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Semafore in Windows problem

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Join Date: Apr 2008

Posts: 47

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Cybulski

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Semafore in Windows problem

May 31st, 2008

I'm trying to implement critical section using objects listed in this article: http://www.mycplus.com/tutorial.asp?TID=290

Somehow, it doesn't work like intended, or maybe I do something wrong. Basically my concept looks like this:

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DirectoryReader::DirectoryReader(char* p_sConfigPath)
{
GetConfig(p_sConfigPath);
   if( hReaderMutex = CreateMutex( NULL, FALSE, "ReaderMutex") )
   cout << "Mutex created!" << endl;
}
 
DirectoryReader::MakeList(string sPath)
{
   While( reading filenames in specified directory )
      {
      WaitForSingleObject( hReaderMutex, INFINITE );
      m_vFilenames.push_back( file name );
      ReleaseMutex( hReaderMutex );
      }
}
DirectoryReader::DirectoryReader(char* p_sConfigPath)
{
GetConfig(p_sConfigPath);
   if( hReaderMutex = CreateMutex( NULL, FALSE, "ReaderMutex") )
   cout << "Mutex created!" << endl;
}

DirectoryReader::MakeList(string sPath)
{
   While( reading filenames in specified directory )
      {
      WaitForSingleObject( hReaderMutex, INFINITE );
      m_vFilenames.push_back( file name );
      ReleaseMutex( hReaderMutex );
      }
}

For some reason blocking doesn't work, for example if I call WaitForSingleObject(); right after cerating mutex (it should put semaphore down), I can still access it later in MakeList method. Do I need pass some options in creating mutex? I don't really know how there security attributes work.
Any ideas?

[edit] I want call MakeList() in multiple threads later, I expect problems too. I also don't want use MFC, since I already started project in WIN32, adding MFC class is impossible it seems.

Last edited by Cybulski; May 31st, 2008 at 8:56 am.

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Ancient Dragon Ancient Dragon is offline

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Re: Semafore in Windows problem

May 31st, 2008

Mutexs work with multiple threads or multiple processes. What you describe is not a problem if there are no other threads or processes that want access to the mutex at the same time. Add a Sleep() before the WaitForSingleObject() to allow time for other threads to gain access to the mutex and see if the wait function blocks until the other thread(s) have released the mutex.

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Join Date: Apr 2008

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Cybulski

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Re: Semafore in Windows problem

Jun 1st, 2008

I'm pretty sure semaphore is not working correctly anyway. There should be no difference between single thread and more threads acquiring semaphore. For example, if I call WaitForSingleObject() twice in row, thread should be locked, but it isn't. Semaphore represents resource.
I need to read more about parameters of these functions when I get back home, but MSDN is pretty hard lecture for me. I must say POSIX is far easier platform for multitasking programming.

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Ancient Dragon Ancient Dragon is offline

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Re: Semafore in Windows problem

Jun 1st, 2008

>>There should be no difference between single thread and more threads acquiring semaphore
Maybe you don't think so, but there is a big difference.

Its working correctly -- Read the MSDN description for that function and you will see that it doesn't block after the first call when called from within the same thread.

•

Originally Posted by MSDN

The thread that owns a mutex can specify the same mutex in repeated wait function calls without blocking its execution

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Join Date: Apr 2008

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Cybulski

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Re: Semafore in Windows problem

Jun 5th, 2008

I am still stuck with this... It seems bit complicated to fill security attributes and security descriptor structures. Anyone have experience with this? Option is to use MFC, but I'm not familiar with GUI controls. Anyway, is it possible to add MFC class to WIN32 project? Seems not.

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mitrmkar

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Re: Semafore in Windows problem

Jun 5th, 2008

Most probably the problems are not about security attributes but about you not using threads. Ancient Dragon made a good point above.
Think what would happen if the wait call would block when you are doing it from the same thread that created the mutex in the first place (and you are not having any other threads operating on the mutex).

I think you can forget the security attributes for now and try grasping the basic understanding how the mutexes work. You can use the mutexes also with NULL security attributes, so you don't have to wrestle with those attributes.

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Cybulski

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Re: Semafore in Windows problem

Jun 5th, 2008

Yeah sorry I missed point. I'm still thinking about semaptores in POSIX manner - acquired means it's acquired regardless of process accessing it.

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Join Date: Apr 2008

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Cybulski

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Re: Semafore in Windows problem

Jun 12th, 2008

Ok, I got working threads, but still can't make proper synchronization. It's hard to find good example.

