Memory leak concepts - Step-by-Step Execution

Concept Flow - Memory leak concepts

Start Program

↓

Allocate Memory

↓

Use Memory

↓

Forget to Free Memory?

No→Free Memory

↓

Memory Released

↓

Memory Leak Occurs

↓

Program Continues or Ends

This flow shows how memory is allocated and used, but if not freed properly, it causes a memory leak.

Execution Sample

C

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

int main() {
  int *ptr = malloc(sizeof(int));
  *ptr = 10;
  // forgot to call free(ptr);
  return 0;
}

This code allocates memory for an integer, assigns 10, but forgets to free the memory, causing a leak.

Execution Table

Step	Action	Memory State	Pointer Value	Notes
1	Start program	No allocated memory	ptr = NULL	Program begins
2	Allocate memory with malloc	Memory block allocated	ptr = address 0x1000	Memory reserved for int
3	Assign value 10 to *ptr	Memory block holds 10	ptr = address 0x1000	Value stored in allocated memory
4	Program ends without free	Memory block still allocated	ptr = address 0x1000	Memory leak: allocated memory not freed

💡 Program ends but allocated memory was not freed, causing a memory leak.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4
ptr	NULL	0x1000 (allocated)	0x1000 (value=10)	0x1000 (leaked)

Key Moments - 3 Insights

Why does forgetting to call free() cause a problem?

Because the allocated memory remains reserved and unusable by the system, as shown in step 4 of the execution_table where memory is still allocated after program ends.

Is the pointer ptr itself leaked if we forget free()?

No, the pointer variable ptr is just a local variable and is cleaned up, but the memory it points to remains allocated, causing the leak (see variable_tracker).

What happens if we call free(ptr) after using the memory?

The memory is released back to the system, preventing leaks. This is shown in the concept_flow where the 'No' branch leads to 'Free Memory' and 'Memory Released'.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value stored at the memory address ptr points to?

A Uninitialized

B NULL

C 10

D 0

💡 Hint

Check the 'Memory State' column at step 3 in the execution_table.

At which step does the memory leak occur according to the execution_table?

A Step 4

B Step 2

C Step 3

D Step 1

💡 Hint

Look for when the memory remains allocated without being freed.

If we add free(ptr) before program ends, how would the 'Memory State' at step 4 change?

A Memory block still allocated

B Memory block freed

C Pointer becomes NULL automatically

D Memory block doubled

💡 Hint

Refer to the concept_flow where freeing memory releases it.

Concept Snapshot

Memory leak happens when allocated memory is not freed.
Use malloc() to allocate memory.
Use free() to release memory.
Forgetting free() causes leaks.
Leaked memory wastes system resources.

Full Transcript

This visual execution shows how a program allocates memory using malloc, stores a value, but forgets to free the memory before ending. The flow starts with program start, then memory allocation, usage, and checks if free is called. If not, memory leak occurs. The execution table traces each step: starting with no memory, then allocation, storing value 10, and finally program ends with memory still allocated, causing a leak. The variable tracker shows pointer ptr changes from NULL to an allocated address holding 10, and remains allocated after program ends. Key moments clarify why forgetting free causes leaks, that the pointer variable itself is not leaked, and that calling free prevents leaks. The quiz tests understanding of stored values, when leaks occur, and effects of calling free. The snapshot summarizes the concept simply: allocate with malloc, free with free, forgetting free causes leaks that waste memory.

Practice

1. What is a memory leak in C programming?

easy

A. When allocated memory is not freed after use

B. When a program runs out of variables

C. When a program crashes due to syntax errors

D. When memory is allocated too quickly

Solution

1. Step 1: Understand memory allocation

   Memory is allocated dynamically using functions like malloc().

2. Step 2: Identify what causes leaks

   If allocated memory is not freed using free(), it causes a memory leak.

3. Final Answer:

   When allocated memory is not freed after use -> Option A

4. Quick Check:

   Memory leak = not freeing allocated memory [OK]

Hint: Memory leak means forgetting to free allocated memory [OK]

Common Mistakes:

• Confusing memory leak with syntax errors
• Thinking memory leaks happen only on crashes
• Believing memory leaks are about variable count

2. Which of the following is the correct way to free dynamically allocated memory in C?

easy

A. free(ptr);

B. release(ptr);

C. delete(ptr);

D. dispose(ptr);

Solution

1. Step 1: Recall C memory management functions

   C uses malloc() to allocate and free() to release memory.

2. Step 2: Identify correct syntax

   free(ptr); is the standard function to free allocated memory.

3. Final Answer:

   free(ptr); -> Option A

4. Quick Check:

   Use free() to release memory [OK]

Hint: Use free() to release memory allocated by malloc() [OK]

Common Mistakes:

• Using delete which is C++ syntax
• Using non-existent functions like release or dispose
• Forgetting to pass the pointer to free()

3. What will be the output of the following C code snippet?

int *ptr = malloc(sizeof(int));
*ptr = 10;
printf("%d\n", *ptr);
// Missing free(ptr);

medium

A. Runtime crash

B. 10

C. Compilation error

D. 0

Solution

1. Step 1: Analyze memory allocation and assignment

   Memory is allocated and 10 is stored at that location.

2. Step 2: Check output and memory freeing

   printf prints 10 correctly; missing free() causes leak but no output error.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Output prints stored value despite missing free() [OK]

Hint: Missing free() leaks memory but doesn't affect immediate output [OK]

Common Mistakes:

• Assuming missing free causes crash or wrong output
• Confusing memory leak with runtime errors
• Expecting zero or garbage output

4. Identify the problem in this C code snippet:

char *str = malloc(20);
strcpy(str, "Hello World");
// No free(str) called
str = malloc(30);
strcpy(str, "New String");
free(str);

medium

A. No problem, code is correct

B. Syntax error in strcpy usage

C. Double free error

D. Memory leak due to first malloc not freed

Solution

1. Step 1: Trace memory allocations

   First malloc(20) is assigned to str, then overwritten by second malloc(30).

2. Step 2: Check freeing of memory

   First allocated memory is lost without free, causing memory leak.

3. Final Answer:

   Memory leak due to first malloc not freed -> Option D

4. Quick Check:

   Overwriting pointer without free causes leak [OK]

Hint: Always free old memory before reassigning pointer [OK]

Common Mistakes:

• Thinking strcpy causes syntax error
• Assuming double free without repeated free calls
• Believing code is leak-free because free is called once

5. You have a function that allocates memory for an integer array and returns it. Which practice helps prevent memory leaks when using this function multiple times?

hard

A. Use global variables to store allocated memory

B. Never call free() because the function handles it internally

C. Always call free() on the returned pointer before reassigning it

D. Allocate memory only once and never reuse pointers

Solution

1. Step 1: Understand ownership of allocated memory

   The caller owns the memory returned and must free it to avoid leaks.

2. Step 2: Prevent leaks by freeing before reassignment

   Calling free() before reassigning pointer prevents losing track of allocated memory.

3. Final Answer:

   Always call free() on the returned pointer before reassigning it -> Option C

4. Quick Check:

   Free before reuse prevents leaks [OK]

Hint: Free returned memory before pointer reuse [OK]

Common Mistakes:

• Assuming function frees memory automatically
• Using global variables without freeing
• Avoiding reuse but causing inefficient memory use