线程等待父线程

您需要的是条件变量。
所有的工作线程都调用wait()方法来挂起它们。

然后，父线程将一个工作项放到队列中，并在条件变量上调用signal。这将唤醒一个正在睡觉的线程。它可以从队列中删除作业，执行作业，然后在条件变量上调用wait以返回睡眠状态。

试题:

#include <pthread.h>
#include <memory>
#include <list>
// Use RAII to do the lock/unlock
struct MutexLock
{
    MutexLock(pthread_mutex_t& m) : mutex(m)    { pthread_mutex_lock(&mutex); }
    ~MutexLock()                                { pthread_mutex_unlock(&mutex); }
    private:
        pthread_mutex_t&    mutex;
};
// The base class of all work we want to do.
struct Job
{
    virtual void doWork()  = 0;
};
// pthreads is a C library the call back must be a C function.
extern "C" void* threadPoolThreadStart(void*);
// The very basre minimal part of a thread pool
// It does not create the workers. You need to create the work threads
// then make them call workerStart(). I leave that as an exercise for you.
class ThreadPool
{
    public:
         ThreadPool(unsigned int threadCount=1);
        ~ThreadPool();
        void addWork(std::auto_ptr<Job> job);
    private:
        friend void* threadPoolThreadStart(void*);
        void workerStart();
        std::auto_ptr<Job>  getJob();
        bool                finished;   // Threads will re-wait while this is true.
        pthread_mutex_t     mutex;      // A lock so that we can sequence accesses.
        pthread_cond_t      cond;       // The condition variable that is used to hold worker threads.
        std::list<Job*>     workQueue;  // A queue of jobs.
        std::vector<pthread_t>threads;
};
// Create the thread pool
ThreadPool::ThreadPool(int unsigned threadCount)
    : finished(false)
    , threads(threadCount)
{
    // If we fail creating either pthread object than throw a fit.
    if (pthread_mutex_init(&mutex, NULL) != 0)
    {   throw int(1);
    }
    if (pthread_cond_init(&cond, NULL) != 0)
    {
        pthread_mutex_destroy(&mutex);
        throw int(2);
    }
    for(unsigned int loop=0; loop < threadCount;++loop)
    {
       if (pthread_create(threads[loop], NULL, threadPoolThreadStart, this) != 0)
       {
            // One thread failed: clean up
            for(unsigned int kill = loop -1; kill < loop /*unsigned will wrap*/;--kill)
            {
                pthread_kill(threads[kill], 9);
            }
            throw int(3);
       }
    }
}
// Cleanup any left overs.
// Note. This does not deal with worker threads.
//       You need to add a method to flush all worker threads
//       out of this pobject before you let the destructor destroy it.
ThreadPool::~ThreadPool()
{
    finished = true;
    for(std::vector<pthread_t>::iterator loop = threads.begin();loop != threads.end(); ++loop)
    {
        // Send enough signals to free all threads.
        pthread_cond_signal(&cond);
    }
    for(std::vector<pthread_t>::iterator loop = threads.begin();loop != threads.end(); ++loop)
    {
        // Wait for all threads to exit (they will as finished is true and
        //                               we sent enough signals to make sure
        //                               they are running).
        void*  result;
        pthread_join(*loop, &result);
    }
    // Destroy the pthread objects.
    pthread_cond_destroy(&cond);
    pthread_mutex_destroy(&mutex);
    // Delete all re-maining jobs.
    // Notice how we took ownership of the jobs.
    for(std::list<Job*>::const_iterator loop = workQueue.begin(); loop != workQueue.end();++loop)
    {
        delete *loop;
    }
}
// Add a new job to the queue
// Signal the condition variable. This will flush a waiting worker
// otherwise the job will wait for a worker to finish processing its current job.
void ThreadPool::addWork(std::auto_ptr<Job> job)
{
    MutexLock  lock(mutex);
    workQueue.push_back(job.release());
    pthread_cond_signal(&cond);
}
// Start a thread.
// Make sure no exceptions escape as that is bad.
void* threadPoolThreadStart(void* data)
{
    ThreadPool* pool = reinterpret_cast<ThreadPool*>(workerStart);
    try
    {
        pool->workerStart();
    }
    catch(...){}
    return NULL;
}
// This is the main worker loop.
void ThreadPool::workerStart()
{
    while(!finished)
    {
        std::auto_ptr<Job>    job    = getJob();
        if (job.get() != NULL)
        {
            job->doWork();
        }
    }
}
// The workers come here to get a job.
// If there are non in the queue they are suspended waiting on cond
// until a new job is added above.
std::auto_ptr<Job> ThreadPool::getJob()
{
    MutexLock  lock(mutex);
    while((workQueue.empty()) && (!finished))
    {
        pthread_cond_wait(&cond, &mutex);
        // The wait releases the mutex lock and suspends the thread (until a signal).
        // When a thread wakes up it is help until it can acquire the mutex so when we
        // get here the mutex is again locked.
        //
        // Note: You must use while() here. This is because of the situation.
        //   Two workers:  Worker A processing job A.
        //                 Worker B suspended on condition variable.
        //   Parent adds a new job and calls signal.
        //   This wakes up thread B. But it is possible for Worker A to finish its
        //   work and lock the mutex before the Worker B is released from the above call.
        //
        //   If that happens then Worker A will see that the queue is not empty
        //   and grab the work item in the queue and start processing. Worker B will
        //   then lock the mutext and proceed here. If the above is not a while then
        //   it would try and remove an item from an empty queue. With a while it sees
        //   that the queue is empty and re-suspends on the condition variable above.
    }
    std::auto_ptr<Job>  result;
    if (!finished)
    {    result.reset(workQueue.front());
         workQueue.pop_front();
    }
    return result;
}

