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• Reading and writing to SysV shared memory without synchronization (use of semaphores, C/C++, Linux)

  - by user363778

  Hi, I use SysV shared memory to let two processes communicate with each other. I do not want the code to become to complex so I wondered if I really had to use semaphores to synchronize the access to the shared memory. In my C/C++ program the parent process reads from the shared memory and the child process writes to the shared memory. I wrote two test applications to see if I could produce some kind of error like a segmentation fault, but I couldn't (Ubuntu 10.04 64bit). Even two processes writing non stop in a while loop to the same shared memory did not produce any error. I hope someone has experience concerning this matter and can tell me if I really must use semaphores to synchronize the access or if I am OK without synchronization. Thanks

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• Does this simple cache class need thread synchronization?

  - by DayOne

  Does this simple cache class need thread synchronization ... if I remove the lock _syncLock statement will encounter any problems? I think i can remove the locks as the references should be updated correctly right? ... BUt i'm think whar happens if client code is iterating over the GetMyDataStructure method and it get replaced? Thanks! public sealed class Cache { private readonly object _syncLock = new object(); private IDictionary<int, MyDataStructure> _cache; public Cache() { Refresh(); } public void Refresh() { lock (_syncLock) { _cache = DAL.GetMyDataStructure(); } } public IDictionary<int, MyDataStructure> **GetMyDataStructure**() { lock (_syncLock) { return _cache; } } }

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• Synchronization between user space process and interupt context code

  - by user1748950

  Recently I attended couple of interviews. Out of all kernel questions which were asked, thr is one specific question which I couldnt find convincing answer of. How will you use different synchronization techniques while sharing data between user space process and interrupt context function? My convincing answer was: In interrupt context code: 1. do *spin_lock_irqsave* 2. access data buffer which is shared between user space app and kernel 3. do *spin_lock_irqrestore* Then this was not convincing answer. Do I have to do irqsave and irqrestore in all the instances of data access? Regards, Yogi

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• Partial sync of Mysql databases on two remote computers

  - by Beck

  Does mysql replication supports delayed sync if remote slave server is OFF? For example I want to make my developing server have a fresh copy of databases each morning. It should be partial sync, not full copy of databases each morning. And synchronization should be only ONE way, not duplex, to avoid deleting something of master server. What utilities are out there or native functionality of mysql itself? Thanks!

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• What's the advantage of synchronizing UID/GID across Linux machines?

  - by alex

  Before I plunge into the depths of how to synchronize UID's/GID's across my different Linux machines, I would like to know what is actually the benefit? I know that this keeps file synchronization relatively easy (as ownership is "naturally" retained). However this can also be achieved otherwise depending on the transmission service. Is there anything else that would benefit from consistent UIDs/GIDs?

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• Data synchronization using XMPP

  - by Jason

  Hi: I'm looking for some insight/advice on synchronizing data over XMPP. I've never developed anything for XMPP before so excuse me if some of my questions seem ridiculous. Basically, what I have is a decentralized social network. Each person has it's own Web site (or server) with a unique URI (one domain could host many servers). Each of these servers can have many clients. E.g., a desktop application, mobile application, etc. What I would like to accomplish is near real-time synchronization/communication between client and server, e.g., I update something on my desktop application, I see it change on my Web site. My server and client code is Python. So, I would like to make use of SleekXMPP if possible (it's license seems to have changed to MIT). I was thinking, and here is where I need advice, that each server would register an account at a dedicated XMPP server, e.g., [email protected]. and then I could use different resources for clients [email protected]/client1, [email protected]/client2, etc. If anyone can register any username, then maybe I also need some intermediate service (since it's decentralized, i'm not sure how to control registrations). Another option, I guess, is that each server runs it's own xmpp server. Assuming, that was all worked out, if I want to broadcast messages to all my resources (except the sending one), how do I do that? Do I have to subscribe to myself? This also seems like a good candidate for publish-subscribe, let me know if you think that could work and what the design/flow of that process would be. thanks :)

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• Thread synchronization and aborting.

