关于 C++多线程的一个问题

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现在学长让我把一个1300多行的代码改成多线程，%>_<%，然后开始看了一些多线程的指导，链接在此
有一个例子如下所示：

#include <iostream>
#include <thread>

static const int num_threads = 10;

//This function will be called from a thread

void call_from_thread(int tid) {
    std::cout << "Launched by thread " << tid << std::endl;
}

int main() {
    std::thread t[num_threads];

    //Launch a group of threads
    for (int i = 0; i < num_threads; ++i) {
        t[i] = std::thread(call_from_thread, i);
    }

    std::cout << "Launched from the main\n";

    //Join the threads with the main thread
    for (int i = 0; i < num_threads; ++i) {
        t[i].join();
    }

    return 0;
}

例子说它的输出应该为：

但是我的输出确实乱掉的：

我是在xcode里面编译的哦。
感谢各位指导。如果能指引一下多线程的方向就更感谢啦！

Supplement 1 · Jun 21, 2015

多谢大家，使用mutex锁住cout即可：
```
static std::mutex mtx;
```
```
mtx.lock();
std::cout << "Launched by thread " << tid << std::endl;
mtx.unlock();
```
```
Launched by thread 4
Launched by thread 3
Launched by thread 1
Launched by thread 5
Launched by thread 0
Launched by thread 2
Launched by thread 6
Launched by thread 7
Launched by thread 8
Launched from the main
Launched by thread 9
```

又一个问题来了，是否可以控制thread输出的顺序呢？

Supplement 2 · Jun 21, 2015

（http://www.cplusplus.com/reference/mutex/mutex/lock/）的例子跟我这个差不多，锁住之后就是正常按照顺序输出，晕.
现在学长要求我讲SVM（支持向量机）一共1500多行放到多thread去执行。里面的算法是搞懂了，但是多个thread执行。感觉是没有什么思路。

thread

int

++i

18 replies • 2015-06-22 10:08:19 +08:00

aheadlead

Jun 20, 2015

请google 互斥锁

https://zh.wikipedia.org/zh/互斥锁

46fo

Jun 20, 2015

0o0 按理说是一次性输出的

limhiaoing

Jun 20, 2015

你看的那篇文章里写了这么一句话
Actually if you run the above code on your system you can get a completely different result or even some mangled characters.

limhiaoing

Jun 20, 2015

我也写过一篇关于C++11 多线程的简单的入门文章
http://blog.poxiao.me/p/multi-threading-in-cpp11-part-1-thread-and-future/
涉及到 std::thread std::future std::async std::promise std::packaged_task等