POSIX Threads Nezer J. Zaidenberg

References  Advanced programming for the UNIX environment (2nd edition chapter This material does not exist in first edition)  Many manuals exist on the web  Yolinux.com : POSIX thread (pthread) libraries (very basic and I found some mistakes there!) Zhttp:// -posix1.html (and chapters 2/3) -posix1.html  Programming with POSIX(R) Threads by David Botenhof (more advanced then this course)

Simple thread program code

#include void *print_message_function( void *ptr ); main() { pthread_t thread1, thread2; char *message1 = "Thread 1"; char *message2 = "Thread 2"; pthread_create( &thread1, NULL, print_message_function, (void*) message1); pthread_create( &thread2, NULL, print_message_function, (void*) message2); pthread_join( thread1, NULL); pthread_join( thread2, NULL); exit(0); } void *print_message_function( void *ptr ) { char *message; message = (char *) ptr; printf("%s \n", message); }

Two important notes  To compile :  C : gcc -lpthread thread.c  C++ : g++ -lpthread thread.cxx  Other platforms and compilers may require -pthread flag instead  Never code like this  You should check pthread create return value

Why do we need thread library ZIn Linux malloc(3) is normally not thread safe to speed things up (no locking and unlocking) ZThat means that when we use threads – we must use different malloc(3) ZThis is also true in Windows while in OSX malloc is always thread safe.

Pthread_create(3) SYNOPSIS #include int pthread_create(pthread_t * thread, const pthread_attr_t * attr, void * (*start_routine)(void *), void *arg); Arguments thread - returns the thread id. (unsigned long int defined in bits/pthreadtypes.h) attr - Set to NULL if default thread attributes are used. (else define members of the struct pthread_attr_t defined in bits/pthreadtypes.h) void * (*start_routine) - pointer to the function to be threaded. Function has a single argument: pointer to void. *arg - pointer to argument of function. To pass multiple arguments, send a pointer to a structure.

Pthread exit(3) – exit(2) for threads SYNOPSIS #include void pthread_exit(void *retval); Arguments: * retval - Return value of thread.

Pthread join(3) – wait(2) for threads SYNOPSIS #include int pthread_join(pthread_t thread, void **value_ptr); Arguments Value_ptr = the value returned by thread This function is BLOCKING!!!

Passing arguments to/from threads ZCan be done using the void pointer (cast the void * into a struct and point into any number of arguments) ZReturn values can be returned in the same way ZShared memory is also used for this (but is less recommended because of problems with syncing)

Example (1/3) #define _MULTI_THREADED #include #include "check.h” typedef struct { int value; char string[128]; } thread_parm_t; void *threadfunc(void *parm) { thread_parm_t *p = (thread_parm_t *)parm; printf("%s, parm = %d\n", p->string, p->value); free(p); return NULL; }

Example 2/3 int main(int argc, char **argv) { pthread_t thread; int rc=0; thread_parm_t *parm=NULL; printf("Create a thread attributes object\n"); parm = malloc(sizeof(thread_parm_t)); parm->value = 5; strcpy(parm->string, "Inside first thread"); rc = pthread_create(&thread, NULL, threadfunc, (void *)parm); parm = malloc(sizeof(thread_parm_t)); parm->value = 77; strcpy(parm->string, "Inside second thread");

Example 3/3 rc = pthread_create(&thread, NULL, threadfunc, (void *)parm); sleep(5); printf("Main completed\n"); return 0; }

Problems with example ZCheck return code (existed in the original example) ZDon’t use sleep() to wait for thread! – use pthread_join(3). ZAvoid heap memory allocation. (It was used in the original example and I wanted to show shared memory and cases where one thread malloc(3)s and another free(3).)

Thread attributes  detached state (joinable? Default: PTHREAD_CREATE_JOINABLE. Other option: PTHREAD_CREATE_DETACHED)  scheduling policy (real-time? PTHREAD_INHERIT_SCHED,PTHREAD_EXPLICIT_SCHE D,SCHED_OTHER)  scheduling parameter  inheritsched attribute (Default: PTHREAD_EXPLICIT_SCHED Inherit from parent thread: PTHREAD_INHERIT_SCHED)  scope (Kernel threads: PTHREAD_SCOPE_SYSTEM User threads: PTHREAD_SCOPE_PROCESS Pick one or the other not both.)  guard size  stack address (See unistd.h and bits/posix_opt.h _POSIX_THREAD_ATTR_STACKADDR)  stack size (default minimum PTHREAD_STACK_SIZE set in pthread.h),

