how to return a value from _beginthread thread

Question

I am in the process of creating a two thread array summation program and i am using windows.h threads. And this is the code that i have so far.

#include "StdAfx.h"
#include <stdio.h>
#include <iostream>
#include <windows.h>
#include <process.h>     // needed for _beginthread()

void  silly( void * );   // function prototype

using namespace std;

int arraySum[100];

int main()
{
    // Our program's first thread starts in the main() function.

    printf( "Now in the main() function.\n" );


    for(int i = 0 ; i  < 100 ; i++){
        arraySum[i] = i;
    }

    // Let's now create our second thread and ask it to start
    // in the silly() function.


    _beginthread( silly, 0, (void*)1 );
    _beginthread( silly, 0, (void*)2 );

    Sleep( 100 );

    int a;
    cin >> a;

}

void  silly( void *arg )
{
    printf( "The silly() function was passed %d\n", (INT_PTR)arg ) ;
    int partialSum = 0;
    for(int i =50*((INT_PTR)arg - 1); i < 50 * ((INT_PTR)arg) ; i++){
    partialSum == arraySum[i];
    }
}

What i seem to find it hard to do is make the function return the partion sum to the main method. Could someone please help me out.

Answer 1

Pass the address of an int to silly(), which can act as both input and output parameter, and have silly() populate it with the value required by the caller:

int silly_result_1 = 1;
int silly_result_2 = 2;

_beginthread( silly, 0, (void*)&silly_result_1 );
_beginthread( silly, 0, (void*)&silly_result_2 );

void silly( void *a_arg )
{
    int* arg = (int*) a_arg;
}

You need to wait for the two threads to complete.

---

Note it is necessary that the variables whose addresses are passed to _beginthread() exist for the lifetime of the thread. For example, the following would result in undefined behaviour:

void _start_my_thread()
{
    int silly_result = 2;
    _beginthread( silly, 0, (void*)&silly_result );
} /* 'silly_result' is out of scope but still used by the thread. */

This can be solved by dynamically allocating memory for the variable (and decide whether the main thread or the new thread is reponsible for destroying the allocated memory).

Answer 2

You cannot make the thread itself return something. Instead you can use a struct in your starting call.

_beginthread( silly, 0, (void*)1 );

If you change that to

typedef struct dataStruct {
    int ID;
    int returnValue;
};

dataStruct thread_1;
thread_1.ID = 1;
thread_1.returnValue = 0;
_beginthread( silly, 0, (void*)&thread_1 );

Inside your thread you then set the returnValue as needed, and can go on from there