  - by kubal5003

  Hello, I've got a little problem with ending the work of one of my threads. First things first so here's the app "layout": Thread 1 - worker thread (C++/CLI) - runs and terminates as expected for(...) { try { if(TabuStop) return; System::Threading::Monitor::Enter("Lock1"); //some work, unmanaged code } finally { if(stop) { System::Threading::Monitor::Pulse("Lock1"); } else { System::Threading::Monitor::Pulse("Lock1"); System::Threading::Monitor::Wait("Lock1"); } } } Thread 2 - display results thread (C#) while (WorkerThread.IsAlive) { lock ("Lock1") { if (TabuEngine.TabuStop) { Monitor.Pulse("Lock1"); } else { Dispatcher.BeginInvoke(RefreshAction); Monitor.Pulse("Lock1"); Monitor.Wait("Lock1", 5000); } } // Thread.Sleep(5000); } I'm trying to shut the whole thing down from app main thread like this: TabuEngine.TabuStop = true; //terminates nicely the worker thread and if (DisplayThread.IsAlive) { DisplayThread.Abort(); } I also tried using DisplayThread.Interrupt, but it always blocks on Monitor.Wait("Lock1", 5000); and I can't get rid of it. What is wrong here? How am I supposed to perform the synchronization and let it do the work that it is supposed to do? //edit I'm not even sure now if the trick with using "Lock1" string is really working and locks are placed on the same object..

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• Turn based synchronization between threads

  - by Amarus

  I'm trying to find a way to synchronize multiple threads having the following conditions: * There are two types of threads: 1. A single "cyclic" thread executing an infinite loop to do cyclic calculations 2. Multiple short-lived threads not started by the main thread * The cyclic thread has a sleep duration between each cycle/loop iteration * The other threads are allowed execute during the inter-cycle sleep of the cyclic thread: - Any other thread that attempts to execute during an active cycle should be blocked - The cyclic thread will wait until all other threads that are already executing to be finished Here's a basic example of what I was thinking of doing: // Somewhere in the code: ManualResetEvent manualResetEvent = new ManualResetEvent(true); // Allow Externally call CountdownEvent countdownEvent = new CountdownEvent(1); // Can't AddCount a CountdownEvent with CurrentCount = 0 void ExternallyCalled() { manualResetEvent.WaitOne(); // Wait until CyclicCalculations is having its beauty sleep countdownEvent.AddCount(); // Notify CyclicCalculations that it should wait for this method call to finish before starting the next cycle Thread.Sleep(1000); // TODO: Replace with actual method logic countdownEvent.Signal(); // Notify CyclicCalculations that this call is finished } void CyclicCalculations() { while (!stopCyclicCalculations) { manualResetEvent.Reset(); // Block all incoming calls to ExternallyCalled from this point forward countdownEvent.Signal(); // Dirty workaround for the issue with AddCount and CurrentCount = 0 countdownEvent.Wait(); // Wait until all of the already executing calls to ExternallyCalled are finished countdownEvent.Reset(); // Reset the CountdownEvent for next cycle. Thread.Sleep(2000); // TODO: Replace with actual method logic manualResetEvent.Set(); // Unblock all threads executing ExternallyCalled Thread.Sleep(1000); // Inter-cycles delay } } Obviously, this doesn't work. There's no guarantee that there won't be any threads executing ExternallyCalled that are in between manualResetEvent.WaitOne(); and countdownEvent.AddCount(); at the time the main thread gets released by the CountdownEvent. I can't figure out a simple way of doing what I'm after, and almost everything that I've found after a lengthy search is related to producer/consumer synchronization which I can't apply here.

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• Offline backup synchronization

  - by Pavan Kumar

  There is a Central Server running Windows Server 2003 and SQL Server 2005 and there are 7 client machines situated in various places and has XP Pro & SQL Server 2005 installed in all of them. They are not interconnected so they are physically seperate. One person goes to each of these centers maybe twice a month and takes the backup (Full database consisting of mdf and ldf files) with a pen drive and brings it to the Central server which contains the central database holding same schema as all the other client databases. I need to synchronize each backup database (belonging to different centers) one by one to update the existing data or inserting new data in the central database . The solution i got was Replication. The pendrive is brought to central server consisting of 7 instances of the databases and then the databases is attached to the central server one by one to the same SQL Server where the central database exists. Then my idea was to replicate the backup database one by one i.e using single subscription (Central Database) and multiple publication ( i.e 7 instances of databases in my case) toplogy by performing replication locally (i.e in the same machine). So i tried to develop a UI in C# .Net to programatically run the Transactional Replication with push subscription using RMO Programming (which is incomplete as of now because there is no point in developing when you already know it is not the solution). Transactional Replication can either be set to initialize with a snapshot or without a snapshot. If i go for the first option i.e with a snapshot , the data whatever is present in Central Database is overwritten by the new data . So the data present initially in the central database is lost. If i try to initialize without snapshot , no data (the data already has the updated and new data) will be sent from the backup database to server. The replication will work in a scenario where any incremental changes is done only after you set the replication . So the initial data whatever was present in the backup database when setting up the replication will not be replicated when running the snapshot agent for the first time to synchronize. Only changes in the backup database thereafter will be reflected to the central database .(Remember I am not going to insert new data or make any changes to the backup database after i attach it to the Central Server. ) So this solution is not feasible. I want a solution for synchronizing from one client database to central database present in the same machine using C#.NET. If you can provide me small example maybe with two databases(with same schema) DB1(Client) to DB2(Server) consisting of one or two tables it will be very helpful. The synchronization is not bidirectional.I want to only update existing data or insert new data from DB1 to DB2 (DB2 may contain some data initially). Thanks and Regards Pavan

