P threads

Operating Systems Multi-threading models Tutorial on pthreads

Multithreading Models Many-to-One One-to-One Many-to-Many

Many-to-One Many user-level threads mapped to single kernel thread Examples: Solaris Green Threads GNU Portable Threads

One-to-One Each user-level thread maps to kernel thread Examples Windows NT/XP/2000 Linux Solaris 9 and later

Many-to-Many Model Allows many user level threads to be mapped to many kernel threads Allows the operating system to create a sufficient number of kernel threads Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package

Thread Libraries Thread library provides programmer with API for creating and managing threads Two primary ways of implementing Library entirely in user space: we will study in the next class how this can be implemented Kernel-level library supported by the OS

Thread Pools Create a number of threads in a pool where they await work Advantages: Usually slightly faster to service a request with an existing thread than create a new thread Allows the number of threads in the application(s) to be bound to the size of the pool

Pthreads May be provided either as user-level or kernel-level A POSIX standard (IEEE 1003.1c) API for thread creation and synchronization API specifies behavior of the thread library, implementation is up to development of the library Common in UNIX operating systems (Solaris, Linux, Mac OS X)

Pthreads: POSIX Threads Pthreads is a standard set of C library functions for multithreaded programming IEEE Portable Operating System Interface, POSIX, section 1003.1 standard, 1995 Pthread Library (60+ functions) Thread management: create, exit, detach, join, . . . Thread cancellation Mutex locks: init, destroy, lock, unlock, . . . Condition variables: init, destroy, wait, timed wait, . . . . . . Programs must include the file pthread.h Programs must be linked with the pthread library (-lpthread)

Pthreads Naming Convention Types: pthread[_object]_t Functions: pthread[_object]_action Constants/Macros: PTHREAD_PURPOSE Examples: pthread_t: the type of a thread pthread_create(): creates a thread pthread_mutex_t: the type of a mutex lock pthread_mutex_lock(): lock a mutex PTHREAD_CREATE_DETACHED

pthread_self() Returns the thread identifier for the calling thread At any point in its instruction stream a thread can figure out which thread it is Convenient to be able to write code that says: “If you’re thread 1 do this, otherwise do that” However, the thread identifier is an opaque object (just a pthread_t value) you must use pthread_equal() to test equality pthread_t pthread_self (void); int pthread_equal (pthread_t id1, pthread_t id2);

pthread_create() Creates a new thread int pthread_create ( pthread_t *thread, pthread_attr_t *attr, void * (*start_routine) (void *) , void *arg); Returns 0 to indicate success, otherwise returns error code thread : output argument for the id of the new thread attr : input argument that specifies the attributes of the thread to be created (NULL = default attributes) start_routine: function to use as the start of the new thread must have prototype: void * foo(void*) arg: argument to pass to the new thread routine If the thread routine requires multiple arguments, they must be passed bundled up in an array or a structure

pthread_create() example Want to create a thread to compute the sum of the elements of an array void *do_work(void *arg); Needs three arguments the array, its size, where to store the sum we need to bundle them in a structure struct arguments { double *array; int size; double *sum; }

pthread_create() example int main(int argc, char *argv) { double array[100]; double sum; pthread_t worker_thread; struct arguments *arg; arg = (struct arguments *)calloc(1, sizeof(struct arguments)); arg->array = array; arg->size=100; arg->sum = ∑ if ( pthread_create (&worker_thread, NULL, do_work, (void *)arg)) { fprintf(stderr,”Error while creating thread\n”); exit(1); } ... }

pthread_create() example void *do_work(void *arg) { struct arguments *argument; int i, size; double *array; double *sum; argument = (struct arguments*)arg; size = argument->size; array = argument->array; sum = argument->sum; *sum = 0; for (i=0;i<size;i++) *sum += array[i]; return NULL; }

Comments about the example The “main thread” continues its normal execution after creating the “child thread” IMPORTANT : If the main thread terminates, then all threads are killed! We will see that there is a join() function Of course, memory is shared by the parent and the child (the array, the location of the sum) nothing prevents the parent from doing something to it while the child is still executing which may lead to a wrong computation we will see that Pthreads provides mutual exclusion mechanisms The bundling and unbundling of arguments is a bit tedious

Memory Management of Args The parent thread allocates memory for the arguments Warning #1: you don’t want to free that memory before the child thread has a chance to read it That would be a race condition Better to let the child do the freeing Warning #2: if you create multiple threads you want to be careful there is no sharing of arguments, or that the sharing is safe For instance, if you reuse the same data structure for all threads and modify its fields before each call to pthread_create(), some threads may not be able to read the arguments destined to them Safest way: have a separate arg structure for each thread

pthread_exit() Terminates the calling thread void pthread_exit (void *retval); The return value is made available to another thread calling a pthread_join() The previous example had the thread just return from function do_work() In this case the call to pthread_exit() is implicit The return value of the function serves as the argument to the (implicitly called) pthread_exit() .

