使用pthread同时push()到一个共享队列?

时间:2013-10-30 23:45:54

标签: c++ multithreading pthreads

我正在练习pthread。

在我的原始程序中,push将共享队列称为request类的实例,但我首先至少要确保将某些内容推送到共享队列。

这是一个非常简单的代码,但它只会引发很多错误,我无法弄清楚原因。

我想这可能是语法,但无论我尝试过什么都行不通。

你知道它为什么不起作用吗?

以下是我一直在尝试的代码。

extern "C" {
    #include<pthread.h>
    #include<unistd.h>
}
#include<queue>
#include<iostream>
#include<string>

using namespace std;

class request {
 public:
    string req;
    request(string s) : req(s) {}

};

int n;
queue<request> q;
pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;

void * putToQueue(string);

int main ( void ) {
    pthread_t t1, t2;

    request* ff = new request("First");
    request* trd = new request("Third");

    int result1 = pthread_create(&t1, NULL, &putToQueue, reinterpret_cast<void*>(&ff));
    if (result1 != 0) cout << "error 1" << endl;
    int result2 = pthread_create(&t2, NULL, &putToQueue, reinterpret_cast<void*>(&trd));
    if (result2 != 0) cout << "error 2" << endl;

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    for(int i=0; i<q.size(); ++i) {
        cout << q.front().req << " is in queue" << endl;
        q.pop();
        --n;
    }

    return 0;
}

void * putToQueue(void* elem) {
    pthread_mutex_lock(&mut);

    q.push(reinterpret_cast<request>(elem));
    ++n;

    cout << n << " items are in the queue." << endl;

    pthread_mutex_unlock(&mut);
    return 0;
}

2 个答案:

答案 0 :(得分:1)

以下代码对必须更改的所有内容进行了评论。我会写一个详细的描述为什么他们必须改变,但我希望代码说明一切。 仍然不是防弹的。有很多事情可以做得不同或更好(失败的new的异常处理等),但至少它编译,运行,并且不会泄漏内存。

#include <queue>
#include <iostream>
#include <string>
#include <pthread.h>
#include <unistd.h>
using namespace std;

// MINOR: param should be a const-ref
class request {
public:
    string req;
    request(const string& s) : req(s) {}
};

int n;
queue<request> q;
pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;


// FIXED: made protoype a proper pthread-proc signature
void * putToQueue(void*);

int main ( void )
{
    pthread_t t1, t2;

    // FIXED: made thread param the actual dynamic allocation address
    int result1 = pthread_create(&t1, NULL, &putToQueue, new request("First"));
    if (result1 != 0) cout << "error 1" << endl;

    // FIXED: made thread param the actual dynamic allocation address
    int result2 = pthread_create(&t2, NULL, &putToQueue, new request("Third"));
    if (result2 != 0) cout << "error 2" << endl;

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    // FIXED: was skipping elements because the queue size was shrinking
    //  with each pop in the while-body.
    while (!q.empty())
    {
        cout << q.front().req << " WAS in queue" << endl;
        q.pop();
    }

    return 0;
}

// FIXED: pretty much a near-total-rewrite
void* putToQueue(void* elem)
{
    request *req = static_cast<request*>(elem);
    if (pthread_mutex_lock(&mut) == 0)
    {
        q.push(*req);
        cout << ++n << " items are in the queue." << endl;
        pthread_mutex_unlock(&mut);
    }
    delete req; // FIXED: squelched memory leak
    return 0;
}

输出(您的可能会变化)

1 items are in the queue.
2 items are in the queue.
Third WAS in queue
First WAS in queue

答案 1 :(得分:1)

如评论中所述,我建议不要直接使用pthread,而是使用C ++ 11线程原语。我将从一个简单的受保护队列类开始:

template <class T, template<class, class> class Container=std::deque>
class p_q {
    typedef typename Container<T, std::allocator<T>> container;
    typedef typename container::iterator iterator;

    container data;
    std::mutex m;
public:
    void push(T a) {
        std::lock_guard<std::mutex> l(m);
        data.emplace_back(a);
    }
    iterator begin() { return data.begin(); }
    iterator end() { return data.end(); }
    // omitting front() and pop() for now, because they're not used in this code
};

使用它,代码的主流保持几乎像单线程代码一样简单和干净,如下所示:

int main() {
    p_q<std::string> q;

    auto pusher = [&q](std::string const& a) { q.push(a); };

    std::thread t1{ pusher, "First" };
    std::thread t2{ pusher, "Second" };

    t1.join();
    t2.join();

    for (auto s : q)
        std::cout << s << "\n";
}

现在看来,这是一个多生产者,单一消费者队列。此外,它取决于生产者在消费发生时不再运行的事实。在这种情况下,这是真的,但不会/不会永远。如果不是这种情况,你需要一个(边缘)更复杂的队列,当它从队列中读取/弹出时进行锁定,而不仅仅是在写入时。