c++ - 尽管尝试创建两个线程，但只创建了一个线程

Question

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <vector>
#include <string>
#include <iostream>

FILE*            fp;
pthread_mutex_t demoMutex         = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t  conditionVariable = PTHREAD_COND_INITIALIZER;
unsigned int    condition         = 0;

struct serverInfo
{
    unsigned int serverId;
    pthread_t    threadId;
    std::vector<std::string> queue;
};
std::vector<serverInfo> serverInfoVector;

void* printHello(void* threadId)
{
    pthread_t* my_tid = (pthread_t*)threadId;

    pthread_mutex_lock(&demoMutex);
    while (condition == 0)
        pthread_cond_wait(&conditionVariable, &demoMutex);

    unsigned int i = 0;
    char found = false;

    if (serverInfoVector.size () > 0) {
       while ((i <= serverInfoVector.size()) && (found == false)) {
          if (*my_tid == serverInfoVector[i].threadId) {
             found = true;
             break;
          }
          else
             i++;
       }
    }

    while (!serverInfoVector[i].queue.empty()) {
       std::cout << "\nThread: " << pthread_self() << ", poped from queue: " << serverInfoVector[i].queue.front();
       serverInfoVector[i].queue.pop_back();
    }

    pthread_mutex_unlock(&demoMutex);
    pthread_exit(NULL);
}

void checkServerExists(unsigned int serverNumber, std::string message)
{
    unsigned int i = 0;
    char found = false;

    pthread_mutex_lock(&demoMutex);

    if (serverInfoVector.size () > 0) {
       while ((i <= serverInfoVector.size()) && (found == false)) {
          if (serverNumber == serverInfoVector[i].serverId) {
             found = true;
             break;
          }
          else
             i++;
       }
    }

    if (found == false) {
       // This server doesn't exist, so create a thread for it, create a queue for it, push the message in the corresponding queue.
       // Push the server number in the serverNumberArray.

       // Create a thread for it.
       pthread_t newThread;
       int returnValue;
       if ((returnValue = pthread_create (&newThread, NULL, printHello, (void*) &newThread)) != 0) {
          printf("\nerror: pthread_create failed with error number %d", returnValue);
       }
       printf("\nIn checkServerExists()`: thread id %ld\n", newThread);

       // Push the message in its queue.
       serverInfo obj;
       obj.serverId = serverNumber;
       obj.threadId = newThread;
       obj.queue.push_back(message);
       serverInfoVector.push_back(obj);

       condition++;
       pthread_cond_signal(&conditionVariable);
       pthread_mutex_unlock(&demoMutex);

       for (unsigned int i = 0; i < serverInfoVector.size(); i++)
          pthread_join(serverInfoVector[i].threadId, NULL);
    }
    else {
       // This server exists, so lookup its thread and queue, push the message in the corresponding queue.
       printf("\nIn else ()`: thread id %ld\n", serverInfoVector[i].threadId);
       serverInfoVector[i].queue.push_back(message);

       condition++;
       pthread_cond_signal(&conditionVariable);
       pthread_mutex_unlock(&demoMutex);

       for (unsigned int i = 0; i < serverInfoVector.size(); i++)
          pthread_join(serverInfoVector[i].threadId, NULL);
    }
}

int main()
{
   fp = fopen("xyz", "w");

   checkServerExists(1, "anisha");
   checkServerExists(2, "kaul");
   checkServerExists(1, "sanjeev");
   checkServerExists(2, "sharma");
}

输出:

In checkServerExists ()`: thread id 140233482061584

Thread: 140233482061584, poped from queue: anisha
In checkServerExists ()`: thread id 140233482061584

In else ()`: thread id 140233482061584

In else ()`: thread id 140233482061584

问题是似乎只创建了一个线程!我在 main() 中调用函数 checkServerExists 4 次，使用不同的 serverID 调用 2 次，所以应该创建两个线程？

我错过了什么？

Answer 1

已编辑:真正的问题是线程终止并连接为正如 hmjd 所指出的那样，它们一经创建。我要离开这个，而不是删除，因为下面也是问题。

我在您发布的输出中看到两个新线程的创建:“In
checkServerExists" 仅在创建新线程时输出。我也查看 printf 中的未定义行为:newThread 具有类型pthread_t，它可以是系统想要的任何东西，并且是可能不是 long，这是您传递给 printf 的格式。据我所知，没有办法(可移植地)输出一个 pthread_t(除了它的十六进制转储字节)；您显示为线程 ID 的值没有任何意义。另外，你不能使用==比较pthread_t，你需要使用pthread_equal。 (至少在我用过的一个平台上，pthread_t 是一个结构。)

您的代码还有许多其他奇怪的地方。为什么申报例如，found 类型为 char，而不是类型 bool。为什么found == false，而不是 !found。以及为什么 break; 在循环，因为你在循环控制变量中有条件。一个多checkServerExists 开头的更惯用形式是:

for ( std::vector<ServerInfo>::iterator current = serverInfoVector.begin();
        current != serverInfoVector.end() && current->serverId != serverNumber;
        ++ current ) {
}
if ( current == serverInfoVector.end() ) {
    //  not found...
} else {
    //  found...
}

假设您没有为查找创建预测对象，而只是使用 std::find。