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

Question

I am a complete beginner in OS. The problem I am having is that I want to receive a value from a thread into the main process. A garbage value is printed in the main. Please elaborate so I can know my mistake.
Here is my code:
#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

You have two ways to do this:
Pass a pointer to an integer as the argument to the thread, and have the thread modify the argument to hold the value you want.
Have the thread allocate a value on the heap and return a pointer to that memory. Then clean it up later.
#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

Try:
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;
}
More info: http://www.cse.cuhk.edu.hk/~ericlo/teaching/os/lab/9-PThread/Pass.html