/* * Futex demonstration program * matthewc@cse.unsw.edu.au */#include <stdio.h>#include <stdint.h>#include <pthread.h>#include <unistd.h>#include <asm/unistd.h>typedef uint32_t u32; /* work around broken futex.h */#include <linux/futex.h>#define futex(a,b,c,d) syscall(__NR_futex,a,b,c,d)#define WORKER_THREADS 3/* shared variables *//* volatile */ int workers_active, ready_futex, run_futex, shutdown;double input;double partial_result[WORKER_THREADS];doubleeval(int thread){        /* example evaluation function */        return input * thread + thread;}voidawait_work(){        int last_run_seq_no = run_futex;        /* if we are the last thread to complete, signal ready_futex */        if (__sync_fetch_and_add(&workers_active, -1) == 1)        {            ready_futex = 1;            futex(&ready_futex, FUTEX_WAKE, 1, NULL);        }        /* now wait for run_futex to increment */        if (run_futex == last_run_seq_no)            futex(&run_futex, FUTEX_WAIT, last_run_seq_no, NULL);}voidthread_pool_wait(){        /* wait for ready_futex */        if (!ready_futex)            futex(&ready_futex, FUTEX_WAIT, 0, NULL);}voidthread_pool_notify(){        /* increment run_futex to start workers */        workers_active = WORKER_THREADS;        ready_futex = 0;        run_futex++;        futex(&run_futex, FUTEX_WAKE, WORKER_THREADS, NULL);}void *worker(void *arg){        int thread = (int)arg;        printf("thread=%d\n", thread);        while (1)        {            await_work();            if (shutdown)                return NULL;            partial_result[thread] = eval(thread+1);        }}voidevaluate(double arg){        double result;        int i;        /* prepare inputs here */        input = arg;        /* start slaves */        thread_pool_notify();        /* might as well run an evaluation on the master too */        result = eval(0);        /* wait on any outstanding slaves */        thread_pool_wait();        /* accumulate results */        for (i = 0; i < WORKER_THREADS; i++)                result += partial_result[i];        printf("result: %lf\n", result);}intmain(){        pthread_t threads[WORKER_THREADS];        int i;        workers_active = WORKER_THREADS;        ready_futex = 0;        shutdown = 0;        for (i = 0; i < WORKER_THREADS; i++)            pthread_create(&threads[i], NULL, worker, (void *)i);        /* wait for threads to start up */        thread_pool_wait();        printf("starting...\n");        evaluate(1);        evaluate(2);        evaluate(3);        evaluate(4);        evaluate(5);        printf("done\n");        /* signal threads to shut down */        shutdown = 1;        thread_pool_notify();        for (i = 0; i < WORKER_THREADS; i++)            pthread_join(threads[i], NULL);        return 0;}