Classical Synchronization Problems CS 537 - Introduction to Operating Systems

Classical Problem 1 Producer-Consumer

#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;

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;

Classical Problem 2 Readers-Writers

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

#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);

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;