Memory Allocation

Question 1

Why use dynamic memory allocation rather then just initializing like this, “ int variable[100]”

Answer 1

The arrays capacity might not be known till runtime, so it might be too much or to little capacity.

Question 2

What is the stack?

Answer 2

changes size continually, out of programmers control

Question 3

Can you manage the Heap? How?

Answer 3

Yes, referenced through pointers with the following library,
#include <stdlib.h>
void *malloc(size_t size);
void free(void * ptr);
void *calloc(size_t nmeb, size_t size);
void *realloc(void *ptr, size_t size);</stdlib.h>

Question 4

In python what happens when objects are no longer referenced

Answer 4

garbage collection

Question 5

what does malloc function do?

Answer 5

allocates a block of memory of specified size, from the heap, its uninitialized.

Question 6

if memory cant be allocated with malloc, what is returned?

Answer 6

NULL (the null pointer value)

Question 7

what does malloc return?

Answer 7

returns a pointer to the allocated block.

Question 8

what does the free() function do?

Answer 8

deallocates the block of memory pointed to by ptr.

Question 9

when will free() work?

Answer 9

only if the ptr is a pointer previously retuned by malloc

Question 10

when will free do nothing?

Answer 10

when the pointer you are freeing is NULL

Question 11

What are the objectives of the lecture on memory allocation?

Answer 11

The objectives include understanding the limitations of automatic memory allocation, explaining how program data is stored in computer memory, reviewing memory management in Python, and introducing C functions to handle dynamic memory.

Question 12

In imperative programming languages, where are a function’s parameters and local variables typically allocated?

Answer 12

They are allocated in an activation frame, which is automatically created when the function is called and deallocated when the function returns.

Question 13

How is memory allocated for local variables in Python?

Answer 13

In Python, a local variable is allocated the first time its name is bound to an object.

Question 14

In C, when are parameters and local variables allocated?

Answer 14

Parameters and local variables whose scope is the entire function are allocated when the function is called.

Question 15

What are automatic variables in the context of memory allocation?

Answer 15

Automatic variables are local variables in functions that are allocated and deallocated automatically when the function is called and returns, respectively.

Question 16

What is the potential problem with the lifetime of automatic variables?

Answer 16

The values stored in automatic variables are not retained when the function where the variable is declared returns.

Question 17

How does virtual memory work in computer systems?

Answer 17

Virtual memory allows the operating system to mimic non-existing memory blocks using a dedicated part of the hard drive, known as swap space or swap file.

Question 18

What are the two main parts of virtual memory?

Answer 18

The two main parts are Kernel Space, where the OS kernel resides and is unavailable to processes, and User Space, where processes stay, work, and interact, each having its own sandbox and unable to access other sandboxes.

Question 19

what should always be done after allocating memory with malloc?

Answer 19

use the assert function to verify that the pointer does not return NULL

Question 20

What does assert function do?

Answer 20

terminates the program if the condition given in parameters does not return true

Question 21

How to you allocate a struct?

Answer 21

typedef struct {
int x;
int y;
} point_t;

point_t *point;
point = malloc(sizeof(point_t));
assert(point != NULL);

Question 22

what should you always do after freeing a block?

Answer 22

set the pointer that you freed to NULL, this will avoid any issues with free in the future regarding that specific block.

Question 23

How do you allocate an array of 10 ints?

Answer 23

int *pa;
pa = malloc(10 * sizeof(int));
assert(pa != NULL);
// Access array elements by using []
// or pointer-plus-offset
pa[0] = 0; // *pa = 0;
*(pa + 1) = 0; // pa[1] = 0;

Question 24

write a function that allocates an array of integers
with a specified capacity, and returns a pointer to the first
element

Answer 24

int *alloc_array_of_ints(int capacity){
int *pa;
assert(capacity > 0);
pa = malloc(capacity * sizeof(int));
assert(pa != NULL);
return pa;
}

Question 25

What error may occur if free is passed a pointer to heap memory that was already deallocated by a previous call to free?

Answer 25

A “double free” error may occur, resulting in undefined behavior.

Question 26

How can the occurrence of a “double free” error be prevented?

Answer 26

After freeing a block of memory, it’s advisable to set the pointer to that block to NULL.

Question 27

What is a “dangling pointer” error in memory management?

Answer 27

A “dangling pointer” error occurs when a pointer still points to deallocated memory after being freed.

Question 28

What precaution should be taken to avoid using freed memory?

Answer 28

It’s important never to use memory that has been freed to avoid accessing dangling pointers

Question 29

What is one defensive programming practice to mitigate the risk of a runtime error after freeing memory?

Answer 29

After freeing a block of memory, setting the pointer to that block to NULL can help avoid potential runtime errors.

Question 30

How can you allocate an array of integers dynamically in C?

Answer 30

You can use malloc to allocate memory for the array, specifying the required size in bytes. For example: pa = malloc(10 * sizeof(int));.

Question 31

What is the purpose of the assert statement in dynamically allocating memory for an array?

Answer 31

The assert statement is used to ensure that memory allocation was successful. It causes the program to terminate if the condition specified is false.

Question 32

How can you allocate an array of integers with a specified capacity using a function?

Answer 32

You can write a function, such as alloc_array_of_ints, that takes the capacity as an argument, allocates memory for the array using malloc, and returns a pointer to the first element.

Question 33

What precaution is taken in the alloc_array_of_ints function to ensure proper memory allocation?

Answer 33

The function asserts that the capacity is greater than 0 and checks if memory allocation was successful before returning the pointer to the allocated memory.

Question 34

What does the sizeof operator yield in C programming?

Answer 34

The sizeof operator yields the amount of memory, in bytes, required to store an object of the type of its operand.

Question 35

What value does malloc return if it fails to allocate memory?

Answer 35

If malloc fails to allocate a block with the specified size, it returns NULL.

Question 36

Why is it important to check the pointer returned by malloc before dereferencing it?

Answer 36

It’s important to check the pointer returned by malloc before dereferencing it because if malloc fails and returns NULL, dereferencing a NULL pointer can lead to undefined behavior.

Question 37

What should a program do if malloc returns NULL?

Answer 37

A program should terminate if malloc returns NULL, unless it’s possible to recover. The assert macro provides a concise way to accomplish this.

Question 38

What is the purpose of the assert macro in C programming?

Answer 38

The assert macro is used to specify a condition that must be true for program execution to continue. If the condition evaluates to false (0), assert displays a descriptive error message and terminates the program.