Threads Thread: an execution within a process A multithreaded - PDF document

Threads • Thread: an execution within a process • A multithreaded process consists of many Pthreads co-existing executions • Separate: – CPU state, stack Operating Systems • Shared: Hebrew University of Jerusalem – Everything else Spring 2004 • Text, data, heap, environment Theading Models - 2 Threading Models - 1 • User Space (N-1) • Kernel (1-1) – Single kernel process, multiple user threads – All threads are first class objects in the kernel – Low kernel overhead – threads are cheap – Scheduling in and by the kernel – Scheduling is determined by the process – Utilizes multi-processors efficiently – Syscalls do not block the other threads – Syscalls block the whole process (and all the threads) – High overhead for large number of threads – No efficiency on multi-processors State of the Art Threading Models - 3 • Linux • Hybrid (M-on-N) – Pre 2.6: LinuxThreads (1-1 with extras) – User both kernel threads and user threads – 2.6 on: NPTL – Native POSIX Thread Library – More complicated to implement (1-1) • Requires changes to libraries • Scheduling is complicated • User space libraries must be synchronized with • Windows kernel version – Threads, but not POSIX (1-1) �

thread creation How to Compile int pthread_create(pthread_t *thread, • #include <pthread.h> pthread_attr_t *attr, • gcc myprog.c –o myprog –l pthread void* (*start_routine)(void*), void *arg); • Create a thread and run the start_routine • attr is usually NULL, don’t mess with it sched_yield Example #include <pthread.h> #include <sched.h> int val = 0; #include <unistd.h> void *thread(void *vargp) { val = (int)vargp; int sched_yield (void); } int main() { • Yield the processor to another thread int i; pthread_t tid; • Useful on uni-processor pthread_create(&tid, NULL, thread, (void *)42); pthread_join(tid, NULL); printf("%d\n",val); } Relationships Who am I • Marriage: • pthread_t pthread_self(void) – pthread_join – I will wait for you forever • Good bye • Uses: – pthread_exit – I am going away now – Debugging • Death – Data structures indexed by thread – pthread_cancel – please die • Divorce: • pthread_equal: compare two pthread_t – pthread_detach – never talk to me again �

pthread_exit pthread_join • int pthread_join(pthread t, void *data) • void pthread_exit(void *data) • Wait until the thread exits and return the exit data. This call blocks! • Performs a detach after the join succeeds • Stops execution of this thread • Return data to anyone trying to join this • Return values: thread – 0: successful completion – EINVAL: thread is not joinable • Don’t call from the main thread, use exit() – ESRCH: no such thread – EDEADLK: a deadlock was detected, or thread specifies the calling thread pthread_cancel Example #include <pthread.h> • Die you !@$#@ void *thread(void *vargp) { • int pthread_cancel(pthread_t thread) pthread_exit((void*)42); } int main() { • return values: int i; – 0: ok pthread_t tid; pthread_create(&tid, NULL, thread, NULL); – EINVAL: thread is invalid pthread_join(tid, (void **)&i); – ESRCH: no such thread printf("%d\n",i); } pthread_detach pthread_cancel • I never want to see this thread again. • Three phases • int pthread_detach(pthread_t thread) – Post a cancel request – Deliver to the target thread at the next • Return values: cancellation point – 0 - ok – EINVAL – thread is not joinable – Call cleanup routines and die – ESRCH – no such thread • On some systems, exit does not cause the program to exit until all non-detached threads are finished �

Review Review: Exiting threads • pthread_create • Four options • pthread_join – Exit from start routine • pthread_detach – Call pthread_exit • pthread_exit – Call exit • pthread_cancel – Killed by pthread_cancel • pthread_self • pthread_equal • sched_yield �