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• How to sync a folder on a remote computer to a server on a domain

  - by Pierre-Alain Vigeant

  We have a small remote office that often share data with us. I learned that the data is shared as a email attachment, but that obviously leads to versioning hell and overriding. I am looking for a way for then to synchronize a folder directly on our main office domain controller. I personally use LiveMesh, but I would like a tool that is synchronized to our server directly without a 3rd party hosting the data, since we already have an online backup service taking care of the offsite backup. What enterprise class tool would let us synchronize a folder from a remote computer that is out of our domain, into our the file server of our domain? The synchronization has to be two-way, e.g.: Someone from the remote office will create an invoice. Someone from our office will review it and make modification to it. The remote office need to see the change. Our server is on Windows 2003.

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• Thunderbird + Gmail, has to send emails twice.

  - by Mohammad

  I've configured Thunderbird to place a copy of my sent emails in my remote "sent" folder of my Gmail account as opposed to the local thunderbird one. This ensures I can completely take advantage of my imap synchronization. And so whenever I send an email, it first sends one to the address list, then it sends a new one to my sent box, however doing this with large attachments seems like a waste of time and bandwidth. Do you guys know of any extension or a combination of a trick plus a Gmail filter that could automate this in one step?

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• Unable to sunchronize local and remote directories ("set times: Operation not permitted")

  - by Tom Auger

  I'm running into FTP errors using software like NetBeans or WinSCP: whenever I attempt to perform a synchronization or update of files from local -- server I get errors on the client saying "set times: Operation not permitted". This is clearly an issue with the way I've configured my Fedora installation. The user that I'm logging in with cannot touch -t any of these files, though he IS part of a group that has r/w access on the files. I do have root / sudo access to this server. What I would like to know is: a) is it likely that this problem would be solved by allowing my FTP user to "touch -t" these files b) how do I enable a certain user to be able to set timestamps on files without giving them ownership of the files (certain of these files need to be owned by Apache, for instance, so I don't want to chown them). Thanks in advance.

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• WS2008 NTP - Using time.windows.com,0x9 - Time always skewed forwards

  - by David

  I have a domain controller configured to use time.windows.com (with 0x09 flags set). I've noticed that frequently the systems' clock is fast - it varies from 10 minutes to even 45 minutes. I always have to keep resetting the system date/time back to what it should be. When I run "w32tm /query /source" it tells me it's using time.windows.com, and obviously I trust Microsoft not to serve incorrect times, but why is my server's clock fast? EDIT: There are a few Time-Service events in the System log: Event ID: 142 Message: The time service has stopped advertising as a time source because the local clock is not synchronized. Event ID: 139 Message: The time service has started advertising as a time source. These two messages appear in pairs every hour or so. Event 142 appears 14 to 16 minutes after 139 appears. Going back a few months, these events appear: Event ID: 35 Message: The time service is now synchronizing the system time with the time source time.windows.com,0x9 (ntp.m|0x9|0.0.0.0:123-65.55.21.21:123). Event ID: 37 Message: The time provider NtpClient is currently receiving valid time data from time.windows.com,0x9 (ntp.m|0x9|0.0.0.0:123-65.55.21.21:123). Event ID: 47 Message: Time Provider NtpClient: No valid response has been received from manually configured peer time.windows.com,0x9 after 8 attempts to contact it. This peer will be discarded as a time source and NtpClient will attempt to discover a new peer with this DNS name. The error was: The time sample was rejected because: The peer is not synchronized, or it has been too long since the peer's last synchronization. These three events only appear once in the log, back in October.

