locking

According to wikipedia, Shared locks are sometimes called "read locks" and exclusive locks are sometimes called "write locks". Can you explain the reasoning behind the terms "shared" and "exclusive"? I wrote this answer down because I thought this would be a fun (and fitting) analogy: Think of a lockable object as a blackboard (lockable) in a class room containing a teacher (writer) and many students (readers). While a teacher is writing something (exclusive lock) on the board: Nobody can read it, because it's still being written, and she's blocking your view => If an object is exclusively

I have three threads and some part of the code can run in parallel, some parts are locked(only one thread at the time). However one lock needs to only let them in in order. Since this is a loop it gets more complex. How do I make this behavior? If i had a print statement I would like to receive the following output: 1,2,3,1,2,3,1,2,3.... currently I receive 2,3,1,3,1,3,2,1,2 A.K.A. random order. The code which is executed in three threads in parallel: while (true){ lock (fetchLock){ if(done){ break; } //Do stuff one at the time } //Do stuff in parralell lock (displayLock){ //Do stuff one at

I would expect the following test to only print "has been locked" once. BUT it consequently prints the line. public class LocKTest { @Test public void testLock() { Lock lock = new ReentrantLock(); while (true) { if (lock.tryLock()) { //lock.lock(); same result if I include an explicit lock here System.out.println("has been locked"); } } } } As far as I understood, tryLock will lock the ReentrantLock if possible (ie if not locked yet). But obviously this is not the case. How can I set such a lock threadsafe? The name is ReentrantLock , meaning you can re-enter if you already own the lock. If

I have a maintenance script in PHP that updates and repairs the database. In theory, running two scripts simultaneously shouldn't be a problem, but I want to be extra safe by putting a lock in a variable in PHP. The problem, of course, is that I can't store it in a $_SESSION variable because sessions only apply to one user. Is there any way I can store this lock in a variable? I'd prefer to not create and delete a file in case the server dies in the middle of the script. MySQL has GET_LOCK() function. Since you're working with the database, you can lock the tables you need while the script

I have a multi-threaded process where a file is shared (read and written) by multiple threads. Is there any way a thread can lock one file segment so that other threads cannot access it? I have tried fcntl(fd, F_SETLKW, &flock) , but this lock only works for processes, not threads (a lock is shared between all threads in an process). Yes - but not with the same mechanism. You'll have to use something like pthread mutexes, and keep track of the bookkeeping yourself. Possible outline for how to make this work Wait on and claim a process-level mutex over a bookkeeping structure make sure no other

In my application I used to lock application with IMEI code. I mean during startup application checks device IMEI and compares it with list of allowed devices. If device is in list user can continue to work, otherwise it bails out: public boolean checkIMEI(Activity activity) { TelephonyManager tm=(TelephonyManager )activity.getSystemService(Context.TELEPHONY_SERVICE); if(tm==null) { Log.v(TAG, "Can't get telephony service"); new MessageBox(activity, "Can't get telephony service. Forcing shut down!"); return false; } //encrypted IMEIs list String[] vals=activity.getResources().getStringArray(R