Multi-Threading Issue in C

Question

I'm trying out Multi-threading in C for the first time, and I seem to be doing something wrong which I hope you could help me with. Here is my code:

#include "stdafx.h"

#define MAX_THREADS 2


int a[100000];
int b[200000];

void startThreads();

DWORD WINAPI populateArrayA(LPVOID data)
{
    int i;
    int* pA = (int*)data;

    for(i = 0; i < sizeof(a) / sizeof(int); i++)
    {
        *pA = i;
        pA++;
    }

    return 0;
}
DWORD WINAPI populateArrayB(LPVOID data)
{
    int i;
    int* pB = (int*)data;

    for(i = 0; i < sizeof(b) / sizeof(int); i++)
    {
        *pB = i;
        pB++;
    }
    return 0;
}

void startThreads()
{
    HANDLE threads[MAX_THREADS];
    DWORD  threadIDs[MAX_THREADS];

    threads[0] = CreateThread(NULL,0,populateArrayA,a,0,&threadIDs[0]);
    threads[1] = CreateThread(NULL,0,populateArrayB,b,0,&threadIDs[1]);

    if(threads[0] && threads[1])
    {
        printf("Threads Created. (IDs %d and %d)",threadIDs[0],threadIDs[1]);
    }

    WaitForMultipleObjects(MAX_THREADS,threads,true,0);

}

int main(int argc, char* argv[])
{
    int i;

    memset(a,0,sizeof(a));
    memset(b,0,sizeof(b));

    startThreads();

    return 0;
}

In this code, Array "b" seems to populate fine, however Array "a" does not. Sorry if the answer is something stupid!

EDIT: I just tried again, and both arrays are all '0's. Not quite sure what's going on. I'm using Visual Studio in case it's a debugging issue or something.

Cheers.

Weird. I just ran this exact code through Visual Studio 2010 and they both looked fine. — Philip
– Philip, Commented Jan 31, 2014 at 16:09
I just tried again, and both arrays are all '0's. Not quite sure what's going on. — Tmarsh2
– Tmarsh2, Commented Jan 31, 2014 at 16:09

manuell · Accepted Answer · 2014-01-31 16:11:22Z

4

The last parameter of WaitForMultipleObjects must be INFINITE, not 0. With 0, the function returns immediatly.

answered Jan 31, 2014 at 16:11

manuell

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1 Comment

Philip Over a year ago

strange that it still runs fine on my system here though. Maybe my processor is just faster and is able to finish the threads.

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