1. Classical Synchronization Problems
CS Introduction to Operating Systems

2. Classical Problem 1
Producer-Consumer

3. #include <stdio.h>
#include <semaphore.h>
#define SIZE 50
#define NUM_THREADS 5
struct object* buffer;
sem_t mutex, empty, full;
void nap() {
int sleepTime = (int)(5 * random());
sleep(sleepTime); }
void enter(struct object item) {
empty.down();
mutex.down();
count++;
buffer[in] = item;
in = (in + 1) % SIZE;
mutex.up();
full.up();
struct object remove() {
full.down();
count--;
struct object item = buffer[out];
out = (out + 1) % SIZE;
empty.up();
return item;

4. void producer() {
struct object newObj;
for(;;) {
nap();
newObj = getObject();
enter(newObj); }
void consumer() {
struct object item;
item = remove();
useItem(item);
int main(int argc, char** argv) {
pthread_t threads[NUM_THREADS];
int i;
sem_init(&mutex, 0, 1);
sem_init(&empty, 0, SIZE);
sem_init(&full, 0, 0);
for(i=0; i<NUM_THREADS; i++) {
if(i % 2)
pthread_create(&threads[i], NULL, (void*)producer, NULL);
else
pthread_create(&threads[i], NULL, (void*)consumer, NULL);
consumer();
return 0;

5. Classical Problem 2
Readers-Writers

6. Readers-Writers Several threads accessing a database
Some threads read the database (readers)
Some threads write the database (writers)
Multiple readers can be accessing database at a time if no writers accessing it
Only one thread can be accessing database if it is a writer
2 major variations
no reader waits if no writer in database
writers can starve
all readers wait if a writer is waiting
writer has to wait until current reader (or writer) is finished with database
readers may starve
Many more variations to deal with starvation issues
Following example is for the first variant
starvation is disregarded

7. #include <stdio.h>
#include <pthreads.h>
#define NUM_THREADS 5
sem_t mutex, database;
void reader() {
while(true) {
nap();
startRead();
endRead(); }
void writer() {
startWrite();
endWrite();
void nap() {
int sleepTime = (int)(5 * random());
sleep(sleepTime);

8. void startRead() {
mutex.down();
readerCount++;
if(readerCount == 1) { database.down(); }
mutex.up(); }
void endRead() {
readerCount--;
if(readerCount == 0) { database.up(); }
void startWrite() { database.down(); }
void endWrite() { database.up(); }
int main() {
pthread_t threads[NUM_THREADS];
int i;
sem_init(&mutex, 0, 1);
sem_init(&database, 0, 1);
for(i=0; i<NUM_THREADS; i++) {
if(i % 2)
pthread_create(&threads[i], NULL, (void*)reader, NULL);
else
pthread_create(&threads[i], NULL, (void*)writer, NULL);
writer();
return 0;