通常的实现方法是有一个未完成工作的队列queue，一个保护队列的互斥锁mutex和一个等待条件queue_not_empty。然后，每个工作线程执行以下操作(使用伪api):

while (true) {
    Work * work = 0;
    mutex.lock();
    while ( queue.empty() )
       if ( !queue_not_empty.wait( &mutex, timeout ) )
           return; // timeout - exit the worker thread
    work = queue.front();
    queue.pop_front();
    mutex.unlock();
    work->perform();
}

wait( &mutex, timeout )调用阻塞，直到满足等待条件或呼叫超时。传递的mutex在wait()内部自动解锁，并在从调用返回之前再次锁定，以便为所有参与者提供一致的队列视图。timeout将被选择为相当大(秒)，并将导致线程退出(如果有更多的工作进入，线程池将启动新线程)。

同时，线程池的工作插入函数这样做:

Work * work = ...;
mutex.lock();
queue.push_back( work );
if ( worker.empty() )
    start_a_new_worker();
queue_not_empty.wake_one();
mutex.unlock();

与多个消费者(工作线程消费工作请求)的经典生产者-消费者同步。众所周知的技术是有一个信号量，每个工作线程执行down()，每次有工作请求时执行up()。然后从互斥锁工作队列中选择请求。因为一个up()只会唤醒一个down()，所以互斥锁上的争用实际上是最小的。

或者你可以用条件变量做同样的事情，在每个线程中做等待，当你有工作时唤醒一个。队列本身仍然被互斥锁(无论如何condvar需要一个互斥锁)。

最后我不完全确定，但我实际上认为你可以使用管道作为队列，包括所有同步(工作线程只是试图"读取(sizeof(request))"))。有点粗糙，但导致更少的上下文切换。

由于网络聊天程序可能是I/o绑定而不是cpu绑定，因此实际上并不需要线程。您可以使用Boost之类的工具在单个线程中处理所有I/O。或GLib主循环。这些是基于平台特定函数的可移植抽象，允许程序阻塞等待任何(可能很大)打开的文件或套接字集的活动，然后在活动发生时唤醒并迅速响应。

最简单的方法是semaphores。信号量是这样工作的:

信号量基本上是一个接受null/正值的变量。进程可以通过两种方式与它交互:增加或减少信号量。

增加信号量给这个神奇的变量加1，仅此而已。在减少计数时，事情变得有趣了:如果计数达到零，并且进程试图再次降低它，因为它不能接受负值，它将阻塞，直到变量上升。

如果多个进程阻塞等待减少信号量值，则每增加一个单位只唤醒一个。

这使得创建一个工作/任务系统非常容易:你的管理进程队列任务，并增加信号量的值来匹配剩余的项目，你的工作进程试图减少计数并不断获取任务。当没有任务可用时，它们将阻塞，并且不消耗cpu时间。当一个进程出现时，只有一个休眠进程会被唤醒。Insta-sync魔法。

不幸的是，至少在Unix世界中，信号量API不是很友好，因为出于某种原因，它处理的是信号量数组而不是单个信号量。但是，您只是一个简单的包装器，而不是一个漂亮的接口!

干杯!