Unhandled exception / Access violation writing location in a Mutex example

Question

I'm working through an example of protecting a global double using mutexes, however I get the error -

Unhandled exception at 0x77b6308e in Lab7.exe: 0xC0000005: Access violation writing location 0x00000068.

I assume this is related to accessing score? (The global double)

#include <windows.h>
#include <iostream>   
#include <process.h>

double score = 0.0; 


HANDLE threads[10];     

CRITICAL_SECTION score_mutex; 


unsigned int __stdcall MyThread(void *data)
{
    EnterCriticalSection(&score_mutex);
    score = score + 1.0; 
    LeaveCriticalSection(&score_mutex); 

    return 0;
}

int main()
{
    InitializeCriticalSection(&score_mutex); 

    for (int loop = 0; loop < 10; loop++)
    {

        threads[loop] = (HANDLE) _beginthreadex(NULL, 0, MyThread, NULL, 0, NULL); 
    }

    WaitForMultipleObjects(10, threads, 0, INFINITE); 

    DeleteCriticalSection(&score_mutex); 

    std::cout << score; 

    while(true);

}

Update:

After fixing the problem with the loop being set to 1000 instead of 10, the error still occured, however when I commented out the pieces of code referring to the mutex the error did not occur.

CRITICAL_SECTION score_mutex; 
EnterCriticalSection(&score_mutex); 
LeaveCriticalSection(&score_mutex); 
InitializeCriticalSection(&score_mutex); 
DeleteCriticalSection(&score_mutex); 

Update 2

The threads return 0 as per convention (It's been a long week!)

I tried adding back in the mutex-related code, and the program will compile and run fine (other than the race condition issues with the double of course) with CRITICAL_SECTION, InitializeCriticalSection and DeleteCriticalSection all added back in. The problem appears to be with EnterCriticalSection or LeaveCriticalSection, as the error reoccurs when I add them.

Answer 1

The remaining bug in your code is in the call to WaitForMultipleObjects(). You set the 3rd parameter to 0 (FALSE) such that the main thread unblocks as soon as any of the 10 threads finishes.

This causes the call to DeleteCriticalSection() to execute before all threads are finished, creating an access violation when one of the (possibly) 9 other threads starts and calls EnterCriticalSection().

Answer 2

You're writing beyond the end of your threads[10] array:

for (int loop = 0; loop < 1000; loop++){
     threads[loop];
}

threads only has size 10!

Answer 3

Your problem is that WaitForMultipleObjects is not waiting for all the threads to complete, causing the critical section to be prematurely deleted. According to MSDN, the third argument is

bWaitAll [in]

If this parameter is TRUE, the function returns when the state of all objects in the >lpHandles array is signaled. If FALSE, the function returns when the state of any one of >the objects is set to signaled. In the latter case, the return value indicates the object >whose state caused the function to return.

You set this to 0, which returns when ANY ONE of your threads completes. This causes the following DeleteCriticalSection to be run while there's still threads waiting to access it.

Answer 4

You should also declare score as a volatile so you don't have cached value problem.