Memory leak concepts

Introduction

A memory leak happens when a program keeps using memory but never gives it back. This can make the program slow or crash.

When writing programs that use dynamic memory (like with malloc).

When you want your program to run smoothly without running out of memory.

When debugging why a program gets slower or crashes after running for a while.

When learning how to manage memory properly in C.

When working on long-running programs like servers or games.

Syntax

C

/* Memory leak example in C */
#include <stdlib.h>
int *ptr = malloc(sizeof(int));
*ptr = 10;
// Forgot to free(ptr);

malloc() reserves memory but does not give it back automatically.

You must use free() to release memory when done.

Examples

This code allocates memory but never frees it, causing a memory leak.

C

#include <stdlib.h>
int *ptr = malloc(sizeof(int));
*ptr = 5;
// No free called here, memory leak occurs

This code correctly frees the memory after use, preventing a leak.

C

#include <stdlib.h>
int *ptr = malloc(sizeof(int));
*ptr = 5;
free(ptr);

Allocating memory for a string but forgetting to free it causes a leak.

C

#include <stdlib.h>
#include <string.h>
char *str = malloc(20 * sizeof(char));
strcpy(str, "hello");
// Missing free(str); causes leak

Sample Program

This program allocates memory for 3 integers, uses them, then frees the memory to avoid a leak.

C

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

int main() {
    int *numbers = malloc(3 * sizeof(int));
    if (numbers == NULL) {
        printf("Memory allocation failed\n");
        return 1;
    }
    numbers[0] = 10;
    numbers[1] = 20;
    numbers[2] = 30;

    printf("Numbers: %d, %d, %d\n", numbers[0], numbers[1], numbers[2]);

    // Forgetting to free here would cause a memory leak
    free(numbers);

    return 0;
}

Important Notes

Always pair each malloc() with a free() to avoid leaks.

Memory leaks can cause your program to use more and more memory over time.

Tools like Valgrind can help find memory leaks in your programs.

Summary

Memory leaks happen when allocated memory is not freed.

Use free() to release memory after use.

Preventing leaks keeps programs fast and stable.

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