Memory leak concepts - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a memory leak in C programming?

A memory leak happens when a program allocates memory but never frees it, causing the memory to stay used and unavailable for other parts of the program or system.

beginner

Why are memory leaks a problem in programs?

Memory leaks reduce the available memory over time, which can slow down or crash programs because they run out of memory to use.

beginner

Which C functions are commonly used to allocate and free memory?

malloc() is used to allocate memory, and free() is used to release that memory back to the system.

intermediate

What happens if you call free() on a pointer twice?

Calling free() twice on the same pointer can cause undefined behavior, which may crash the program or corrupt memory.

intermediate

How can you detect memory leaks in C programs?

You can use tools like Valgrind or AddressSanitizer to find memory leaks by tracking allocated memory that was never freed.

What does a memory leak cause in a program?

A Program runs faster

B Memory stays allocated and is never freed

C Memory is freed too early

D Memory is shared between programs

A memory leak means memory is allocated but not freed, causing it to stay used.

Which function is used to free allocated memory in C?

A free()

B delete()

C alloc()

D malloc()

free() releases memory that was allocated with malloc() or similar functions.

What is a common cause of memory leaks?

A Calling free() too many times

B Using global variables

C Forgetting to call free() after malloc()

D Writing to a file

Not freeing allocated memory causes memory leaks.

Which tool can help detect memory leaks in C programs?

A Valgrind

B GCC

C Make

D Git

Valgrind tracks memory usage and can find leaks.

What happens if you free memory twice?

A Memory is automatically reallocated

B Program runs normally

C Memory is doubled

D Undefined behavior, possible crash

Freeing memory twice can cause crashes or corrupt memory.

Explain what a memory leak is and why it is harmful in C programs.

Think about what happens when memory is allocated but never given back.

Describe how you can prevent memory leaks when using malloc() in C.

Consider the steps after allocating 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