C++ Semaphore, limit 2 instances at the same time

Hello
I'm trying to make a loop create 10 threads but only 2 working threads at the same time, wait until one finish the job then start another thread but never exceed 2 at same time. I'm doing this with semaphores.

Code:

---

#include <iostream>
#include <Windows.h>

typedef struct structure1 { int threadnr; } structure1;

HANDLE ghSemaphore = CreateSemaphore(NULL, 2, 2, NULL);

DWORD WINAPI ThreadProc(LPVOID param)
{
        structure1 * t1 = (structure1 *)param;
        printf("Thread %i\n",t1->threadnr);
        Sleep(1000);
        return 0;
}

int main()
{
        for (int i = 0; i < 10; i++)
        {
                WaitForSingleObject(ghSemaphore, INFINITE);
                structure1 * t1 = new structure1;
                t1->threadnr = i;
                CreateThread(NULL, 0, ThreadProc, t1, NULL, NULL);
                ReleaseSemaphore(ghSemaphore, 1, 0);
        }
        system("PAUSE");
        return 0;
}

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I set the semaphore max value at 2 but it start all threads at same time instead of only 2.
I tried to set WaitForSingleObject at the beginning of ThreadProc and ReleaseSemaphore at the end and it works but it will create all threads in buffer. I want to create just 2 at same time and wait for one to finish then start another one
Any idea ?
Thank you

One way to do this could be

Code:

---

#include <stdio.h>
#include <Windows.h>

const int MAXTHREAD = 2;
const int THREADS = 10;

struct structure1 {
        structure1(int th = 0) : threadnr(th) {}
        int threadnr;
};

DWORD WINAPI ThreadProc(LPVOID param)
{
structure1 *t1 = (structure1 *)param;

        printf("Thread %i started\n", t1->threadnr);
        Sleep(3000);
        delete t1;
        return 0;
}

HANDLE CreateThread(int th)
{
structure1 * t1 = new structure1(th);

        return CreateThread(NULL, 0, ThreadProc, t1, NULL, NULL);
}

int main()
{
HANDLE hlds[MAXTHREAD] = {0};

        for (int i = 0; i < THREADS; ++i) {
                bool create = false;

                for (int t = 0; (t < MAXTHREAD) && (create == false); ++t)
                        if (hlds[t] == 0) {
                                hlds[t] = CreateThread(i);
                                create = true;
                        }

                if (create == false) {
                        DWORD result = WaitForMultipleObjects(MAXTHREAD, hlds, FALSE, INFINITE);
                        hlds[result - WAIT_OBJECT_0] = CreateThread(i);
                }
        }

        WaitForMultipleObjects(MAXTHREAD, hlds, FALSE, INFINITE);
        return 0;
}

---

PS I would consider using beginthreadex() rather than CreateThread(). See http://stackoverflow.com/questions/3...createthread-c

2kaud - thank you for the code. it works exactly as i wanted :thumb:
I have only one confusion: i read that waitformultipleobject() can wait for maximum 64 objects so if i have 100 maxthreads and 500 threads it won't work ?

Quote:

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Originally Posted by eclessiastes

... i read that waitformultipleobject() can wait for maximum 64 objects so if i have 100 maxthreads and 500 threads it won't work ?

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Just read the WaitForMultipleObjects function article, section Remarks

Quote:

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Originally Posted by eclessiastes

I have only one confusion: i read that waitformultipleobject() can wait for maximum 64 objects so if i have 100 maxthreads and 500 threads it won't work ?

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C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Include\WinNT.h

Code:

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#define MAXIMUM_WAIT_OBJECTS 64    // Maximum number of wait objects

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WaitForMultipleObjects function

Quote:

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nCount [in]

The number of object handles in the array pointed to by lpHandles. The maximum number of object handles is MAXIMUM_WAIT_OBJECTS. This parameter cannot be zero.

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Quote:

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Originally Posted by eclessiastes

Hello
I'm trying to make a loop create 10 threads but only 2 working threads at the same time, wait until one finish the job then start another thread but never exceed 2 at same time. I'm doing this with semaphores.

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Frankly, I can see no point in having number of threads more than the number allowed to execute simultaneously.

For some reason when i have
[CODE]for (size_t i=0; i<10; ++i) [CODE]
it doesn't show 1,2,3,4 etc but it appears random like 4,2,1,3,7,6,8,9
is there any way to make it loop line by line instead of random ?

Rather than rolling your own thread pool, check out QueueUserWorkItem api in msdn.