destruction of a shared semaphore

gcote · 02-06-2007, 10:55 AM

Hello!

I'm a newbie at programming in Linux.

I'm trying to create a semaphore shared amongst multiple processes for testing purposes. I want to see if I can create something similar to Named Mutexes and Named Events that are available in Windows.

I created a small class CNamedSemaphore. The class exposes 2 methods, Lock and Unlock. Lock decreases the semaphore count and Unlock increases it. If 2 processes try to Lock the same shared semaphore, the second one has to wait until the first one unlocks it and so on. Everything works well. If I use IPCS, I can see my shared semaphore.

The problem I'm facing is when the destructor of my class is called, it calls
semctl( m_shr_sem, 0, IPC_RMID );
The call destroys it completely. After running IPCS, I don't see it anymore even if I still have multiple processes running with a reference to that semaphore.
I assumed that there was some kind of reference count on it and only the last call would really destroy it but it is not the case. I looked inside the structures but I didn't see anything.

I can easily create shared memory to store my own count but it is an overhead I'd like to avoid if possible. Am I missing something? Is there a better way to do this.

Here is the code for my class:

Code:

class CNamedSemaphore
{
public:
	CNamedSemaphore(long in_lValue, int *out_piReturn);

	~CNamedSemaphore();
	
	int CreateNamedSemaphore(long in_lValue);
	int	Lock();
	int	TryLock();	
	int	Unlock();
private:
	int	       		m_shr_sem;
	key_t 	       	m_semKey;
	struct sembuf   m_semBuf;
	int				m_flag;
};

#include "cnamedsemaphore.h"

 #include <sys/mman.h>

union semun {
	int val;		/* value for SETVAL */
	struct semid_ds *buf;	/* buffer for IPC_STAT & IPC_SET */
	unsigned short *array;	/* array for GETALL & SETALL */
	struct seminfo *__buf;	/* buffer for IPC_INFO */
	void *__pad;
};

CNamedSemaphore::CNamedSemaphore(long in_lValue, int *out_piReturn)
: 
m_shr_sem(-1),
m_semKey(0)
{
	*out_piReturn = CreateNamedSemaphore(in_lValue);
}

CNamedSemaphore::~CNamedSemaphore()
{
	if (m_shr_sem < 0)
	{
		return;
	}
	semctl( m_shr_sem, 0, IPC_RMID );
}
	
int CNamedSemaphore::CreateNamedSemaphore(long in_lValue)
{
	if (m_shr_sem != -1)
		return -1;
	
	m_semKey = (key_t) in_lValue;
	
	m_flag = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP;
	
	m_shr_sem  = (int) semget( m_semKey, 1, m_flag );
	
	if (m_shr_sem < 0)
	{
		m_flag |= IPC_CREAT;
		
		m_shr_sem  = (int) semget( m_semKey, 1, m_flag );
		
		if (m_shr_sem < 0)
		{
			return -1;
		}
	}
	
	if (m_flag & IPC_CREAT)
	{
		union semun      arg;
	
		arg.val = 1;
		if (semctl(m_shr_sem, 0, SETVAL, arg) == -1)
		{
			return -1;
		}
	}

	return 0;
}


int	CNamedSemaphore::Lock()
{
	if (m_shr_sem < 0)
	{
		return -1;
	}

	m_semBuf.sem_num = 0;
	m_semBuf.sem_op = -1;
	m_semBuf.sem_flg = SEM_UNDO;
	if (semop(m_shr_sem, &m_semBuf, 1) != 0)
		return -1;

	return 0;
}

int	CNamedSemaphore::TryLock()
{
	if (m_shr_sem < 0)
	{
		return -1;
	}

	if (m_shr_sem < 0)
	{
		return -1;
	}

	int	iReturn = 0;
	m_semBuf.sem_num = 0;
	m_semBuf.sem_op = -1;
	m_semBuf.sem_flg = SEM_UNDO | IPC_NOWAIT;
	errno = 0;
	iReturn = semop(m_shr_sem, &m_semBuf, 1);
	if (iReturn == -1)
	{
		if (errno == EAGAIN)
		{
			return -1;
		}
	}
	return 0;
}

int	CNamedSemaphore::Unlock()
{
	if (m_shr_sem < 0)
	{
		return -1;
	}

	m_semBuf.sem_num = 0;
	m_semBuf.sem_op  = 1;
	m_semBuf.sem_flg = SEM_UNDO;
	
	if (semop(m_shr_sem, &m_semBuf, 1) != 0)
		return -1;

	return 0;
}

In the app, I would use

Code:

    int iReturn;
    pNamedSemaphore = (CNamedSemaphore *) new CNamedSemaphore(1000, &iReturn);

    // Other parts of the code
    /* Lock button was pressed */
    pNamedSemaphore->Lock();

    // Other parts of the code
    /*Unlock button was pressed */
    pNamedSemaphore->Unlock();

    // Other parts of the code
    /* Close button was pressed */
    delete pNamedSemaphore;

Thank you!

paulsm4 · 02-06-2007, 11:46 PM

Hi -

In Windows, named semaphores (and synchronization objects in general) are reference-counted: the object remains until the last person using it cleans up.

SysV IPCs, however, are very different. If *anybody* deletes the object (given, of course, the privilege to delete it) - boom! It's gone.