So, there is header file:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
class MultiReader :
	public DirectoryReader
{
public:
	MultiReader(char* p_sConfigPath);
	~MultiReader(void);
 
	static const HANDLE hReaderMutex;
	void CreateThreads(void);
	static DWORD WINAPI ReaderThread(PVOID p_pVoid);
};
class MultiReader :
	public DirectoryReader
{
public:
	MultiReader(char* p_sConfigPath);
	~MultiReader(void);

	static const HANDLE hReaderMutex;
	void CreateThreads(void);
	static DWORD WINAPI ReaderThread(PVOID p_pVoid);
};

And that's how I'm trying synchronize threads:

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MultiReader::MultiReader(char* p_sConfigPath):DirectoryReader(p_sConfigPath)
{
	hReaderMutex = CreateMutex( NULL, TRUE, "ReaderMutex");
}
 
MultiReader::~MultiReader(void)
{
}
 
void MultiReader::CreateThreads()
{
	unsigned int nThreadNo;
 
	for( nThreadNo = 0; nThreadNo < vDirectories.size(); nThreadNo++)
	{
		CreateThread( NULL, 0, MultiReader::ReaderThread, (void*)vDirectories.at(nThreadNo).sDirectoryName.c_str(), 0, NULL);
	}
}
 
DWORD WINAPI MultiReader::ReaderThread(LPVOID p_pVoid)
{
	WaitForSingleObject( hReaderMutex, 0);
	cout << "Thread: " << (char*)p_pVoid << endl;
	ReleaseMutex( hReaderMutex );
	return NULL;
}
MultiReader::MultiReader(char* p_sConfigPath):DirectoryReader(p_sConfigPath)
{
	hReaderMutex = CreateMutex( NULL, TRUE, "ReaderMutex");
}

MultiReader::~MultiReader(void)
{
}

void MultiReader::CreateThreads()
{
	unsigned int nThreadNo;

	for( nThreadNo = 0; nThreadNo < vDirectories.size(); nThreadNo++)
	{
		CreateThread( NULL, 0, MultiReader::ReaderThread, (void*)vDirectories.at(nThreadNo).sDirectoryName.c_str(), 0, NULL);
	}
}

DWORD WINAPI MultiReader::ReaderThread(LPVOID p_pVoid)
{
	WaitForSingleObject( hReaderMutex, 0);
	cout << "Thread: " << (char*)p_pVoid << endl;
	ReleaseMutex( hReaderMutex );
	return NULL;
}

I get this:

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error LNK2001: unresolved external symbol "public: static void * MultiReader::hReaderMutex" (?hReaderMutex@MultiReader@@2PAXA)
C:\Documents and Settings\hp\My Documents\Visual Studio 2008\Projects\FileFactory\Debug\FileFactory.exe : fatal error LNK1120: 1 unresolved externals
error LNK2001: unresolved external symbol "public: static void * MultiReader::hReaderMutex" (?hReaderMutex@MultiReader@@2PAXA)
C:\Documents and Settings\hp\My Documents\Visual Studio 2008\Projects\FileFactory\Debug\FileFactory.exe : fatal error LNK1120: 1 unresolved externals

It is after taking compiler advice to make hReaderMutex handle static. I don't want it static too! Class will be part of singleton.

Last edited by Cybulski; Jun 12th, 2008 at 10:20 am.

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Join Date: Nov 2007

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mitrmkar

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Re: Semafore in Windows problem

Jun 12th, 2008

hReaderMutex is not defined anywhere, hence the error. To get over this problem you need something like ...

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// in MultiReader.cpp ...
const HANDLE MultiReader::hReaderMutex = CreateMutex( NULL, TRUE, "ReaderMutex");
// in MultiReader.cpp ...
const HANDLE MultiReader::hReaderMutex = CreateMutex( NULL, TRUE, "ReaderMutex");

However, this is a bit bad construct, because when the CreateMutex() fails, you will not know the reason for the failure (GetLastError()).

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Join Date: Apr 2008

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Cybulski

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Re: Semafore in Windows problem

#10

Jun 12th, 2008

Ok I figured out that static const must be initialize within class body, but still:

 Help with Code Tags 
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class MultiReader :
	public DirectoryReader
{
public:
	MultiReader(char* p_sConfigPath);
	~MultiReader(void);
 
	static const HANDLE hReaderMutex = CreateMutex( NULL, TRUE, "ReaderMutex");
};
class MultiReader :
	public DirectoryReader
{
public:
	MultiReader(char* p_sConfigPath);
	~MultiReader(void);

	static const HANDLE hReaderMutex = CreateMutex( NULL, TRUE, "ReaderMutex");
};