如何将值从线程传递到C++中的主线程

Question

我是操作系统的完整初学者。我遇到的问题是我想从线程接收一个值到主进程。一个垃圾值被打印在 main 中。请详细说明，以便我知道我的错误。

这是我的代码：

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <iostream>
#include<string.h>
#include <sys/wait.h>
#include<pthread.h>
#include <sys/types.h>
#include<sys/stat.h>
#include<errno.h>
#include<fcntl.h>
#include<time.h>
#include <fstream>
#include<vector>

using namespace std;

void *thread_1(void *arg);

int main(int argc, char* argv[])
{
    void *temp;
    pthread_t t1;
    pthread_create(&t1, NULL,thread_1,NULL);
    pthread_join(t1,&temp);
    int *num = (int*)temp;
    cout<<"Value in main: "<<*num<<endl;
}

void *thread_1(void *arg)
{
    int number = 5;
    int* value = &number;
    cout<<"Value in thread: "<<*value<<endl;
    pthread_exit((void*)value);
}

Answer 1

您有两种方法可以做到这一点：

1. 将指向整数的指针作为参数传递给线程，并让线程修改参数以保存您想要的值。

2. 让线程在堆上分配一个值并返回一个指向该内存的指针。以后再清理吧。

   #include <iostream>
   #include <pthread.h>
   
   void *thread_1(void *arg);
   void *thread_2(void *arg);
   
   int main(int argc, char* argv[])
   {
       int value = 0;
       pthread_t t1;
       pthread_create(&t1, NULL,thread_1, &value);
       pthread_join(t1, NULL);
   
       std::cout<<"Value in main: "<<value<<std::endl;
   
   
       int* value2 = NULL;
       pthread_t t2;
       pthread_create(&t2, NULL,thread_2, NULL);
       pthread_join(t2, reinterpret_cast<void**>(&value2));
   
       if (value2) {
           std::cout<<"Value in main: "<<*value2<<std::endl;
           delete value2;
       }
   }
   
   void *thread_1(void *arg)
   {
       int* value = static_cast<int*>(arg);
       *value = 5;
   
       std::cout<<"Value in thread: "<<*value<<std::endl;
       pthread_exit(NULL);
   }
   
   void *thread_2(void *arg)
   {
       int* value = new int(10);
       std::cout<<"Value in thread: "<<*value<<std::endl;
       pthread_exit(static_cast<void*>(value));
   }
   

Answer 2

试试：

void *thread_1(void *arg)
{
    
    int value = *((int *)arg);
    cout<<"Value in thread: "<<value<<endl;
    int new_val = 345;
    pthread_exit(&new_val );
}

int main(int argc, char* argv[])
{
    int *temp=15;
    void *return_val;
    pthread_t t1;
    pthread_create(&t1, NULL,thread_1,(void *)temp);
    pthread_join(t1,&return_val);

    
    cout<<"Value in main: "<<*temp<<endl;
    cout<<"Value from loop: "<<*(int*)return_val<<endl;
}

更多信息：http://www.cse.cuhk.edu.hk/~ericlo/teaching/os/lab/9-PThread/Pass.html