我应该如何使用条件变量解决哲学家就餐问题？
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【中文标题】我应该如何使用条件变量解决哲学家就餐问题？【英文标题】：How should I solve starving in Dinning Philosophers problem using conditional variables? 【发布时间】：2019-09-29 13:46:04 【问题描述】：我用condition_variables写了一个模拟哲学家就餐的简单程序，问题是有些哲学家从来没有机会吃饭。
我尝试在 _condVar.Wait lambda 函数中放置断点，以确保 putdownForks 方法通知其他等待线程，我确实这样做了，但我仍然不明白为什么其他线程永远不会吃东西。
#include <condition_variable>
#include <iostream>
#include <thread>
#include <unistd.h>
#define thread_num 5
#define HUNGRY 0
#define EATING 1
#define THINKING 2
using namespace std;
thread _threads[thread_num];
mutex _mutex;
condition_variable _condVar;
int _states[thread_num];
bool _isCancelRequested;
void think(int id)
usleep(rand() % 1000000 + 2000000);
void eat(int id)
usleep(rand() % 1000000 + 2000000);
void putdownForks(int id)
unique_lock<mutex> lck(_mutex);
_states[id] = THINKING;
_condVar.notify_all();
void pickupForks(int id)
int leftNeighbourId = (id -1 )%thread_num;
if(leftNeighbourId<0)leftNeighbourId+=thread_num;
int rightNeighbourId= (id+1)%thread_num;
unique_lock<mutex> lck(_mutex);
_states[id] = HUNGRY;
bool isLeftNeighbourEating = _states[leftNeighbourId] == EATING;
bool isRightNeighbourEating = _states[rightNeighbourId] == EATING;
_condVar.wait(lck,
[isLeftNeighbourEating, isRightNeighbourEating]
return !(isLeftNeighbourEating || isRightNeighbourEating);
);
_states[id] = EATING;
void philosopher(int id)
while(!_isCancelRequested)
think(id);
pickupForks(id);
eat(id);
putdownForks(id);
void print_info()
unique_lock<mutex> lck(_mutex);
for(int i=0;i<thread_num;i++)
cout << "Id: "<< i<< " State: "<< _states[i]<<endl;
cout << endl;
void info()
while (!_isCancelRequested)
usleep(1000000);
print_info();
int main()
for(int i=0; i<thread_num;i++)
_threads[i]= thread(philosopher,i);
thread info_thread(info);
for(int i=0; i<thread_num;i++)
_threads[i].join();
info_thread.join();
这是状态随时间变化的输出
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0
Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0
如您所见，id 为 1、2 和 4 的哲学家从不吃东西。
【问题讨论】：
【参考方案1】：哇，这次 *** 效果对我来说很难。 通知后，等待条件需要重新评估，因此每次通知条件变量时我都需要“更新”邻居的状态。 PickupForks 方法应该是这样的：
void pickupForks(int id)
int leftNeighbourId = (id -1 )%thread_num;
if(leftNeighbourId<0)leftNeighbourId+=thread_num;
int rightNeighbourId= (id+1)%thread_num;
unique_lock<mutex> lck(_mutex);
_states[id] = HUNGRY;
_condVar.wait(lck,
[leftNeighbourId, rightNeighbourId]
bool isLeftNeighbourEating = _states[leftNeighbourId] == EATING;
bool isRightNeighbourEating = _states[rightNeighbourId] == EATING;
return !(isLeftNeighbourEating || isRightNeighbourEating);
);
_states[id] = EATING;
【讨论】：
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