Threads and Thread Control Thread Concepts Pthread Creation and Termination Pthread synchronization Threads and Signals

Overview Threads are concurrent lines of execution within a process. Sometimes called “lightweight processes” All the threads of a process are in its memory space, but each thread has its own Thread ID Stack Register values Set of blocked and pending signals Thread specific data errno variable There also must be a scheduling policy for thread execution

Overview A typical UNIX process contains only a single thread of execution Multiple threads allow us to take advantage of parallel programming without the overhead of a new process May make programming for asynchronous events easier

POSIX Threads pthreads are the optional POSIX standardization for threads Must include and link against the pthread library -lpthread when compiling a C/C++ program

Thread Identification Each thread has an identifier Only unique within the process to which the thread belongs IDs can be represented by pthread_t which can be a struct Implementation of struct is system dependant A thread can get its own ID structure with pthread_t pthread_self(void); Compare two thread IDs with int pthread_equal(pthread_t t1, pthread_t t2);

Thread Creation int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void*), void *arg); Arguments thread points to the new thread ID pthread_attr_t set of thread attributes. Use NULL for default attributes start_routine is a function pointer to the function that the new thread will begin executing when it is created arg is the single argument that may be passed to the start routine. If you need to pass multiple parameters, bundle them as a struct and pass that

Thread Termination If any thread calls exit, _exit or Exit, the entire process will terminate Thread may terminate due to exec 3 ways a single thread can terminate Return from the start routine Call pthread_exit void pthread_exit(void *value_ptr); Thread cancelled by another thread with int pthread_cancel(pthread_t thread);

Joining and Detaching int pthread_join(pthread_t thread, void **value_ptr); int pthread_detach(pthread_t thread); Join is similar to waitpid. Allows a thread to wait on another to terminate Termination status of thread waited on returned in value_ptr if specified in call to pthread_exit pthread_detach allows us to put a thread in the detached state

Joinable Attribute We can specify that a newly created thread is joinable or detached when we create the thread with the pthread_attr_t parameter Threads created as detached can not be joined Default is joinable Detached thread resources are released as soon as thread terminates Creating a thread as detached prevents synchronization (since we can’t join it)

Mutexes Mutex – “mutual exclusion” Only one thread at a time can own a particular mutex variable A thread aquires the mutex by “locking” it. When it is done, it releases it by “unlocking” it Mutex variable is of type pthread_mutex_t

Mutexes int pthread_mutex_init(pthread_mutex_t *mutex, const pthread_mutexattr_t *attr); mutex is a pointer to a variable of type pthread_mutex_t attr is a pointer to a type that holds attributes for non default mutexes. We can pass in NULL for a default mutex

Mutex Use Declare mutex variable pthread_mutex_t myMutex; Initialize pthread_mutex_create(&myMutex, NULL); A thread locks the mutex pthread_mutex_lock(&myMutex); Thread executes critical section and then releases the mutext pthread_mutex_unlock(&myMutex); When mutex not longer needed, it can be destroyed pthread_mutex_destroy(&myMutex);

Condition Variables Allows threads to synchronize based on value of data Avoids busy loop where threads acquire lock, check value of a variable and repeat until the value changes Condition variables should always be protected by mutex locks

Condition Variables int pthread_cond_init(pthread_cond_t *cond, const pthread_condattr_t *attr); int pthread_cond_destroy(pthread_cond_t *cond); int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t * mutex); int pthread_cond_broadcast(pthread_cond_t *cond); Notify all threads waiting on this condition int pthread_cond_signal(pthread_cond_t *cond); Notify a single thread waiting on this condition See page for example

Threads and Signals Signals are delivered to a single thread If signal is the result of hardware fault, it is usually sent to the thread that caused it Otherwise signal sent to arbitrary thread Each thread has its own signal mask int pthread_sigmask(int how, const sigset_t *set, sigset_t *oset); Signal dispositions shared between all threads in a process

Threads and Signals int sigwait(const sigset_t *set, int *sig); Allows a thread to wait for a signal from the set given by set parameter The particular signal received is returned through the sig parameter To prevent race condition, the signal being waited on must be blocked before calling this function int pthread_kill(pthread_t thread, int sig); Sends a signal to a specified thread