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• Options for synchronizing Palm Desktop calendars

  - by Al Everett

  My wife and I each have Palm Centro phones and very full calendars. We've been using Palm products for years and are happy with them and are not looking to switch (and don't have the budget for it even if we wanted to). What is a viable way we can synchronize our calendars? Back when we had Palm Z22s we used AirSet. It worked great in synchronizing our desktop calendars. Unfortunately, their sync software does not support Palm Desktop v6 and there is nothing in the pipeline to support it. (The third party vendor is apparently not interested in updating for the newer Palm desktop.) I would love to be able to get back to having each other's appointments appear on the other's calendar. What can we do? Some limitations: A data plan is not an option (so this precludes over-the-air synchronization options) No Microsoft Outlook Edit: Syncing my Palm to my Palm Desktop is not the issue. Being able to sync, in some way, the Palm calendar databases of my wife's and my calendars is what's desired.

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• Mesh Networked servers via vpn

  - by microspino

  I got a design idea and I would like to have some advice from SF about It. I have 5 customers with small real-estate databases. I've built for them a desktop app and now they would like to merge their database to share their data. I don't want to centralize everything in one place nor I want to do maintenance for servers. They told me also, that all of them in their offices, have little servers and maintenance guys available. Although everything seems suitable for web application, I had the idea to experiment something new: Any customer small-server wild be connected to the others in a sort of mesh network without a single point of failure and through VPNs. If one of the servers went down the customers could still connect to their databases from one of the other mesh networked servers instead of from the local one that is down. During normal operations all the servers sync the db with the others through VPNs. I can accept a half-day timing window of NON synched data, in other words, since I don't need real time synchronization, the server don't have to always stay in synch. I can migrate my data over to other Non-Sql technologies like CouchDB or Redis or whatever you suggest. As you can see I don't have a lot of constraints and although I could go with a web application I would like to delegate and decentralize support, data-privacy and management, as more as I can to my customers offices. Is that a crazy idea? Do you know If something similar exist? Which technology would you suggest?

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• WinSCP equivalent for Linux/Ubuntu

  - by Shashank

  I'm shifting most of my projects to a Linux machine, and one of the things that I miss is WinSCP. I've found other answers saying that nautilus, FileZilla etc. can be used for SFTP, but something that I loved about WinSCP was that it has two panes (FileZilla's got that) and I could start synchronization from any directory. Unison or Rsync could work, but I'd have to create a folder pair every time I want to sync two folders. Is there an SFTP client for Linux that has a two-paned view and allows ad-hoc synchronization? Thanks!

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• What happens if two COM classes each without a threading model are implemented in one in-proc COM se

  - by sharptooth

  Consider a situation. I have an in-proc COM server that contains two COM classes. Both classes are marked as "no threading model" in the registry - the "ThreadingModel" value is just absent. Both classes read/write the same set of global variable without any synchronization. As far as I know "no threading model" will enforce COM to disallow concurrent access to the same or different instances of the same class by different threads. Will COM prevent concurrent access to instances of the two abovementioned different classes? Do I need synchronization when accessing the global variables from two different COM classes in this situation?

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• using git on android

  - by Dave

  Hi all, I have a desktop application using git for synchronization. I have also an android application which do the same as the desktop, but I don't know how to do the synchronization part on it. I haven't found any implementation of git on android. I found a jgit, but its unwell documented and I was not able to do even a commit with that. I was thinking about remote commands to my git server from my android app, but this is really a tease for me. Could you tell me if any other git implementation which can be used on android exists? If you have any idea how to solve that, please tell me. Thanks for reply, Dave

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• Java: serial thread confinement question

  - by denis

  Assume you have a Collection(ConcurrentLinkedQueue) of Runnables with mutable state. Thread A iterates over the Collection and hands the Runnables to an ExecutorService. The run() method changes the Runnables state. The Runnable has no internal synchronization. The above is a repetitive action and the worker threads need to see the changes made by previous iterations. So a Runnable gets processed by one worker thread after another, but is never accessed by more than one thread at a time - a case of serial thread confinement(i hope ;)). The question: Will it work just with the internal synchronization of the ConcurrentLinkedQueue/ExecutorSerivce? To be more precise: If Thread A hands Runnable R to worker thread B and B changes the state of R, and then A hands R to worker thread C..does C see the modifications done by B?