You need to keep your own reference count on the semaphore. Frankly, I find it useful to create a second, "counting semaphore" to keep the reference count on my "working semaphore". Your suggestion about a shared global is another solution (although you're probably going to need to create a new semaphore anyway, just to protect the global variable).

Just a thought .. PSM

gcote · 02-07-2007, 12:13 PM

Thank you!

This is a great idea!

I modified my class to create a semaphore set of 2 semaphores instead of only 1
semget(m_semKey, 2, flag);

It seems to be working... Cool!

Here's my modified class

Code:

#include "cnamedsemaphore.h"

 #include <sys/mman.h>

union semun {
	int val;			/* value for SETVAL */
	struct semid_ds *buf;	/* buffer for IPC_STAT & IPC_SET */
	unsigned short *array;	/* array for GETALL & SETALL */
	struct seminfo *__buf;	/* buffer for IPC_INFO */
	void *__pad;
};

CNamedSemaphore::CNamedSemaphore(long in_lValue, int *out_piReturn)
: 
m_shr_sem(-1),
m_semKey(0)
{
	*out_piReturn = CreateNamedSemaphore(in_lValue);
}

CNamedSemaphore::~CNamedSemaphore()
{
	if (m_shr_sem < 0)
	{
		return;
	}
	union semun      arg;
	Lock();
	arg.val = semctl(m_shr_sem, 1, GETVAL);
	//check for > 0???
	assert(arg.val);
	// cout doesn't work well with threads
	fprintf(stdout, "CNamedSemaphore::~CNamedSemaphore count=%d\n", arg.val);
	arg.val--;
	if (semctl(m_shr_sem, 1, SETVAL, arg) == -1)
	{
		arg.val = 0;
	}
	Unlock();
	fprintf(stdout, "CNamedSemaphore::~CNamedSemaphore count=%d\n", arg.val);
	if (arg.val <= 0)
	{
		semctl( m_shr_sem, 0, IPC_RMID );
	}
}
	
int CNamedSemaphore::CreateNamedSemaphore(long in_lValue)
{
	if (m_shr_sem != -1)
		return -1;
	
	int				flag;
	m_semKey = (key_t) in_lValue;
	
	flag = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP;
	
	m_shr_sem  = (int) semget( m_semKey, 2, flag );
	
	if (m_shr_sem < 0)
	{
		flag |= IPC_CREAT;
		
		m_shr_sem  = (int) semget( m_semKey, 2, flag );
		
		if (m_shr_sem < 0)
		{
			return -1;
		}
	}
	
	union semun      arg;
	if (flag & IPC_CREAT)
	{
		// could use SETALL?
		arg.val = 1;
		semctl(m_shr_sem, 1, SETVAL, arg);
		semctl(m_shr_sem, 0, SETVAL, arg);
		fprintf(stdout, "CNamedSemaphore::CreateNamedSemaphore IPC_CREAT count=%d\n", arg.val);
		// for debugging purposes, check value
		arg.val = semctl(m_shr_sem, 1, GETVAL);
		fprintf(stdout, "CNamedSemaphore::CreateNamedSemaphore IPC_CREAT count=%d\n", arg.val);
	}
	else
	{
		Lock();
		arg.val = semctl(m_shr_sem, 1, GETVAL);
		arg.val++;
		if (semctl(m_shr_sem, 1, SETVAL, arg) == -1)
		{
			Unlock();
			return -1;
		}
		Unlock();
		// for debugging purposes, check value
		fprintf(stdout, "CNamedSemaphore::CreateNamedSemaphore count=%d\n", arg.val);
	}
	// for debugging purposes, check value
	arg.val = semctl(m_shr_sem, 1, GETVAL);
	fprintf(stdout, "CNamedSemaphore::CreateNamedSemaphore GETVAL count=%d\n", arg.val);

	return 0;
}


int	CNamedSemaphore::Lock()
{
	if (m_shr_sem < 0)
	{
		return -1;
	}

	struct sembuf   semBuf;
	semBuf.sem_num = 0;
	semBuf.sem_op = -1;
	semBuf.sem_flg = SEM_UNDO;
	if (semop(m_shr_sem, &semBuf, 1) != 0)
		return -1;

	return 0;
}

int	CNamedSemaphore::TryLock()
{
	if (m_shr_sem < 0)
	{
		return -1;
	}

	if (m_shr_sem < 0)
	{
		return -1;
	}

	struct sembuf   semBuf;
	semBuf.sem_num = 0;
	semBuf.sem_op = -1;
	semBuf.sem_flg = SEM_UNDO | IPC_NOWAIT;
	errno = 0;
	if (semop(m_shr_sem, &semBuf, 1) == -1)
	{
		if (errno == EAGAIN)
		{
			return -1;
		}
		// ??? Check errno!!!
		assert(false);
	}
	return 0;
}

int	CNamedSemaphore::Unlock()
{
	if (m_shr_sem < 0)
	{
		return -1;
	}

	struct sembuf   semBuf;
	semBuf.sem_num = 0;
	semBuf.sem_op  = 1;
	semBuf.sem_flg = SEM_UNDO;
	
	if (semop(m_shr_sem, &semBuf, 1) != 0)
		return -1;

	return 0;
}

One thing about the shared memory that I didn't like is the fact that the minimum size is 4096 (getpagesize()). If at one point, I need to create many shared semaphores, one 4k block of shared memory per semaphore adds up quickly. I would have to implement my own shared memory manager.

Thank you very much for the help!

G.