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• Help me find article on Multi-threading and Event Handling in Java

  - by JDR

  I once read an article on how to properly write event handlers for multi-threading in Java, but I can't for the life of me find it anymore. It described the pitfalls and potentials for deadlocks that can occur when firing events (not Swing events mind you, but general events like model update notifications). To clarify, the situation would be as such: // let's say this is code from an MVC model somewhere public void setSomeProperty(String myProperty){ if(!this.myProperty.equals(myProperty)){ this.myProperty = myProperty; fireMyPropertyChangedEvent(...); } } The article described how passing control to arbitrary external listener code was a potential cause for deadlock. I now find myself in a situation where I need to fire such events in a multithreaded environment and I would very much like to read the article again to see what it has to say before I continue. Does anyone know the article I'm referring to? I believe it came as a (fairly short) PDF. It started off with an initial naive implementation and incrementally pointed out flaws and improved upon it. It ended with a sort of final proper-way-to-fire-multithreaded-events. I've searched endlessly in my browse history and on google, but all I could find were endless amounts topics on Swing event dispatch threads. Thank you.

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• How to end a thread in perl

  - by user1672190

  I am new to perl and i have a question about perl thread. I am trying to create a new thread to check if the running function is timed out, and my way of doing it is as below. Logic is 1.create a new thread 2.run the main function and see if it is timed out, if ture, kill it Sample code: $exit_tread = false; # a flag to make sure timeout thread will run my $thr_timeout = threads->new( \&timeout ); execute main function here; $exit_thread = true # set the flag to true to force thread ends $thr_timeout->join(); #wait for the timeout thread ends Code of timeout function sub timeout { $timeout = false; my $start_time = time(); while (!$exit_thread) { sleep(1); last if (main function is executed); if (time() - $start_time >= configured time ) { logmsg "process is killed as request timed out"; _kill_remote_process(); $timeout = true; last; } } } now the code is running as i expected, but i am just not very clear if the code $exit_thread = true works because there is a "last" at the end of while loop. Can anybody give me a answer? Thanks

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• throwing exception from APCProc crashes program

  - by lazy_banana

  I started to do some research on how terminate a multithreaded application properly and I found those 2 post(first, second) about how to use QueueUserAPC to signal other threads to terminate. I thought I should give it a try, and the application keeps crashing when I throw the exception from the APCProc. Code: #include <stdio.h> #include <windows.h> class ExitException { public: char *desc; DWORD exit_code; ExitException(char *desc,int exit_code): desc(desc), exit_code(exit_code) {} }; //I use this class to check if objects are deconstructed upon termination class Test { public: char *s; Test(char *s): s(s) { printf("%s ctor\n",s); } ~Test() { printf("%s dctor\n",s); } }; DWORD CALLBACK ThreadProc(void *useless) { try { Test t("thread_test"); SleepEx(INFINITE,true); return 0; } catch (ExitException &e) { printf("Thread exits\n%s %lu",e.desc,e.exit_code); return e.exit_code; } } void CALLBACK exit_apc_proc(ULONG_PTR param) { puts("In APCProc"); ExitException e("Application exit signal!",1); throw e; return; } int main() { HANDLE thread=CreateThread(NULL,0,ThreadProc,NULL,0,NULL); Sleep(1000); QueueUserAPC(exit_apc_proc,thread,0); WaitForSingleObject(thread,INFINITE); puts("main: bye"); return 0; } My question is why does this happen? I use mingw for compilation and my OS is 64bit. Can this be the reason?I read that you shouldn't call QueueApcProc from a 32bit app for a thread which runs in a 64bit process or vice versa, but this shouldn't be the case.

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• Threading in Android

  - by virsir

  I am currently developing Android app, it needs download content from internet. I use thread to do that and then call runOnUiThread method to update GUI. I placed a refresh menu on it, if user tried to refresh the content, the download thread will be created and started. The problem is that how can I control the thread order, I need to accept the latest request's response and abandon previous thread requests if there were some other requests still running because the request parameters may have been changed by user. Currently I was using a threadId to do this thing, when a thread finished, it will check its threadId, if it was the latest recored one, it then takes control and render the response. My question is that is there any other proper better solution for this? Do I need to stop threads when user exit the app? I remember that some book said that do not try stop thread manually and wait itself finish is a good practice, is that true? Should I stop them by calling "stop" or "interrupt" method? I read some documents around threading in Android and found the class HandlerThread, what is it? In what kind of situation I need to use it?

