Interprocess nonblocking semaphore / mutex / shared memory.

[m.p]

Hello!

I need lock some resources (e.g. files) used by my programs. I've got few process and I want synchronize beetwen them. QSystemSemaphore or QSharedMemory look nice, but unfortunetly those classes don't have non-blocking function to lock (like QMutex's tryLock() ). Could anyone pass me a nice multiplatform solution for that?

best regards
m.p

[high_flyer]

QSharedMemory look nice, but unfortunetly those classes don't have non-blocking function to lock

QSharedMemory::lock()

[m.p]

QSharedMemory::lock()

No, no, no. QSharedMemory::lock() is locking function, from doc:

"This is a semaphore that locks the shared memory segment for access by this process and returns true. If another process has locked the segment, this function blocks until the lock is released."

I need something non blocking.

[high_flyer]

Sorry read to fast.
But you can do it your self.
For example, you can override QSharedMemory so:

// in the header:
bool m_bLocked;
 
//and
bool isLocked() {return m_bLocked; }
 
 
//in the implementation:
//initialize in the ctor
 
bool MyQSharedMemory::tryLock()
{
    if(!m_bLocked){
       m_bLocked = QSharedMemory::lock();
       return m_bLocked;
    }
 
     return false;       
}

// in the header:
bool m_bLocked;

//and
bool isLocked() {return m_bLocked; }


//in the implementation:
//initialize in the ctor

bool MyQSharedMemory::tryLock()
{
    if(!m_bLocked){
       m_bLocked = QSharedMemory::lock();
       return m_bLocked;
    }
 
     return false;       
}

To copy to clipboard, switch view to plain text mode 

But I would go through the QSharedMemory doc thoroughly, as my guess is that the trolls didn't build it in in the first place because probably it conflicts with the way QSharedMemory work, and the way it should be used.
Since I didn't work with it yet, I can't say.

Another way is to use to protect the resource with a mutex, and only do something with it if you managed to lock the mutex.

if(mutex.tryLock()){
   //Do something with resource
}

if(mutex.tryLock()){
   //Do something with resource
}

To copy to clipboard, switch view to plain text mode 

[m.p]

Yours solutions are fine, but not for me. They wont work for more than one process.

[high_flyer]

What do you mean "more than one process"?
Can you explain a bit more?

[m.p]

What do you mean "more than one process"?
Can you explain a bit more?

Yes. I have two or more separate application wrote by me (or more than one instance of one app), and the can use common resources (e.g. files), and I want provide exclusive acces at the time to them. So I must synchronize those processes.

[high_flyer]

One relative easy way of doing this is by creating a temp file, that all sharing applications know about.
If the file exist, the application knows the resource is used by another application.
So you can in your application just use QFileInfo::exists(), to know if other applications are using the resource.
Something like:
1. look if file exist.
2. If exist, continue with something else.
3. if does not exist, create file, and access shared resource.
4. delete file when shared resource when finished dealing with shared resource.

[m.p]

I have been thinking about that solution, but what if one of process that acquired acces to resources and created temp file crashed before it can delete this temp file? Other proces will be thinking, that resources are loocked.

[high_flyer]

You can handle that by catching and handling SIGSEGV signal.
You can search in the forum there are several threads about it, or google it.

[m.p]

SIGSEGV is a POSIX signal. I need multiplatform solution.