Week 3 - Pointers and Memory Addressing

Question 1

Name the 5 base data types in C.

Answer 1

Char, int, float, double, pointer

Question 2

How would you declare a pointer p to a character in C?

Answer 2

char p;
or
char p;

Question 3

How would you assign p to the address of/as a pointer to ch in C?

Answer 3

p = &ch;

Question 4

char ch = ‘A’;
char *p;
p = &ch;
What does p contain?

Answer 4

The address of where ch is stored in memory

Question 5

If x is any datatype, then what is &x in C?

Answer 5

The location in memory (its “address”) where x is currently stored

Question 6

If x is a pointer in C, then what does <datatype> *x; declare?</datatype>

Answer 6

Declares x to contain the memory address of a variable of type <datatype></datatype>

*x says: go to the memory address stored in x and bring back the value (of type <datatype> stored there</datatype>

Question 7

What is “dereferencing the pointer” in C?

Answer 7

*x, which says: go to the memory address stored in x and bring back the value (of type <datatype> stored there</datatype>

Question 8

How do you calculate the amount of memory used to store a pointer?

Answer 8

sizeof(), normally dependant on the OS

Question 9

What can void pointers do in C?

Answer 9

Point to any datatype, but need to be cast when used

Question 10

What are the two runtime errors when using pointers in C, and how are they caused?

Answer 10

Memory fault & segmentation fault

Caused by trying to access memory which it is not allowed to access, and trying to access memory in a way that is not allowed, e.g. trying to overwrite the operating system

Question 11

Sometimes in C, you don’t know at compile time how much memory you will need at runtime. What are the 3 possible solutions?

Answer 11

1. Declare an array whose size is decided at runtime (but no way of knowing how much free contiguous memory is available or how much is required before declaring the array)
2. Process the data in smaller chunks (possible with lots of extra programming)
3. Dynamically allocate memory at runtime using pointers (so can check whether memory allocation was successful)

Question 12

What C header file is needed to include NULL and malloc?

Answer 12

stdlib.h

Question 13

When using malloc in C, what must you always trap?

Answer 13

The fact that memory was not allocated

Question 14

When using malloc in C, when must you always free the memory?

Answer 14

Before the variable goes out of scope

Question 15

How do you free memory stored in pointer p in C?

Answer 15

free(p);

Question 16

Why should you always free memory in C?

Answer 16

Freeing memory returns it to the system, allowing it to be re-used

Question 17

In C, if a pointer p is freed, what does p become?

Answer 17

A dangling pointer

Question 18

In C, what can you use to test that all memory is freed properly?

Answer 18

The valgrind utility

Question 19

In C, what does malloc do?

Answer 19

Allocate memory to a variable at runtime, e.g.

int *p = NULL;
p = (int *)malloc(23)

^ this allocates 23 bytes!