Memory allocation flow - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is memory allocation in C?

Memory allocation is the process of reserving a portion of computer memory for use by a program. In C, this can be done statically, automatically, or dynamically.

beginner

What function is used to allocate memory dynamically in C?

The malloc() function is used to allocate a block of memory dynamically during program execution.

intermediate

Describe the flow of memory allocation using malloc().

1. Program calls malloc() requesting memory size.
2. The system checks for available memory.
3. If enough memory is free, it reserves the block.
4. malloc() returns a pointer to the start of this block.
5. If not enough memory, malloc() returns NULL.

beginner

What should you do after finishing using dynamically allocated memory?

You should free the memory using the free() function to avoid memory leaks and allow the system to reuse that memory.

intermediate

What happens if you forget to free dynamically allocated memory?

The program will have a memory leak, meaning it keeps using more memory over time, which can slow down or crash the system.

Which function in C is used to allocate memory dynamically?

A malloc()

B printf()

C scanf()

D free()

malloc() allocates memory dynamically. free() releases it.

What does malloc() return if it cannot allocate memory?

A A pointer to allocated memory

B An integer 0

C NULL

D A random value

malloc() returns NULL when it fails to allocate memory.

What is the purpose of the free() function?

A Allocate new memory

B Release previously allocated memory

C Check memory size

D Print memory contents

free() releases memory allocated by malloc() or similar functions.

What happens if you use memory after freeing it?

A Undefined behavior, possible crash

B Program runs faster

C Memory is automatically reallocated

D Nothing happens

Using freed memory causes undefined behavior and can crash the program.

Which of these is NOT a type of memory allocation in C?

A Static allocation

B Automatic allocation

C Dynamic allocation

D Virtual allocation

Virtual allocation is not a standard C memory allocation type.

Explain the flow of dynamic memory allocation using malloc() in C.

Think about what happens step-by-step when you ask for memory.

Why is it important to use free() after dynamic memory allocation?

Consider what happens if memory is never released.

Practice

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

easy

A. Frees previously allocated memory

B. Allocates a block of memory on the heap

C. Declares a variable

D. Copies data from one memory location to another

Solution

1. Step 1: Understand the purpose of malloc

   malloc is used to allocate memory dynamically on the heap during program execution.

2. Step 2: Differentiate from other memory functions

   free releases memory, declaration creates variables, and copying data is done by functions like memcpy.

3. Final Answer:

   Allocates a block of memory on the heap -> Option B

4. Quick Check:

   malloc = Allocate memory [OK]

Hint: Remember: malloc = get memory, free = release memory [OK]

Common Mistakes:

• Confusing malloc with free
• Thinking malloc declares variables
• Assuming malloc copies data

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

easy

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

B. int *ptr = malloc(10);

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

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

Solution

1. Step 1: Calculate total memory needed

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

2. Step 2: Check pointer type and assignment

   The pointer must be of type int * and assigned the result of malloc. int *ptr = malloc(10 * sizeof(int)); matches this.

3. Final Answer:

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

4. Quick Check:

   Correct malloc size and pointer type = int *ptr = malloc(10 * sizeof(int)); [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 this C code snippet?

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

medium

A. 10 20

B. 0 0

C. Allocation failed

D. Compilation error

Solution

1. Step 1: Check memory allocation and NULL condition

   The code allocates memory for 4 integers. If allocation fails, it prints "Allocation failed". Otherwise, it proceeds.

2. Step 2: Assign and print values

   Since allocation likely succeeds, ptr[0] = 10 and ptr[1] = 20 are set, then printed as "10 20".

3. Final Answer:

   10 20 -> Option A

4. Quick Check:

   Successful malloc prints assigned values [OK]

Hint: Check if malloc returns NULL before using pointer [OK]

Common Mistakes:

• Ignoring NULL check
• Assuming uninitialized values print zero
• Forgetting to free memory

4. Identify the error in this code snippet related to memory allocation:

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

medium

A. Memory allocated is too small for the string

B. Missing check if malloc returned NULL

C. Using strcpy without including

D. No error, code is correct

Solution

1. Step 1: Check allocated memory size

   Memory allocated is 5 bytes, but the string "Hello World" needs 12 bytes (11 chars + 1 null terminator).

2. Step 2: Understand consequences of insufficient memory

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

3. Final Answer:

   Memory allocated is too small for the string -> Option A

4. Quick Check:

   Allocated size < string length = Memory allocated is too small for the string [OK]

Hint: Allocate enough space including null terminator [OK]

Common Mistakes:

• Ignoring string length vs allocated size
• Not checking malloc return value
• Assuming strcpy auto-expands memory

5. You want to allocate memory for an array of 5 integers, initialize them to zero, and then safely release the memory. Which code snippet correctly implements this flow?

hard

A. int *arr = malloc(5 * sizeof(int));
for(int i=0; i<5; i++) arr[i] = 0;

B. int arr[5];
for(int i=0; i<5; i++) arr[i] = 0;

C. int *arr = calloc(5, sizeof(int));
free(arr);

D. int *arr = malloc(5);
memset(arr, 0, 5 * sizeof(int));
free(arr);

Solution

1. Step 1: Understand memory allocation and initialization

   calloc allocates and initializes memory to zero automatically, perfect for this task.

2. Step 2: Check safe memory release

   free(arr); correctly releases the allocated memory after use.

3. Final Answer:

   int *arr = calloc(5, sizeof(int)); free(arr); -> Option C

4. Quick Check:

   calloc zero-initializes and malloc does not [OK]

Hint: Use calloc for zero-initialized memory allocation [OK]

Common Mistakes:

• Using malloc without initializing memory
• Allocating too little memory
• Not freeing dynamically allocated memory