Malloc and free

  #include <stdio.h>

#include <stdlib.h> 

int main() {

    int *ptr; 

   ptr = (int*) malloc(sizeof(int));

    if (ptr == NULL) {

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

        return 1; 

    }

    printf("Memory successfully allocated using malloc.\n");

     printf("enter a value.\n");

    scanf("%d",ptr);   

        printf("value = %d ", *ptr);    

    printf("\n");

free(ptr);

 printf("after free ptr new value = %d ", *ptr);    

           return 0; 

}

🟦 Step 1: Include Header Files

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

📖 Explanation

Header File	Purpose
📘 stdio.h	Used for printf() and scanf()
📗 stdlib.h	Used for malloc(), calloc(), realloc(), and free()

---

🟦 Step 2: Declare Pointer

int *ptr;

Explanation

• ptr is an integer pointer.
• It will store the address of dynamically allocated memory.

Initially

ptr

↓

Uninitialized

---

🟦 Step 3: Allocate Memory

ptr = (int*) malloc(sizeof(int));

Explanation

malloc() allocates memory for one integer.

If

sizeof(int) = 4 bytes

then

malloc(4)

allocates 4 bytes of memory.

---

Memory Diagram

ptr

↓

+---------+
| Garbage |
+---------+

Important: Unlike calloc(), malloc() does not initialize memory. The allocated memory contains a garbage value until you assign one.

---

🟦 Step 4: Check Memory Allocation

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

Explanation

If the operating system cannot allocate memory,

ptr

↓

NULL

The program terminates safely.

---

🟦 Step 5: Read User Input

scanf("%d", ptr);

Suppose the user enters

50

Memory becomes

ptr

↓

+------+
|  50  |
+------+

---

🟦 Step 6: Print the Value

printf("Value = %d", *ptr);

*ptr means:

The value stored at the memory location pointed to by ptr.

Output

Value = 50

---

🟦 Step 7: Free the Memory

free(ptr);

Explanation

The allocated memory is returned to the operating system.

Before free()

ptr

↓

+------+
|  50  |
+------+

After free()

Memory Released

ptr

↓

Dangling Pointer

The pointer still contains the old address, but the memory is no longer valid.

---

🟥 Step 8: Accessing Memory After free()

printf("After free ptr new value = %d", *ptr);

❌ This is Incorrect

After free(), the pointer refers to memory that has already been released.

Accessing it causes Undefined Behaviour.

Possible outputs:

After free ptr new value = 50

or

After free ptr new value = 0

or

After free ptr new value = -125678

or

Segmentation Fault

There is no guaranteed result.

---

🌈 Memory Flow

🔹 Before malloc()

ptr

↓

Uninitialized

⬇

🔹 After malloc()

ptr

↓

+---------+
| Garbage |
+---------+

⬇

🔹 After User Input

ptr

↓

+------+
|  50  |
+------+

⬇

🔹 After free()

Memory Released

ptr

↓

Dangling Pointer

⬇

🔹 Accessing *ptr

❌ Undefined Behaviour

---

🖥 Sample Execution

Input

Enter a value:
50

Possible Output

Memory successfully allocated using malloc.
Enter a value:
50
Value = 50
After free ptr new value = 50

On another system or another run, the last line may show a different value or the program may crash.

---

⚠ Problems in the Program

Problem

free(ptr);

printf("%d", *ptr);

The memory has already been released.

The pointer becomes a dangling pointer.

Reading from it is undefined behaviour according to the C standard.

---

✅ Correct Version

free(ptr);
ptr = NULL;

if(ptr != NULL)
{
    printf("%d", *ptr);
}
else
{
    printf("Pointer is NULL. Memory has been freed.");
}

Output

Memory successfully allocated using malloc.
Enter a value:
50
Value = 50
Pointer is NULL. Memory has been freed.