Inconsistent output from c++ multithreaded program - c++

Question

I have the following program in C++:
// multithreading01.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <string>
#include <iostream>
#include <process.h>
using namespace std;
bool threadFinished = false;
struct params {
string aFile;
bool tf;
};
void WriteToFile(void *p)
{
params* a = (params*)p;
cout<<a->aFile<<endl;
a->tf = true;
_endthread();
}
int main(int argc, char* argv[])
{
params *param01 = new params;
params *param02 = new params;
param01->aFile = "hello from p1";
param01->tf = false;
param02->aFile = "hello from p2";
param02->tf = false;
_beginthread(WriteToFile,0,(void *) param01);
_beginthread(WriteToFile,0,(void *) param02);
while(!param01->tf || !param02->tf)
{
}
cout << "Main ends" << endl;
system("pause");
return 0;
}
However, I am getting inconsistent outputs such as
output 1:
hello from p1
hello from p2
output 2:
hello from p1hello from p2
output 3:
hhello from p2ello from p1
How can I get a consistent output from this code? I am using Visual C++ 6.0 Standard Edition.

Answer 1

Read this small writeup
Like everyone mentioned in the comment, when you create threads, generally speaking, idea is to separate tasks and thusly increasing performance on modern multicore architecture CPUs which could one thread per core.
If you want to access same resource (same file in your case) from two different threads then you need to make sure that simultaneous access from two threads doesnt happen otherwise you would see the problem that you are seeing.
Your provide safe simultaneous access by protecting shared resource using some locks (e.g POSIX locks or you could chose your platform specific lock implementation).
Common mistake beginners do is that they lock the "code" not "resource".
Dont do this:
void WriteToFile(void *p)
{
pthread_mutex_lock(var); //for example only
params* a = (params*)p;
cout<<a->aFile<<endl;
a->tf = true;
_endthread();
pthread_mutex_unlock(var); //for example only
}
You should instead put a lock in your resource
struct params {
lock_t lock; //for example only not actual code
string aFile;
bool tf;
};
void WriteToFile(void *p)
{
params* a = (params*)p;
pthread_mutex_lock(a->lock); //Locking params here not the whole code.
cout<<a->aFile<<endl;
a->tf = true;
pthread_mutex_unlock(a->lock); //Unlocking params
_endthread();
}