malloc function - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the malloc function do in C?

The malloc function allocates a block of memory of a specified size on the heap and returns a pointer to the beginning of this block.

beginner

What header file must be included to use malloc?

You must include the <stdlib.h> header file to use malloc in C.

beginner

What does malloc return if it fails to allocate memory?

malloc returns NULL if it cannot allocate the requested memory.

beginner

How do you properly free memory allocated by malloc?

You use the free function and pass the pointer returned by malloc to release the allocated memory back to the system.

beginner

Example: What does this code do?

int *p = malloc(5 * sizeof(int));

This code allocates memory for an array of 5 integers on the heap and stores the pointer to this memory in p. The cast to (int *) is not necessary in C and is omitted here.

What is the correct way to allocate memory for 10 doubles using malloc?

A malloc(10);

B malloc(10 * sizeof(int));

C malloc(10 * sizeof(double));

D malloc(sizeof(double));

You must multiply the number of elements (10) by the size of each element (sizeof(double)) to allocate enough memory.

If malloc fails, what value does it return?

A Garbage value

B 0

C -1

D NULL

malloc returns NULL to indicate failure to allocate memory.

Which header file is required to use malloc?

A <code>&lt;stdlib.h&gt;</code>

B <code>&lt;string.h&gt;</code>

C <code>&lt;stdio.h&gt;</code>

D <code>&lt;malloc.h&gt;</code>

The <stdlib.h> header contains the declaration for malloc.

What must you do after you finish using memory allocated by malloc?

A Nothing, it frees automatically

B Call <code>free</code> with the pointer

C Call <code>delete</code>

D Set pointer to NULL

You must call free to release the allocated memory and avoid memory leaks.

What type of pointer does malloc return?

A void *

B int *

C char *

D float *

malloc returns a void * pointer which can be cast to any other pointer type.

Explain how malloc works and why it is important in C programming.

Think about how programs get memory dynamically.

Describe the steps to safely allocate and free memory using malloc.

Consider what happens if malloc fails and how to avoid memory leaks.

Practice

1. What does the malloc function do in C programming?

easy

A. Declares a variable

B. Frees previously allocated memory

C. Allocates a block of memory dynamically during program execution

D. Prints output to the screen

Solution

1. Step 1: Understand the purpose of malloc

   The malloc function is used to allocate memory dynamically while the program is running, not at compile time.

2. Step 2: Differentiate malloc from other functions

   Unlike free which releases memory, or printf which outputs text, malloc only allocates memory.

3. Final Answer:

   Allocates a block of memory dynamically during program execution -> Option C

4. Quick Check:

   malloc = allocate memory [OK]

Hint: malloc is for getting memory, free is for releasing it [OK]

Common Mistakes:

• Confusing malloc with free
• Thinking malloc prints output
• Assuming malloc declares variables

2. Which of the following is the correct syntax to allocate memory for 10 integers using malloc?

easy

A. int *ptr = malloc(10 * sizeof(int));

B. int *ptr = malloc(10);

C. int ptr = malloc(10 * sizeof(int));

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

Solution

1. Step 1: Calculate correct size for allocation

   To allocate memory for 10 integers, multiply 10 by the size of an integer using sizeof(int).

2. Step 2: Use correct pointer type and syntax

   The pointer must be declared as int * and assigned the result of malloc. int *ptr = malloc(10 * sizeof(int)); correctly does this.

3. Final Answer:

   int *ptr = malloc(10 * sizeof(int)); -> Option A

4. Quick Check:

   malloc size = count * sizeof(type) [OK]

Hint: Multiply count by sizeof(type) for malloc size [OK]

Common Mistakes:

• Not multiplying by sizeof(int)
• Using wrong pointer type
• Allocating too little memory

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

int *p = malloc(4 * sizeof(int));
if (p == NULL) {
    printf("Allocation failed\n");
} else {
    p[0] = 10;
    p[1] = 20;
    printf("%d %d\n", p[0], p[1]);
}
free(p);

medium

A. Compilation error

B. Allocation failed

C. 0 0

D. 10 20

Solution

1. Step 1: Check if malloc succeeded

   The code checks if p is NULL. If not NULL, allocation succeeded and memory can be used.

2. Step 2: Assign and print values

   Values 10 and 20 are assigned to p[0] and p[1]. Then they are printed, so output is "10 20".

3. Final Answer:

   10 20 -> Option D

4. Quick Check:

   malloc success prints assigned values [OK]

Hint: Check for NULL before using malloc memory [OK]

Common Mistakes:

• Ignoring NULL check
• Printing uninitialized memory
• Forgetting to free memory

4. Identify the error in this code snippet:

char *str = malloc(5);
strcpy(str, "Hello World");
printf("%s", str);
free(str);

medium

A. strcpy is not allowed with malloc

B. malloc size is too small for the string

C. free should be called before strcpy

D. printf format specifier is wrong

Solution

1. Step 1: Check allocated memory size

   The string "Hello World" needs 12 bytes (11 chars + 1 for '\0'), but only 5 bytes are allocated.

2. Step 2: Understand consequences of small allocation

   Copying a larger string into smaller allocated memory causes buffer overflow and undefined behavior.

3. Final Answer:

   malloc size is too small for the string -> Option B

4. Quick Check:

   Allocate enough memory for string + '\0' [OK]

Hint: Allocate string length + 1 for null terminator [OK]

Common Mistakes:

• Allocating less memory than string length
• Calling free before using memory
• Wrong printf format for strings

5. You want to create a dynamic array of integers that doubles its size when full. Which approach correctly uses malloc and realloc to achieve this?

hard

A. Start with malloc for initial size, then use realloc to double size when needed

B. Use malloc once with maximum size needed upfront

C. Use realloc first, then malloc to allocate memory

D. Use malloc repeatedly without freeing old memory

Solution

1. Step 1: Allocate initial memory with malloc

   Start by allocating memory for the initial array size using malloc.

2. Step 2: Use realloc to resize when array is full

   When the array is full, use realloc to double the memory size safely without losing data.

3. Step 3: Avoid incorrect usage

   Using malloc repeatedly without freeing old memory causes leaks, allocating maximum size upfront wastes memory, and using realloc before malloc is incorrect.

4. Final Answer:

   Start with malloc for initial size, then use realloc to double size when needed -> Option A

5. Quick Check:

   malloc first, then realloc to grow [OK]

Hint: malloc once, then realloc to grow array safely [OK]

Common Mistakes:

• Calling realloc before malloc
• Allocating max size upfront wasting memory
• Not freeing old memory causing leaks