C Programming Week 5

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Questions and Answers

What does the malloc() function return upon a successful memory allocation?

A pointer to the end of the allocated block

A pointer that points to zero

A pointer to the beginning of the allocated block (correct)

NULL

Which function initializes allocated memory elements to zero?

calloc() (correct)

realloc()

malloc()

free()

What is the purpose of the realloc() function?

To allocate a new block of memory

To change the size of a previously allocated memory block (correct)

To deallocate previously allocated memory

To allocate memory and initialize it to zero

What is the result of calling free() on a pointer that has not been allocated memory?

It leads to undefined behavior Signup and view all the answers

Which of the following statements is true regarding memory allocation functions?

malloc() and realloc() should always be followed by free() Signup and view all the answers

What parameter does the malloc() function require to specify the amount of memory to allocate?

size_t size Signup and view all the answers

What does the malloc() function return if it fails to allocate memory?

NULL Signup and view all the answers

Which of the following functions is used specifically for deallocating memory?

free() Signup and view all the answers

What is the main difference between malloc() and calloc()?

malloc() allocates memory but doesn't initialize it, while calloc() initializes allocated memory to zero. Signup and view all the answers

What should be checked after calling malloc() or calloc()?

If the pointer returned is NULL, indicating a failed allocation. Signup and view all the answers

In which scenario would realloc() be particularly useful?

To shrink or expand the size of an already allocated memory block. Signup and view all the answers

What is the primary purpose of the free() function?

To release memory that is no longer needed. Signup and view all the answers

What is the correct prototype for the calloc() function?

void *calloc(size_t num, size_t size); Signup and view all the answers

If realloc() fails to allocate the requested memory, what happens to the original pointer?

It still points to the original memory block, which remains unchanged. Signup and view all the answers

Which of the following statements about memory allocation functions is true?

calloc() uses parameters to define both the quantity and size of allocated elements. Signup and view all the answers

What happens if you fail to call free() on dynamically allocated memory?

This may lead to memory leaks and inefficient memory usage. Signup and view all the answers

What will the return value of malloc(0) be?

A non-NULL pointer with a valid memory allocation. Signup and view all the answers

What should be the data type of the pointer returned from malloc()?

void * Signup and view all the answers

Study Notes

C Programming - Pointers

Pointers: C pointers are powerful tools for direct memory manipulation. They store memory addresses. The type of a pointer indicates the type of data it points to (e.g., char*, int*).

Memory Allocation Functions

malloc(): This function allocates a specified number of bytes of memory. It returns a pointer to the beginning of the allocated block. Crucially, if the allocation fails, malloc() returns NULL.
calloc(): This function is similar to malloc() but also initializes all allocated memory to zero. Like malloc, if allocation fails, calloc returns NULL.
realloc(): This function changes the size of previously allocated memory. It can enlarge or reduce the allocated block and returns a pointer to the new block of memory. If reallocation fails, it returns NULL, and the original block remains unchanged.
free(): This function deallocates memory that was previously allocated with malloc(), calloc(), or realloc(). This is vital to avoid memory leaks. Calling free() on a pointer that wasn't previously allocated results in undefined behaviour

Pointers - Important Concepts

NULL Pointer: A pointer that does not point to a valid memory location. Initialising pointers to NULL is good practice.
Pointer to Pointer (Double Pointer): A pointer that stores the address of another pointer. int **ptr2 = &ptr; makes ptr2 a pointer to the address stored in ptr.
Constant Pointers: A pointer that cannot be changed to point to a different memory location. const int *p = &var; means the pointer p cannot be changed, but the variable it points to might be modifiable.

Pointers vs Arrays

Memory Differences: Arrays usually have fixed memory size allocated on the stack, that cannot be resized or reassigned. Pointers can be assigned to different memory locations and resized.
Semantic Differences: Arrays represent contiguous blocks of memory containing elements. Pointers hold memory addresses.

Benefits of Passing by Reference (Using Pointers)

Memory Efficiency: Passing large data structures by reference (via pointers) is more efficient than copying the entire structure (passing by value). It avoids unnecessary memory usage and processing.
Performance Improvement: Passing pointers to large, complex data reduces the time needed to perform operations on that data.
Data Modification: Data passed by reference can be directly modified within the function and the changes persist in the original variable.

Pointers - Use Cases

Dynamic Memory Allocation: Using functions like malloc() and free() with pointers facilitates dynamic allocation of memory, allocating only the necessary space when needed. Critical for applications working with varying amounts of data.
Arrays and Strings: Arrays are essentially pointers to their first element. Pointer arithmetic can efficiently access elements within the array.
Function Arguments: Passing pointers as arguments allows functions to modify the original data, eliminating the need to return values.

Arrays of Pointers

Declaration and Usage: Arrays of pointers can store multiple pointers to other data types. Pointers can be initialized to point to statically declared variables or dynamically allocated memory.
Dynamic Allocation: Within arrays of pointers each element of the array is a pointer that contains the memory address, this allows memory to be allocated at runtime, which is essential for handling varying amounts of data.

Pointers to Arrays

Declaration: A pointer can point to the first element of an array. int *p = arr; points to the first element of array arr.
Array Decay: When an array is passed to a function, it "decays" to a pointer to its first element.
Accessing Elements: Pointer arithmetic allows for efficient element access.
Example Code: Code examples demonstrate how pointers are used to point to arrays and perform various operations like accessing data and modifying the original array's data within the function.

Arrays of Strings

Declaration: Arrays of String pointers enable straightforward storage and use of strings in an array.
Accessing Strings: The array elements can directly used to access strings

Pointers to Pointers (Double Pointers)

Declaration: Double (or more) pointers enable more complex and sophisticated memory manipulation.
Memory Structure: To understand how double pointers work think of their memory locations as layers upon layers of memory addresses.
Example: Demonstrates the use of double pointers to dynamically allocate arrays of strings and demonstrate the memory structure of these pointers.

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Description

Explore the concepts of pointers and memory allocation in C programming. This quiz covers essential functions like malloc(), calloc(), realloc(), and free(). Test your understanding of how these functions manipulate memory management in C.