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• Synchronization requirements for FileStream.(Begin/End)(Read/Write)

  - by Doug McClean

  Is the following pattern of multi-threaded calls acceptable to a .Net FileStream? Several threads calling a method like this: ulong offset = whatever; // different for each thread byte[] buffer = new byte[8192]; object state = someState; // unique for each call, hence also for each thread lock(theFile) { theFile.Seek(whatever, SeekOrigin.Begin); IAsyncResult result = theFile.BeginRead(buffer, 0, 8192, AcceptResults, state); } if(result.CompletedSynchronously) { // is it required for us to call AcceptResults ourselves in this case? // or did BeginRead already call it for us, on this thread or another? } Where AcceptResults is: void AcceptResults(IAsyncResult result) { lock(theFile) { int bytesRead = theFile.EndRead(result); // if we guarantee that the offset of the original call was at least 8192 bytes from // the end of the file, and thus all 8192 bytes exist, can the FileStream read still // actually read fewer bytes than that? // either: if(bytesRead != 8192) { Panic("Page read borked"); } // or: // issue a new call to begin read, moving the offsets into the FileStream and // the buffer, and decreasing the requested size of the read to whatever remains of the buffer } } I'm confused because the documentation seems unclear to me. For example, the FileStream class says: Any public static members of this type are thread safe. Any instance members are not guaranteed to be thread safe. But the documentation for BeginRead seems to contemplate having multiple read requests in flight: Multiple simultaneous asynchronous requests render the request completion order uncertain. Are multiple reads permitted to be in flight or not? Writes? Is this the appropriate way to secure the location of the Position of the stream between the call to Seek and the call to BeginRead? Or does that lock need to be held all the way to EndRead, hence only one read or write in flight at a time? I understand that the callback will occur on a different thread, and my handling of state, buffer handle that in a way that would permit multiple in flight reads. Further, does anyone know where in the documentation to find the answers to these questions? Or an article written by someone in the know? I've been searching and can't find anything. Relevant documentation: FileStream class Seek method BeginRead method EndRead IAsyncResult interface

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• Synchronization between Client database and Central Database

  - by Indranil Mutsuddy

  Hello, I am trying to develop UI in C# .NET to synchronize 7 instances of backup databases with the central database one by one (All holding same schema) .The backup database( all 7 instances client databases) which is brought to the central server in a removable device such pendrive will consist of mdf and ldf files from each client and will be attached to the server where the central database resides. After all the client backup databases are attached i need to synchronize(update existing data or insert new data to the central database residing in server) each backup database one by one to central database. I want to know as how i can synchronize betweeen a backup database with a central database using C# .NET

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• Effective thread Synchronization in C#

  - by n0vic3c0d3r

  I have a scenario where I need to search from many binary files (using keys) and combine the results (strings). Until now, I have been doing it in a for loop one file after the other. foreach (string file in FileSources.Keys) { aggregatedDefinitions.Append(DefinitionLookup(txtSearchWord.Text, file)); } Since this operation is very slow, I was thinking of using threads, so that I could do IO operations in parallel. Is threading the right way to go. If I use threading, how can I ensure that I get the results in the order I want. I haven't used Threading until now. It would be very helpful if you could suggest some materials/books that would help me solve my problem.

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• Synchronization in Robotium

  - by Vicente

  How long does robotium wait for a new activity to show up? Is it possible to set the timeout manually for the Solo instance?

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• 2 way synchronization of in-house gitosis repositories server with Github repositories

  - by Robert J Berger

  We use gitosis as our local in-house shared repository and also have a private Github account that mirrors our local repository. If one does a git push to the in-house repository the post-update hook updates the Github repository. How can I make this two way without causing "loops"? I.e. if someone pushes to the Github repository, I would like it to also update the in-house gitosis repository. A pointer to an example of how to do it would be greatly appreciated. Or if there are recommendations of alternatives to gitosis that would make this kind of thing easy, I would consider migrating to that.

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• Boost Thread Synchronization

  - by Dave18

  I don't see synchronized output when i comment the the line wait(1) in thread(). can I make them run at the same time (one after another) without having to use 'wait(1)'? #include <boost/thread.hpp> #include <iostream> void wait(int seconds) { boost::this_thread::sleep(boost::posix_time::seconds(seconds)); } boost::mutex mutex; void thread() { for (int i = 0; i < 100; ++i) { wait(1); mutex.lock(); std::cout << "Thread " << boost::this_thread::get_id() << ": " << i << std::endl; mutex.unlock(); } } int main() { boost::thread t1(thread); boost::thread t2(thread); t1.join(); t2.join(); }

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