[i] = i *10+10;

    }

    printf("After assigning values:\n");

    for (int i = 0; i <n ; i++) {

        printf("%d ", ptr[i]); 

    }

    printf("\n");

    printf(" values with extra index:\n");

    for (int i = 0; i <n +5; i++) {

        printf("%d ", ptr[i]); 

    }

    free(ptr);

       

         printf("\n");

        printf("After free values:\n");

    for (int i = 0; i <n ; i++) {

        printf("%d ", ptr[i]); 

    }

    return 0;

}

🟦 Step 1: Include Header Files

#include <stdio.h>
#include <stdlib.h>

Explanation

Header File	Purpose
stdio.h	Provides input/output functions (printf(), scanf())
stdlib.h	Provides memory management functions (calloc(), malloc(), realloc(), free())

---

🟦 Step 2: Main Function

int main()

Every C program starts execution from the main() function.

---

🟦 Step 3: Declare Variables

int *ptr;
int n = 5;

Explanation

• ptr is an integer pointer.
• It will point to dynamically allocated memory.
• n stores the number of integers to allocate.

Initially:

ptr
 ↓
NULL (not allocated yet)

n = 5

---

🟦 Step 4: Read Number of Elements

printf("Enter number of values:\n");
scanf("%d",&n);

Example Input

5

Now,

n = 5

---

🟦 Step 5: Allocate Memory using calloc()

ptr = (int *)calloc(n, sizeof(int));

Syntax

calloc(Number_of_elements, Size_of_each_element)

If

n = 5
sizeof(int) = 4 bytes

Then memory allocated is

5 × 4 = 20 bytes

Memory Diagram

ptr
 ↓

+----+----+----+----+----+
| 0  | 0  | 0  | 0  | 0  |
+----+----+----+----+----+

Important: calloc() initializes all allocated memory to 0.

---

🟦 Step 6: Check Allocation

if(ptr == NULL)
{
    printf("Memory allocation failed!\n");
    return 1;
}

Why?

Sometimes the operating system cannot provide memory.

If allocation fails,

ptr = NULL

The program exits safely.

---

🟦 Step 7: Store Values

for(int i=0;i<n;i++)
{
    ptr[i]=i*10+10;
}

Calculations

i	Formula	Stored Value
0	0×10+10	10
1	1×10+10	20
2	2×10+10	30
3	3×10+10	40
4	4×10+10	50

Memory

ptr
 ↓

+----+----+----+----+----+
|10  |20  |30  |40  |50  |
+----+----+----+----+----+

---

🟦 Step 8: Print Values

for(int i=0;i<n;i++)
{
    printf("%d ",ptr[i]);
}

Output

10 20 30 40 50

---

🟦 Step 9: Print Beyond Allocated Memory

for(int i=0;i<n+5;i++)
{
    printf("%d ",ptr[i]);
}

Here,

n = 5

Loop runs from

0 to 9

But allocated memory only contains

Index

0
1
2
3
4

The program also accesses

5
6
7
8
9

which are outside the allocated memory.

Memory Representation

Allocated Memory

+----+----+----+----+----+
|10  |20  |30  |40  |50  |
+----+----+----+----+----+

Invalid Access

Index

5
6
7
8
9

---

⚠ Undefined Behaviour

Possible outputs:

10 20 30 40 50 0 1024 -5 12345

or

Segmentation Fault

or

Garbage Values

The result is unpredictable.

---

🟦 Step 10: Free Allocated Memory

free(ptr);

What happens?

The memory allocated by calloc() is returned to the operating system.

Before free():

ptr
 ↓

+----+----+----+----+----+
|10  |20  |30  |40  |50  |
+----+----+----+----+----+

After free():

Memory Released

ptr
 ↓
Dangling Pointer

The pointer still contains the old address, but that memory is no longer valid.

---

🟦 Step 11: Access Memory After free()

for(int i=0;i<n;i++)
{
    printf("%d ",ptr[i]);
}

This is incorrect.

The memory has already been released.

Possible output:

10 20 30 40 50

or

0 0 0 0 0

or

Garbage Values

or

Segmentation Fault

This is another example of Undefined Behaviour.

---

📊 Complete Memory Flow

1 Before Allocation

ptr

↓

NULL

---

2. After calloc()

ptr

↓

+----+----+----+----+----+
| 0  | 0  | 0  | 0  | 0  |
+----+----+----+----+----+

---

3.After Storing Values

ptr

↓

+----+----+----+----+----+
|10  |20  |30  |40  |50  |
+----+----+----+----+----+

4.After free()

Memory Returned to OS

ptr

↓

Dangling Pointer

---

🖥 Sample Execution

Input

Enter number of values:
5

Output

After assigning values:
10 20 30 40 50

Values with extra index:
10 20 30 40 50 0 -1452672 8 0 245

After free values:
-1452672 8 0 245 1