我有一个使用互斥锁和两个条件变量进行同步的线程生产者/消费者系统。当有物品要使用时，生产者向消费者线程发出信号，而消费者在消费完物品时向生产者线程发出信号。线程继续生产和消费，直到析构函数通过设置布尔变量要求它们退出。因为任何一个线程都可能在等待条件变量，所以我必须对 quit 变量进行第二次检查，这感觉是错误和混乱的......
我已将问题简化为以下（使用 g++4.7 在 GNU/Linux 上工作）示例：
// C++11and Boost required.
#include <cstdlib> // std::rand()
#include <cassert>
#include <boost/circular_buffer.hpp>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <thread>
#include <vector>
// Creates a single producer and single consumer thread.
class prosumer
{
public:
// Create the circular buffer and start the producer and consumer thread.
prosumer()
: quit_{ false }
, buffer_{ circular_buffer_capacity }
, producer_{ &prosumer::producer_func, this }
, consumer_{ &prosumer::consumer_func, this }
{}
// Set the quit flag and wait for the threads to exit.
~prosumer()
{
quit_ = true;
producer_.join();
consumer_.join();
}
private:
// Thread entry point for the producer.
void producer_func()
{
// Value to add to the ringbuffer to simulate data.
int counter = 0;
while ( quit_ == false )
{
// Simulate the production of some data.
std::vector< int > produced_items;
const auto items_to_produce = std::rand() % circular_buffer_capacity;
for ( int i = 0; i < items_to_produce; ++i )
{
produced_items.push_back( ++counter );
}
// Get a lock on the circular buffer.
std::unique_lock< std::mutex > lock( buffer_lock_ );
// Wait for the buffer to be emptied or the quit flag to be set.
buffer_is_empty_.wait( lock, [this]()
{
return buffer_.empty() == true || quit_ != false;
} );
// Check if the thread was requested to quit.
if ( quit_ != false )
{
// Don't let the consumer deadlock.
buffer_has_data_.notify_one();
break;
}
// The buffer is locked by this thread. Put the data into it.
buffer_.insert( std::end( buffer_ ), std::begin( produced_items ), std::end( produced_items ) );
// Notify the consumer that the buffer has some data in it.
buffer_has_data_.notify_one();
}
std::cout << "producer thread quit\n";
}
// Thread entry for the consumer.
void consumer_func()
{
int counter_check = 0;
while ( quit_ == false )
{
std::unique_lock< std::mutex > lock( buffer_lock_ );
// Wait for the buffer to have some data before trying to read from it.
buffer_has_data_.wait( lock, [this]()
{
return buffer_.empty() == false || quit_ != false;
} );
// Check if the thread was requested to quit.
if ( quit_ != false )
{
// Don't let the producer deadlock.
buffer_is_empty_.notify_one();
break;
}
// The buffer is locked by this thread. Simulate consuming the data.
for ( auto i : buffer_ ) assert( i == ++counter_check );
buffer_.clear();
// Notify the producer thread that the buffer is empty.
buffer_is_empty_.notify_one();
}
std::cout << "consumer thread quit\n";
}
// How many items the circular buffer can hold.
static const int circular_buffer_capacity = 64;
// Flag set in the destructor to signal the threads to stop.
std::atomic_bool quit_;
// Circular buffer to hold items and a mutex for synchronization.
std::mutex buffer_lock_;
boost::circular_buffer< int > buffer_;
// Condition variables for the threads to signal each other.
std::condition_variable buffer_has_data_;
std::condition_variable buffer_is_empty_;
std::thread producer_;
std::thread consumer_;
};
int main( int argc, char **argv )
{
(void)argc; (void) argv;
prosumer test;
// Let the prosumer work for a little while.
std::this_thread::sleep_for( std::chrono::seconds( 3 ) );
return EXIT_SUCCESS;
}
如果查看 producer_func 和 consumer_func 线程函数，您可以看到它们循环直到退出变量由产消者析构函数设置，但它们在锁定循环缓冲区后还会再次检查退出变量。如果设置了退出变量，它们会相互发送信号以防止死锁。
我的另一个想法是在析构函数的条件变量上调用 notify_one()，这会是更好的解决方案吗？
有一个更好的方法吗？
更新 1：我忘了提到，在这种情况下，当线程被请求退出时，消费者不需要消耗循环缓冲区中的任何剩余数据，如果生产者也产生更多数据就可以了。只要他们都退出并且不陷入僵局，一切都会好起来的。