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• New to JEE; architecture suggestions for a service/daemon?

  - by Kate

  I am brand new to the JEE world. As an exercise to try and familiarize myself with JEE, I'm trying to create a tiered web-app, but I'm getting a little stuck on what the best way is to spin up a service in the background that does work. Parameters of the service: It must open and hold a socket connection and receive information from the connected server. There is a 1-to-1 correlation between a user and a new socket connection. So the idea is the user presses a button on the web-page, and somewhere on the server a socket connection is opened. For the remainder of the users session (or until the user presses some sort of disconnect button) the socket remains open and pushes received information to some sort of centralized store that servlets can query and return to the user via AJAX. Is there a JEE type way to handle this situation? Naturally what I would think to do is to just write a Java application that listens on a port that the servlets can connect to and spawns new threads that open these sockets, but that seems very ad-hoc to me. (PS: I am also new to Stack Overflow, so forgive me if it takes me some time to figure the site out!)

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• Is this use of PreparedStatements in a Thread in JAVA correct?

  - by Gormcito

  I'm still an undergrad just working part time and so I'm always trying to be aware of better ways to do things. Recently I had to write a program for work where the main thread of the program would spawn "task" threads (for each db "task" record) which would perform some operations and then update the record to say that it has finished. Therefore I needed a database connection object and PreparedStatement objects in or available to the ThreadedTask objects. This is roughly what I ended up writing, is creating a PreparedStatement object per thread a waste? I thought static PreparedStatments could create race conditions... Thread A stmt.setInt(); Thread B stmt.setInt(); Thread A stmt.execute(); Thread B stmt.execute(); A´s version never gets execed.. Is this thread safe? Is creating and destroying PreparedStatement objects that are always the same not a huge waste? public class ThreadedTask implements runnable { private final PreparedStatement taskCompleteStmt; public ThreadedTask() { //... taskCompleteStmt = Main.db.prepareStatement(...); } public run() { //... taskCompleteStmt.executeUpdate(); } } public class Main { public static final db = DriverManager.getConnection(...); }

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• .NET 4 ... Parallel.ForEach() question

  - by CirrusFlyer

  I understand that the new TPL (Task Parallel Library) has implemented the Parallel.ForEach() such that it works with "expressed parallelism." Meaning, it does not guarantee that your delegates will run in multiple threads, but rather it checks to see if the host platform has multiple cores, and if true, only then does it distribute the work across the cores (essentially 1 thread per core). If the host system does not have multiple cores (getting harder and harder to find such a computer) then it will run your code sequenceally like a "regular" foreach loop would. Pretty cool stuff, frankly. Normally I would do something like the following to place my long running operation on a background thread from the ThreadPool: ThreadPool.QueueUserWorkItem( new WaitCallback(targetMethod), new Object2PassIn() ); In a situation whereby the host computer only has a single core does the TPL's Parallel.ForEach() automatically place the invocation on a background thread? Or, should I manaully invoke any TPL calls from a background thead so that if I am executing from a single core computer at least that logic will be off of the GUI's dispatching thread? My concern is if I leave the TPL in charge of all this I want to ensure if it determines it's a single core box that it still marshalls the code that's inside of the Parallel.ForEach() loop on to a background thread like I would have done, so as to not block my GUI. Thanks for any thoughts or advice you may have ...

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• Communicating with a running python daemon

  - by hanksims

  I wrote a small Python application that runs as a daemon. It utilizes threading and queues. I'm looking for general approaches to altering this application so that I can communicate with it while it's running. Mostly I'd like to be able to monitor its health. In a nutshell, I'd like to be able to do something like this: python application.py start # launches the daemon Later, I'd like to be able to come along and do something like: python application.py check_queue_size # return info from the daemonized process To be clear, I don't have any problem implementing the Django-inspired syntax. What I don't have any idea how to do is to send signals to the daemonized process (start), or how to write the daemon to handle and respond to such signals. Like I said above, I'm looking for general approaches. The only one I can see right now is telling the daemon constantly log everything that might be needed to a file, but I hope there's a less messy way to go about it. UPDATE: Wow, a lot of great answers. Thanks so much. I think I'll look at both Pyro and the web.py/Werkzeug approaches, since Twisted is a little more than I want to bite off at this point. The next conceptual challenge, I suppose, is how to go about talking to my worker threads without hanging them up. Thanks again.

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• How do I make a lock that allows only ONE thread to read from the resource ?