Thread Cancellation Terminating a thread before it has finished Two general approaches: Asynchronous cancellation terminates the target thread immediately Deferred cancellation allows the target thread to periodically check if it should be cancelled

pthread_kill() Causes the termination of a thread int pthread_kill ( pthread_t thread, int sig); thread : input parameter, id of the thread to terminate sig: signal number returns 0 to indicate success, error code otherwise

pthread_join() Causes the calling thread to wait for another thread to terminate int pthread_join ( pthread_t thread, void **value_ptr); thread : input parameter, id of the thread to wait on value_ptr : output parameter, value given to pthread_exit() by the terminating thread (which happens to always be a void * ) returns 0 to indicate success, error code otherwise multiple simultaneous calls for the same thread are not allowed

pthread_join() example int main(int argc, char *argv) { double array[100]; double sum; pthread_t worker_thread; struct arguments *arg; void *return_value; arg = (struct arguments *)calloc(1,sizeof(struct arguments)); arg->array = array; arg->size=100; arg->sum = ∑ if ( pthread_create (&worker_thread, NULL, do_work, (void *)arg)) { fprintf(stderr,”Error while creating thread\n”); exit(1); } ... if ( pthread_join (worker_thread, &return_value)) { fprintf(stderr,”Error while waiting for thread\n”); exit(1); } }

pthread_join() Warning This is a common “bug” that first-time pthread programmers encounter Without the call to pthread_join() the previous program may end immediately, with the main thread reaching the end of main() and exiting, thus killing all other threads perhaps even before they have had a chance to execute

pthread_join() Warning When creating multiple threads be careful to store the handle of each thread in a separate variable Typically one has an array of thread handles That way you’ll be able to call pthread_join() for each thread Also, note that the following code is sequential! for (i=0; i < num_threads; i++) { pthread_create(&(threads[i]),...) pthread_join(threads[i],...) }

Thread Attributes One of the parameters to pthread_create() is a thread attribute In all our previous examples we have set it to NULL But it can be very useful and provides a simple way to set options: Initialize an attribute Set its value with some Pthread API call Pass it to Pthread API functions like pthread_create()

pthread_attr_init() Initialized the thread attribute object to the default values int pthread_attr_init ( pthread_attr_t *attr); Return 0 to indicate success, error code otherwise attr : pointer to a thread attribute

Detached Thread One option when creating a thread is whether it is joinable or detached Joinable : another thread can call join on it By default a thread is joinable Detached : no thread can call join on it Let’s look at the function that allows to set the “detached state”

pthread_attr_setdetachstate() Sets the detach state attribute int pthread_attr_setdetachstate ( pthread_attr_t *attr, int detachstate); returns 0 to indicate success, error code otherwise attr: input parameter, thread attribute detachstate: can be either PTHREAD_CREATE_DETACHED PTHREAD_CREATE_JOINABLE (default)

Detach State Detached threads have all resources freed when they terminate Joinable threads have state information about the thread kept even after they finish To allow for a thread to join a finished thread So-called “no rush to join” So, if you know that you will not need to join a thread, create it in a detached state so that you save resources This is lean-and-mean C, as opposed to hand-holding Java, and every little saving is important

Creating a Detached Thread #include <pthread.h> #define NUM_THREAD 25 void * thread_routine (void *arg) { printf(“Thread %d, my TID is %u\n”, (int)arg, pthread_self()); pthread_exit(0); }

Creating a Detached Thread int main() { pthread_attr_t attr; pthread_t tids[NUM_THREADS]; int x; pthread_attr_init (&attr); pthread_attr_setdetachstate (&attr, PTHREAD_CREATE_DETACHED ); phthread_create (&(tids[x]), &attr, thread_routine, (void *)x); . . . // should take a while otherwise . . . // the child may not have time to run }

P threads

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