Unit of Study Survey
Please take a few moments to complete the end of semester survey:
http://sydney.edu.au/itl/surveys/complete
Tutorial 13 Revision
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Exercise 1: Memory, sizeof and pointers
1. Assume a 64 bit system, where the sizeof all pointers are 8 bytes, and sizeof(int) is 4 byte. The
following questions refer to the code below:
void f(void) {
struct int_array {
int* data;
int len; };
struct int_array arr;
struct int_array* arr_ptr = &arr; struct int_array arrs[100];
(a) What are the initial values of arr.data and arr.len (b) What is sizeof(struct int_array)?
(c) What is sizeof(arr_ptr)?
(d) What is sizeof(arr_ptr->data)?
(e) What is sizeof(arrs)?
(f) Suppose the address of arrs[0] is 0x1000, what is the address of arrs[50]?
(g) Suppose the address of arrs[50] is 0x1500, what is the address of arrs[20]?
(h) Supposetheaddressofarrs[50]->lenis0x1500,whatistheaddressofarrs[50]->data?
2. Assume a 64 bit system, where the sizeof(int) is 4 bytes, and sizeof(float) is also 4 bytes. The following questions refer to the code below:
void* g(int x) { union merged {
float y; };
static int counter = 0; ++count;
union merged* ptr = malloc(sizeof(merged)); if (!ptr) {
return NULL; }
ptr->x = x;
return ptr; }
(a) What is the sizeof(union merged)?
(b) Which variables are allocated on the stack, which variables are allocated on the heap and which
variables are allocated in the static storage area?
(c) What are the lifetimes of each of the variables listed above?
Exercise 2: C preprocessor
1. What is wrong with the following macro of a MAX function? #define MAX(x, y) x > y ? x : y
2. What are include guards?
Exercise 3: Declarations, definitions and linkage
Classify each of the following as a declaration or definition. What are their linkages?
static int* y;
const int z = 0; extern const int w;
static void f(void); static void f(void) {
return 0; };
void g(void); void h(void) {
return; };
int main(void) { return 1;
COMP2017 Revision
Operating Systems and Machine Principles
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Exercise 4: Control flow
Trace the following program by hand. What do they output? 1. #include
int main(void) {
for (unsigned i = 5; i– > 0;) {
printf(“%u\n”, i); if (i) {
for (int j = 0; j < i; ++j) { printf("%d\n", i + j);
2. #include
void cpy2(char* dest, char* src, size_t n) { size_t i = 0;
for (; i < n; ++i) { dest[i] = src[i];
int main(void) {
// why does this segfault when x is declared as a char*? char x[] = "123456";
cpy2(x + 1, x, 4);
COMP2017 Revision
Operating Systems and Machine Principles
Page 3 of 4
Exercise 5: Strings
Implement the following functions from the string.h:
void memcpy(void* dest, const void* src, size_t len); void memmove(void* dest, const void* src, size_t len); int memcmp(const void* s1, const void* s2, size_t len);
void strcpy(char* dest, const char* src);
void strcat(char* dest, const char* src);
char* strchr(const char* s, int c);
char* strrchr(const char* s, int c);
size_t strspn(const char* s, const char* accept);
Exercise 6: Parallelism and concurrency
1. Why is x += 1 not an atomic operation?
2. What are the four conditions of a dead lock?
3. What is a critical section?
4. What is a live lock?
5. What is starvation in term of multi-threading?
6. What does it mean if a resource is highly contended, why could this cause a performance issue?
7. What is false sharing?
8. How can a binary semaphore be used as a mutex?
9. The Senate Bus problem from the Little Book of Semaphores
¡°This problem was originally based on the Senate bus at Wellesley College. Riders come to a bus stop and wait for a bus. When the bus arrives, all the waiting riders invoke boardBus, but anyone who arrives while the bus is boarding has to wait for the next bus. The capacity of the bus is 50 people; if there are more than 50 people waiting, some will have to wait for the next bus. When all the waiting riders have boarded, the bus can invoke depart.
If the bus arrives when there are no riders, it should depart immediately.
Puzzle: Write synchronization code that enforces all of these constraints. ¡±
Represent each rider as a thread, write synchronisation code to model the above problem.
COMP2017 Revision
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