Lecture 3 - Memory & Pointers

Question 1

What is memory?

Answer 1

Part of the computer which is used to store data and program instructions for retrieval.

Question 2

What can we think of memory as

Answer 2

a sorting cabinet, where each box stores the value and the label is the variable name

Question 3

What notion do we have in memory?

Answer 3

A notion of spatial locality. This is an important property of which caches make use of. This is when a line of memory is fetched,so close by data that may be needed soon can be loaded quickly (is cached).

• We are making a guess as to what data will be needed

Question 4

How long are addresses in 64 bit architecture?

Answer 4

64 bits or 8 bytes

Question 5

How big is a memory addressed by an address?

Answer 5

a byte (8 bits)

Question 6

How many addresses can a 64 bit architecture have ?

Answer 6

2^64 addresses to bytes

Question 7

Do we need to get the full byte to modify a single bit ?

Answer 7

YES

Question 8

What doe (1 &laquo_space;3) mean ?

Answer 8

A left shift (multiplication by 2).

Question 9

What does & do?

Answer 9

Gets the address of a variable -> pointer.

Question 10

what is sizeof()

Answer 10

A function that gets the size of a variable in bytes

Question 11

What is a pointer?

Answer 11

A variable which has the value , which is the address of a memory location. It essentially points to where the data is.

Question 12

What is * called.

Answer 12

The dereference operator?

Question 13

What does * do?

Answer 13

It gets the value at the address.

Question 14

Is every pointer the same size?

Answer 14

YES

Question 15

How big are pointers in 64 bit architecture?

Answer 15

8 bytes (64 bits)

Question 16

Is pointer a normal variable.

Answer 16

Yes in a sense that is also has to be stored at some memory location.

Question 17

What is int * * name?

Answer 17

A pointer to a pointer which points to data.

Question 18

How are arguments passed to a function in C?

Answer 18

By value?

Question 19

Are the any exceptions to call by value?

Answer 19

Yes, arrays. Pointers are also copied by the value they point to can be manipulated directly.

Question 20

Are int param[] and int * param interchangeable?

Answer 20

YES

Question 21

What do we use to represent that a pointer points to nothing.

Answer 21

NULL or 0

Question 22

What will happen if we try to dereference a NULL pointer?

Answer 22

The program will crash.

Question 23

What is const?

Answer 23

a type qualifier, which indicates the content of the variable cannot be changed. This is enforced by the compiler.

Question 24

What are the 3 ways that a pointer is unmodifiable.

Answer 24

• pointer itself cannot be changed via float * const ptr;
• the value we are pointing to cannot be changed via const float * ptr;
• both via const float * const ptr;

Question 25

What does the name of the array refer to ?

Answer 25

The memory location at which the first element of the array is stored.

Question 26

Are ptr[i] and *(ptr+i) equivalent

Answer 26

YES

Question 27

Can an array be changed?

Answer 27

No, but its values can.

Question 28

What can pointer arithmetic be used for?

Answer 28

Modifying value of the pointer.

Question 29

What can we do via pointer arithmetic?

Answer 29

Subtract/Add from/to pointer
Subtract 2 pointers from each other
compare pointers

Question 30

Does pointer arithmetic take into account the size of the type of data it is storing?

Answer 30

YES, it does so automatically.

Question 31

What is ptr->hello?

Answer 31

it is equivalent to (*ptr).hello. It is the notation for accessing a member of a struct.

Question 32

What does main look like when we want to process command line arguments?

Answer 32

int main(int argc , char * argv[])

Question 33

What is argc

Answer 33

argc specifies the number of command line arguments that have been written in the command

Question 34

What is argv?

Answer 34

The actual command line arguments as strings.

Question 35

What is the alternative way of writing char * []?

Answer 35

char * *

Question 36

What is void * used for?

Answer 36

Writing generic code that works with all data types.

Question 37

Can all pointers be converted to void *?

Answer 37

Yes

Question 38

When would void * prove to be useful?

Answer 38

When swapping variables of differnt types, this will simply swap the bytes, where the bytes have no meaning

Question 39

Can we get the value pointed to by void *?

Answer 39

No , the pointer type has to be changed first.

Question 40

What variables are stored on the stack?

Answer 40

Automatic variables, the size of these are known statically

Question 41

What is stored on the heap?

Answer 41

Dynamically allocated variables , where we have to manage allocation and freeing of the memory

Question 42

Do heap and stack share the same memory space?

Answer 42

Yes , stack goes top to bottom, while heap bottom to top. They grow towards each other as they fill up.

Question 43

How can we request a new chunk of memory from the heap?

Answer 43

using malloc with number of bits we need, which returns a void pointer, so remember to convert

Question 44

What is returned if malloc fails?

Answer 44

NULL

Question 45

What command is used to free memory?

Answer 45

free

Question 46

What happens if free is not called?

Answer 46

We leak memory, so that it is wasted.

Question 47

What is good to do after freeing memory?

Answer 47

Setting the pointer to it to NULL.

Question 48

What should we never return from a function?

Answer 48

A pointer to a function local variable, this is because it’s lifetime will end at the end of the block.

We can deal with this by allocating memory and passing the pointer back. Or manipulating it in the function in relation to e.g. root node