C++ 如何将一个值从一个线程传递给main

Question

我是 OS 的初学者。我遇到的问题是我想从线程接收一个值到主进程中。垃圾值打印在 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

尝试：

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

Answer 2

您有两种方法可以做到这一点：

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。

让线程在堆上分配一个值并 return 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));
}

C++ 如何将一个值从一个线程传递给main

How to pass a value from a thread to main in c++

c++

linux

multithreading

operating-system