In the scope of this course we will always use NULL for thread attributes

Sync using mutex (1/2) #include pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER; int counter = 0; void *functionC() { pthread_mutex_lock( &mutex1 ); counter++; printf("Counter value: %d\n",counter); pthread_mutex_unlock( &mutex1 ); }

Syncing 2/2 int main() { int rc1, rc2; pthread_t thread1, thread2; if( (rc1=pthread_create( &thread1, NULL, &functionC, NULL)) ) { printf("Thread creation failed: %d\n", rc1); } if( (rc2=pthread_create( &thread2, NULL, &functionC, NULL)) ) { printf("Thread creation failed: %d\n", rc2); } // ++counter pthread_join( thread1, NULL); pthread_join( thread2, NULL); exit(0); }

Pthread_mutex_lock SYNOPSIS #include int pthread_mutex_lock(pthread_mutex_t *mutex); int pthread_mutex_trylock(pthread_mutex_t *mutex); int pthread_mutex_unlock(pthread_mutex_t *mutex); Lock function is blocking Try lock function is not blocking (it would lock if it can) Unlock release

Using cond (1/7) #include #define NUM_THREADS 3 #define TCOUNT 10 #define COUNT_LIMIT 12 int count = 0; int thread_ids[3] = {0,1,2}; pthread_mutex_t count_mutex; pthread_cond_t count_threshold_cv;

Using cond 2/7 int main(int argc, char *argv[]) { int i, rc; pthread_t threads[3]; /* Initialize mutex and condition variable objects */ pthread_mutex_init(&count_mutex, NULL); pthread_cond_init (&count_threshold_cv, NULL); pthread_create(&threads[0], NULL, inc_count, (void *)&thread_ids[0]); pthread_create(&threads[1], NULL, inc_count, (void *)&thread_ids[1]); pthread_create(&threads[2], NULL, watch_count, (void *)&thread_ids[2]);

Cond 3/7 /* Wait for all threads to complete */ for (i = 0; i < NUM_THREADS; i++) { pthread_join(threads[i], NULL); } printf ("Main(): Waited on %d threads. Done.\n", NUM_THREADS); /* Clean up and exit */ pthread_mutex_destroy(&count_mutex); pthread_cond_destroy(&count_threshold_cv); pthread_exit (NULL); }

Using cond 4/7 void *inc_count(void *idp) { int j,i; double result=0.0; int *my_id = idp; for (i=0; i < TCOUNT; i++) { pthread_mutex_lock(&count_mutex); count++; /* Check the value of count and signal waiting thread when condition is reached. Note that this occurs while mutex is locked. */

Cond 5/7 if (count == COUNT_LIMIT) { pthread_cond_signal(&count_threshold_cv); printf("inc_count(): thread %d, count = %d Threshold reached.\n", *my_id, count); } printf("inc_count(): thread %d, count = %d, unlocking mutex\n”, *my_id, count); pthread_mutex_unlock(&count_mutex); /* Do some work so threads can alternate on mutex lock */ for (j=0; j < 10000; j++) result = result + (double)random(); } pthread_exit(NULL); }

Using cond 6/7 void *watch_count(void *idp) { int *my_id = idp; printf("Starting watch_count(): thread %d\n", *my_id); /* Lock mutex and wait for signal. Note that the pthread_cond_wait routine will automatically and atomically unlock mutex while it waits. Also, note that if COUNT_LIMIT is reached before this routine is run by the waiting thread, the loop will be skipped to prevent pthread_cond_wait from never returning. */

Using Cond 7/7 pthread_mutex_lock(&count_mutex); if (count < COUNT_LIMIT) { pthread_cond_wait(&count_threshold_cv, &count_mutex); printf("watch_count(): thread %d Condition signal received.\n", *my_id); } pthread_mutex_unlock(&count_mutex); pthread_exit(NULL); }

Things to note ZCond modify a mutex. ZCond operations are protected by mutex.

Good and bad things ZCheck return code ZIn your practice code add some work so threads will alternate

Mutex and cond summary MutexCond standaloneRequires mutex When locking – I lock other threads When locking – I lock myself When unlocking – I allow other threads to access the critical section Another thread allows me to access the critical section Advisory locking – only lock threads that participate in the game Typically used by peersTypically used by consumer thread