Why does thread synchronization not work?

Question

I wrote a multi-threaded program in which three threads try to save text in the same file. I applied a critical section. And under windows 7 it works fine, but in CE 6.0 it does not synchronize, i.e. Each thread tries to simultaneously save:

Now it works !!! Thank you all for your help!

Critical Section:

InitializeCriticalSection(&CriticalSection); // Create worker threads for( i=0; i < THREADCOUNT; i++ ) { aThread[i] = CreateThread( NULL, 0, (LPTHREAD_START_ROUTINE) WriteToFile, NULL, 0, &ThreadID); if( aThread[i] == NULL ) { printf("CreateThread error: %d\n", GetLastError()); return 1; } } // Wait for all threads to terminate for( i=0; i < THREADCOUNT; i++ ) { WaitResult = WaitForSingleObject(aThread[i], INFINITE); switch(WaitResult) { case WAIT_OBJECT_0: printf("Thread %d has terminated...\n", i); break; // Time out case WAIT_TIMEOUT: printf("The waiting is timed out...\n"); break; // Return value is invalid. default: printf("Waiting failed, error %d...\n", GetLastError()); ExitProcess(0); } } // Close thread handles for( i=0; i < THREADCOUNT; i++ ) CloseHandle(aThread[i]); // Release resources used by the critical section object. DeleteCriticalSection(&CriticalSection); 

The function called by the thread:

 DWORD WINAPI WriteToFile( LPVOID lpParam ) { // lpParam not used in this example UNREFERENCED_PARAMETER(lpParam); DWORD dwCount=1, dwWaitResult; HANDLE hFile; char DataBuffer[30]; DWORD dwBytesToWrite; DWORD dwBytesWritten; // Request ownership of the critical section. EnterCriticalSection(&CriticalSection); // Write to the file printf("Thread %d writing to file...\n", GetCurrentThreadId()); hFile = CreateFile(TEXT("file.txt"), GENERIC_WRITE, 0, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL); SetFilePointer(hFile, 0, NULL, FILE_END); while( dwCount <= 3 ) { sprintf(DataBuffer, "Theard %d writing %d\n", GetCurrentThreadId(), dwCount); dwBytesToWrite = (DWORD)strlen(DataBuffer); WriteFile( hFile, DataBuffer, dwBytesToWrite, &dwBytesWritten, NULL); printf("Theard %d wrote %d successfully.\n", GetCurrentThreadId(), dwCount); } } dwCount++; } CloseHandle(hFile); // Release ownership of the critical section. LeaveCriticalSection(&CriticalSection); return TRUE; } 

Answer 1

The problem is that you pass TRUE to the fWaitAll flag for WaitForMultipleObjects . This is not supported on Windows CE: MSDN documentation says this flag should be FALSE . WaitForMultipleObjects thus does not wait, but instead returns an error, but you do not check the return code. The main thread, thus, runs straight through, closes the descriptors, and deletes the critical section while the worker threads are still running. Once DeleteCriticalSection been called, the critical section “can no longer be used for synchronization,” so the EnterCriticalSection calls EnterCriticalSection probably no longer blocked, and you end up with the script that you have here.

On Windows 7, everything works because the WaitForMultipleObjects call really waits for all threads to complete.

Instead of using WaitForMultipleObjects simply use WaitForSingleObject in a loop to wait for each thread in turn.