Thread Synchronization C++

I recently met a developer who was complaining about the lack of  material and articles available online regarding thread synchronization . According to him either the articles were too detailed or simply too vague.

I therefore decided to give my two cents on the topic , since I seem to have some extra time on my hands these days. This topic could be very advanced and I would more than appreciate and welcome other developers to comment on this topic and if you believe I am going wrong somewhere putting me write would be worth the 25 minutes it took me to write about this topic.

Threads are synchronized using the following methods:

Events,Mutex,Semaphores and Critical Sections

The following diagram shows what each method does and how its implemented

 

Example of Using a Triggering Event:

HANDLE HEvent;

void MyFunction(void *p)

{

Start:

WaitForSingleObject ( HEvent , INFINITE );      

cout << "\n Event Triggered" ;     

goto Start;

cout << "This Line would never be called \n";

return;

}

void main()

{

cout << "Main Thread Started";

HEvent = CreateEvent(NULL,false,false,"NAME"); 

_beginthread(MyFunction,0,0);

Sleep(3000);

SetEvent(HEvent); //Make Event Block

cin.get();

}       

Example of Using a Mutex:

HANDLE hMutex; 

void threadA(void* dummy)    //Thread A Procedure

{

WaitForSingleObject(hMutex, INFINITE);  //Check if handle is in use  released ?

for(int i = 0; i < 10; i++)              

cout << "testA" << endl;  //will block other wise it will continue

ReleaseMutex(hMutex);

}

void threadB(void* dummy)

{

WaitForSingleObject(hMutex, INFINITE);

for(int i = 0; i < 10; i++)

cout << "testB" << endl;

ReleaseMutex(hMutex);

}

void main()

{

hMutex = CreateMutex(NULL, false, "fileMutex");

_beginthread(threadA,0,NULL);

_beginthread(threadB,0,NULL);

}

Example of Using a Semaphore. 

I picked and modified this example from an online source , I cant seem to find the link to that site (Apologies to the original author) but I found a trace of that code in one of my projects.You don't really find good examples of semaphores online.

 

#define MAX_SEM_COUNT 10

#define THREADCOUNT 12 //No of threads to be created

HANDLE ghSemaphore;

DWORD WINAPI ThreadProc( LPVOID );

void main()

{

  HANDLE aThread[THREADCOUNT];

  DWORD ThreadID;

  int i;

  // Create a semaphore with initial and max counts of MAX_SEM_COUNT

  ghSemaphore = CreateSemaphore(NULL,MAX_SEM_COUNT,MAX_SEM_COUNT,NULL);     

  // Create worker threads

  for( i=0; i < THREADCOUNT; i++ )

  {

    aThread[i] = CreateThread(NULL,0,ThreadProc,NULL,0,&ThreadID);

  }

  // Wait for all threads to terminate

  WaitForMultipleObjects(THREADCOUNT, aThread, TRUE, INFINITE);

  // Close thread and semaphore handles

  for( i=0; i < THREADCOUNT; i++ )

    CloseHandle(aThread[i]);

    CloseHandle(ghSemaphore);

}

DWORD WINAPI ThreadProc( LPVOID lpParam )

{

  DWORD dwWaitResult;

  BOOL bContinue=TRUE;

  while(bContinue)

  {

    // Try to enter the semaphore gate.

    dwWaitResult = WaitForSingleObject(ghSemaphore,INFINITE); // zero-second time-out interval

    switch (dwWaitResult)

    {

      // The semaphore object was signaled.

      case WAIT_OBJECT_0:

        // TODO: Perform task

        printf("Thread %d: wait succeeded\n", GetCurrentThreadId());

        bContinue=FALSE;

        // Simulate thread spending time on task

        Sleep(5);

        // Release the semaphore when task is finished

        if (!ReleaseSemaphore(ghSemaphore,1,NULL) )

        {

          printf("ReleaseSemaphore error: %d\n", GetLastError());

        }

        break;

      // The semaphore was nonsignaled, so a time-out occurred.

      case WAIT_TIMEOUT:

        printf("Thread %d: wait timed out\n", GetCurrentThreadId());

        break;

    }

  }

  return TRUE;