我正在尝试设置多个线程以等待状态，直到收到pthread_cond_broadcast()。
完成一份工作后，我希望线程回到等待状态。
我还希望调用pthread_cond_broadcast()的进程在继续之前等待所有线程返回其等待状态。在这种情况下，它是调用广播的main（）函数。我正在尝试使用main（0在调用广播后执行pthread_cond_wait()。
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;
}
NUM_THREADS 为4， job_lock 为pthread_mutex_t， can_work 且 jobs_done 为{{ 1}}， jobs_completed 是pthread_cond_t， jobs_complete 是bool。
int现在，我正在通过调用// 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
然后调用pthread_cond_broadcast()来实现此目的，但这似乎已陷入僵局。
任何人都可以解释我应该怎么做或者我哪里出错了？我很感激你的帮助。
谢谢！
答案 0 :(得分：3)
我只发布这个（这几乎都是C代码，但是pthreads也是如此，所以请求一点松弛）来演示一种方法来做我认为你正在尝试的事情去完成。显然，您希望将大部分内容正确地封装在适当的类中等。这有望向您展示条件变量，互斥量及其与谓词管理和通知的关系是如何工作的。
我希望你觉得它很有用。祝你有美好的一天。
#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
修正批量大小
相同的代码，但更改此内容：
for (int i=0; i<64; ++i)
run_batch(i);
到此：
for (int i=0; i<64; ++i)
run_batch(N);
给出以下内容，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
答案 1 :(得分：1)
在函数末尾有3个可能的pthread_mutex_unlock连续调用，这会导致未定义的行为。你实际上并不需要两个内在的。如果jobs_complete为true，则线程将退出循环并释放锁定，否则它将循环并需要它在can_work条件下等待。
此外，还有
pthread_cond_wait(&jobs_complete);
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，代码也会被清除。
请注意，条件变量上的信号或广播只会对已经等待变量的线程产生影响。信号不会保留以供将来等待。
因此，当线程在jobs_complete上循环并且再次等待时，必须再次发信号以恢复工作。
另一件事：您将job_complete的类型称为int，将job_completed称为bool，但您的代码似乎并不同意：
if(jobs_completed == NUM_THREADS)
{
jobs_complete = true;
以下是我的建议：了解信号量和障碍抽象模型，如果可以，请使用现有的实现（C ++ 11中的boost或std）或重新实现他们使用pthread API。这些将帮助您比操纵cond变量更容易处理这种情况。在这个网站上查找现有解决方案。例如，this question处理一个非常类似的问题，我可以轻松修改我提供的解决方案，以使用pthread API来满足您的要求。