CENG460
Operating Systems
Spring 2017 -2018
Synchronization (3)
Practical exercises in Linux
Dr. Omar Chebaro
[Operating Systems Concepts, 8th Edition, by Silberschatz, Galvin,
and Gagne, 2009]
1
Thread Synchronization
O The ultimate cause of most bugs involving
threads is that the threads are accessing the
same data. That’s one of the powerful aspects
of threads, but it can also be dangerous
O If one thread is only partway through updating a
data structure when another thread accesses the
same data structure, problems are likely to arise
O These bugs are called race conditions; the
threads are racing one another to change the
same data structure
2
Example of Race Conditions
3
Solution for Race Conditions
O In previous example we saw that the log ‘Job
2 finished’ is repeated twice while no log for
‘Job 1 finished’ is seen
O Therefore, threads need to synchronize their
access to the shared variable “counter”
O Use the system calls for thread
synchronization as stated in next slide
4
System Calls for Thread Synchronization
We have to include this library:
#include <semaphore.h>
int sem_init(sem_t *sem, int pshared, unsigned int value);
O This function initializes a semaphore object pointed to by sem
O value: specifies the initial value of the semaphore
O The pshared parameter controls the type of semaphore
O If pshared is 0, the semaphore is local to the current process (and
its threads)
O For now, we are interested only in semaphores from type 0
int sem_wait(sem_t * sem); //
function atomically decreases
the value of the semaphore by 1
int sem_post(sem_t * sem); // atomically increases the
value of the semaphore by one, but always waits until
the semaphore has a nonzero count first
int sem_destroy(sem_t * sem);
O Again, this function takes a pointer to a semaphore and cleans
any resources that it may have
5
Example of
Synchroniz
ed Threads
6