线程广播，然后等待

void* Work::job(void* id)
{
    int idx = (long)id;
    while(1)
    {
        pthread_mutex_lock(&job_lock);
        while(!jobs_complete)
        {
            // wait for main to broadcast
            pthread_cond_wait(&can_work, &job_lock);
            pthread_mutex_unlock(&job_lock);
            // work here
            pthread_mutex_lock(&job_lock);
            ++jobs_completed;
            if(jobs_completed == NUM_THREADS)
            {
                jobs_complete = true;
                pthread_cond_signal(&jobs_done);
                pthread_mutex_unlock(&job_lock);
            }
            pthread_mutex_unlock(&job_lock);
        }
        pthread_mutex_unlock(&job_lock);
    }
    return NULL;
}

// work
jobs_completed = false;
jobs_complete = 0;
pthread_mutex_lock(&job_lock);
pthread_cond_broadcast(&can_work);
pthread_cond_wait(&jobs_complete);
pthread_mutex_unlock(&job_lock);
// work that depends on jobs_complete

我只是发布这个(这几乎是所有的C代码，但pthreads也是，所以有点宽松是善意的请求)来演示一种方法做我认为你试图完成。很明显，你会想要在适当的类中适当地封装其中的大部分。这里希望向您展示的是条件变量、互斥锁以及它们与谓词管理和通知的关系是如何工作的。

我希望它对你有用。祝你今天愉快。

#include <iostream>
#include <unistd.h>
#include <pthread.h>
using namespace std;
// our global condition variable and mutex
pthread_cond_t cv = PTHREAD_COND_INITIALIZER;
pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
// our predicate values.
bool finished = false;
int jobs_waiting = 0;
int jobs_completed = 0;
// our thread proc
static void *worker_proc(void* p)
{
    intptr_t id = (intptr_t)p;  // our id
    size_t n_completed = 0;     // our job completion count
    // always latch prior to eval'ing predicate vars.
    pthread_mutex_lock(&mtx);
    while (!finished)
    {
        // wait for finish or work-waiting predicate
        while (!finished && jobs_waiting == 0)
            pthread_cond_wait(&cv, &mtx);
        // we own the mutex, so we're free to look at, modify
        //  etc. the values(s) that we're using for our predicate
        if (finished)
            break;
        // must be a job_waiting, reduce that number by one, then
        //  unlock the mutex and start our work. Note that we're
        //  changing the predicate (jobs_waiting is part of it) and
        //  we therefore need to let anyone that is monitoring know.
        --jobs_waiting;
        pthread_cond_broadcast(&cv);
        pthread_mutex_unlock(&mtx);
        // DO WORK HERE (this just runs a lame summation)
        for (int i=0,x=0;i<1048576; x += ++i);
        ++n_completed;
        // finished work latch mutex and setup changes
        pthread_mutex_lock(&mtx);
        ++jobs_completed;
        pthread_cond_broadcast(&cv);
    }
    // final report
    cout << id << ": jobs completed = " << n_completed << endl;
    // we always exit owning the mutex, so unlock it now. but
    //  let anyone else know they should be quitting as well.
    pthread_cond_broadcast(&cv);
    pthread_mutex_unlock(&mtx);
    return p;
}
// sets up a batch of work and waits for it to finish.
void run_batch(int num)
{
    pthread_mutex_lock(&mtx);
    jobs_waiting = num;
    jobs_completed = 0;
    pthread_cond_broadcast(&cv);
    // wait or all jobs to complete.
    while (jobs_completed != num)
        pthread_cond_wait(&cv, &mtx);
    // we own this coming out, so let it go.
    pthread_mutex_unlock(&mtx);
}
// main entry point.
int main()
{
    // number of threads in our crew
    static const size_t N = 7;
    pthread_t thrds[N] = {0};
    // startup thread crew.
    intptr_t id = 0;
    for (size_t i=0; i<N; ++i)
        pthread_create(thrds + i, NULL, worker_proc, (void*)(++id));
    // run through batches. each batch is one larger
    //  than the prior batch. this should result in some
    //  interesting job-counts per-thread.
    for (int i=0; i<64; ++i)
        run_batch(i);
    // flag for shutdown state.
    pthread_mutex_lock(&mtx);
    finished = true;
    pthread_cond_broadcast(&cv);
    pthread_mutex_unlock(&mtx);
    for (size_t i=0; i<N; pthread_join(thrds[i++], NULL));
    return 0;
}

示例输出#1

3: jobs completed = 256
6: jobs completed = 282
5: jobs completed = 292
2: jobs completed = 242
1: jobs completed = 339
4: jobs completed = 260
7: jobs completed = 409

示例输出#2

6: jobs completed = 882
1: jobs completed = 210
4: jobs completed = 179
5: jobs completed = 178
2: jobs completed = 187
7: jobs completed = 186
3: jobs completed = 194

示例输出#3

1: jobs completed = 268
6: jobs completed = 559
3: jobs completed = 279
5: jobs completed = 270
2: jobs completed = 164
4: jobs completed = 317
7: jobs completed = 159

---

固定批量大小

相同的代码，但改变了这个:

for (int i=0; i<64; ++i)
    run_batch(i);

:

for (int i=0; i<64; ++i)
    run_batch(N);

给出了以下内容，这可能更接近您真正想要的内容:

示例输出#1

4: jobs completed = 65
2: jobs completed = 63
5: jobs completed = 66
3: jobs completed = 63
1: jobs completed = 64
7: jobs completed = 63
6: jobs completed = 64

示例输出#2

3: jobs completed = 65
5: jobs completed = 62
1: jobs completed = 67
7: jobs completed = 63
2: jobs completed = 65
6: jobs completed = 61
4: jobs completed = 65

示例输出#3

2: jobs completed = 58
4: jobs completed = 61
5: jobs completed = 69
7: jobs completed = 68
3: jobs completed = 61
1: jobs completed = 64
6: jobs completed = 67

在函数末尾有3个可能的连续调用pthread_mutex_unlock，这将导致未定义的行为。实际上你不需要这两个内层。如果jobs_complete是true，线程将退出循环并释放锁，否则它将循环并需要等待can_work条件。

也,

 pthread_cond_wait(&jobs_complete);

你的意思可能是:

pthread_cond_wait(&jobs_complete,&job_lock);

此外，该函数需要pthread_cond_t *和pthread_mutex_t *，而不是int，所以即使这样，代码也明显被破坏了。

请注意，条件变量上的信号或广播只会对已经等待该变量的线程产生影响。该信号不被保留以供将来等待。因此，当线程在jobs_complete上循环while阻塞并再次等待时，它们将不得不再次发出信号以恢复工作。

另一件事:你提到job_complete的类型是int, job_completed的类型是bool，但你的代码似乎不一致:

        if(jobs_completed == NUM_THREADS)
        {
            jobs_complete = true;

我的建议是:学习信号量和屏障抽象模型，如果可以的话，使用现有的实现(c++ 11中的boost或std)，或者使用pthread API重新实现它们。这些将帮助您比操作其他变量更容易地处理这种情况。在这个网站上查找现有的解决方案。例如，这个问题处理的是一个非常相似的问题，我提供的解决方案可以很容易地修改为使用pthread API来满足您的需求。