【发布时间】：2018-06-24 13:41:21
【问题描述】：
我一直在尝试编写餐饮哲学家的代码，以便更好地使用多线程编程。在我的代码中，我有一个condition_variable，它会停止线程，直到创建了所有线程。但是，似乎当我调用condition_variable::notify_all 通知所有线程都已创建并开始“吃”时，只通知一个线程。例如：
我有一个 Philosophers 类，它有这些成员变量：
static std::condition_variable start;
static std::mutex start_mutex;
还有这些成员函数。
static void start_eating() {
start.notify_all();
}
void dine() {
signal(SIGINT, ctrl_c_catch);
std::unique_lock lk{ start_mutex };
start.wait(lk);
std::cout << id << "started\n";
// see end for complete class...
每个线程都在 condition_variable start 上等待，并且在我调用 start_eating() 之前不会继续。问题是当我调用start.notify_all(); 时，只有一个线程得到通知并继续。但是，当我在等待后更改代码以解锁互斥锁时，一切运行正常（所有线程继续）：
std::unique_lock lk{ start_mutex };
start.wait(lk);
lk.unlock();
我不明白这里发生了什么。为什么需要解锁互斥锁？
完整代码：
#include <chrono>
#include <mutex>
#include <vector>
#include <thread>
#include <condition_variable>
#include <atomic>
#include <signal.h>
#include <iostream>
#include <shared_mutex>
#include <ctime>
namespace clk = std::chrono;
const auto EAT_SLEEP_TIME = clk::milliseconds{1}; // 5 seconds
const auto NUM_SEATS = 5U;
using Fork = std::mutex; // is the fork being used or not
std::mutex cout_mutex;
void ctrl_c_catch(int dummy);
class Philosopher {
Fork& left;
Fork& right;
unsigned id;
unsigned times_eaten;
static std::condition_variable start;
static std::mutex start_mutex;
static std::atomic_bool end;
public:
Philosopher(Fork& l, Fork& r, unsigned i) : left{ l }, right{ r }, id{ i }, times_eaten{} {}
static void start_eating() {
start.notify_all();
}
static void stop_eating() {
end = true;
}
void dine() {
signal(SIGINT, ctrl_c_catch);
std::unique_lock lk{ start_mutex };
start.wait(lk);
// lk.unlock(); // uncommenting this fixes the issue
std::cout << id << " started\n";
while (!end) {
if (&right < &left) {
right.lock();
left.lock();
} else {
left.lock();
right.lock();
}
cout_mutex.lock();
std::clog << id << " got both forks, eating\n";
cout_mutex.unlock();
++times_eaten;
std::this_thread::sleep_for(EAT_SLEEP_TIME * (rand() % 50));
right.unlock();
left.unlock();
std::this_thread::sleep_for(EAT_SLEEP_TIME * (rand() % 50));
}
cout_mutex.lock();
std::cout << id << " stopped, terminating thread. Eaten " << times_eaten << "\n";
cout_mutex.unlock();
delete this;
}
};
std::atomic_bool Philosopher::end = false;
std::condition_variable Philosopher::start{};
std::mutex Philosopher::start_mutex{};
template <size_t N, typename T = unsigned>
constexpr std::array<T, N> range(T b = 0, T s = 1) {
std::array<T, N> ret{};
for (auto& i : ret) {
i = b;
b += s;
}
return ret;
}
void ctrl_c_catch(int dummy) {
std::cout << "Caught ctrl-c or stop\nStoping Philosophers\n";
Philosopher::stop_eating();
std::this_thread::sleep_for(clk::seconds{5});
exit(0);
}
int main() {
srand(time(NULL));
signal(SIGINT, ctrl_c_catch);
std::vector<Fork> forks{ NUM_SEATS }; // 5 forks
std::vector<std::thread> phil; // vector of philosophers
for (unsigned i : range<NUM_SEATS - 1>()) {
auto p = new Philosopher{forks[i], forks[i + 1], i};
phil.emplace_back(&Philosopher::dine, p);
}
auto p = new Philosopher{forks[NUM_SEATS - 1], forks[0], NUM_SEATS - 1};
phil.emplace_back(&Philosopher::dine, p);
std::clog << "Waiting for 5 seconds\n";
std::this_thread::sleep_for(clk::seconds{10});
std::clog << "Starting Philosophers\n Type 'stop' to stop\n";
Philosopher::start_eating();
for (auto& t : phil)
t.detach();
std::this_thread::sleep_for(clk::seconds{15});
ctrl_c_catch(0);
std::string dummy;
std::cin >> dummy;
if (dummy == "stop")
ctrl_c_catch(0);
return 0;
}
【问题讨论】：
标签： c++ multithreading dining-philosopher