  - by mare

  I have a file that holds an integer ID value. Currently reading the file is protected with ReaderWriterLockSlim as such: public int GetId() { _fileLock.EnterUpgradeableReadLock(); int id = 0; try { if(!File.Exists(_filePath)) CreateIdentityFile(); FileStream readStream = new FileStream(_filePath, FileMode.Open, FileAccess.Read); StreamReader sr = new StreamReader(readStream); string line = sr.ReadLine(); sr.Close(); readStream.Close(); id = int.Parse(line); return int.Parse(line); } finally { SaveNextId(id); // increment the id _fileLock.ExitUpgradeableReadLock(); } } The problem is that subsequent actions after GetId() might fail. As you can see the GetId() method increments the ID every single time, disregarding what happens after it has issued an ID. The issued ID might be left hanging (as said, exceptions might occur). As the ID is incremented, some IDs might be left unused. So I was thinking of moving the SaveNextId(id) out, remove it (the SaveNextId() actually uses the lock too, except that it's EnterWriteLock). And call it manually from outside after all the required methods have executed. That brings out another problem - multiple threads might enter the GetId() method before the SaveNextId() gets executed and they might all receive the same ID. I don't want any solutions where I have to alter the IDs after the operation, correcting them in any way because that's not nice and might lead to more problems. I need a solution where I can somehow callback into the FileIdentityManager (that's the class that handles these IDs) and let the manager know that it can perform the saving of the next ID and then release the read lock on the file containing the ID. Essentialy I want to replicate the relational databases autoincrement behaviour - if anything goes wrong during row insertion, the ID is not used, it is still available for use but it also never happens that the same ID is issued. Hopefully the question is understandable enough for you to provide some solutions..

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• C# cross thread dialogue co-operation

  - by John Attridge

  K I am looking at a primarily single thread windows forms application in 3.0. Recently my boss had a progress dialogue added on a separate thread so the user would see some activity when the main thread went away and did some heavy duty work and locked out the GUI. The above works fine unless the user switches applications or minimizes as the progress form sits top most and will not disappear with the main application. This is not so bad if there are lots of little operations as the event structure of the main form catches up with its events when it gets time so minimized and active flags can be checked and thus the dialog thread can hide or show itself accordingly. But if a long running sql operation kicks off then no events fire. I have tried intercepting the WndProc command but this also appears queued when a long running sql operation is executing. I have also tried picking up the processes, finding the current app and checking various memory values isiconic and the like inside the progress thread but until the sql operation finishes none of these get updated. Removing the topmost causes the dialog to disappear when another app activates but if the main app is then brought back it does not appear again. So I need a way to find out if the other thread is minimized or no longer active that does not involve querying the actual thread as that locks until the sql operation finishes. Now I know that this is not the best way to write this and it would be better to have all the heavy processing on separate threads leaving the GUI free but as this is a huge ancient legacy app the time to re-write in that fashion will not be provided so I have to work with what I have got. Any help is appreciated

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• Lazy load images in UITableViewCell

  - by lostInTransit

  Hi I have some 50 custom cells in my UITableView. I want to display an image and a label in the cells where I get the images from URLs. I want to do a lazy load of images so the UI does not freeze up while the images are being loaded. I tried getting the images in separate threads but I have to load each image every time a cell becomes visible again (Otherwise reuse of cells shows old images) Apps like Facebook load images only for cells currently visible and once the images are loaded, they are not loaded again. Can someone please tell me how to duplicate this behavior. Thanks. Edit Trying to cache images in an NSMutableDictionary object creates problems when the user scrolls fast. I am getting images only when scrolling completely stops and clearing out the cache on memory warning. But the app invariably gets a memory warning (due to size of images being cached) and clears the cache before reloading. If scrolling is very fast, it crashes. Any other suggestions are welcome

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• Multithreading: Read from / write to a pipe

  - by Tero Jokinen

  I write some data to a pipe - possibly lots of data and at random intervals. How to read the data from the pipe? Is this ok: in the main thread (current process) create two more threads (2, 3) the second thread writes sometimes to the pipe (and flush-es the pipe?) the 3rd thread has infinite loop which reads the pipe (and then sleeps for some time) Is this so far correct? Now, there are a few thing I don't understand: do I have to lock (mutex?) the pipe on write? IIRC, when writing to pipe and its buffer gets full, the write end will block until I read the already written data, right? How to check for read data in the pipe, not too often, not too rarely? So that the second thread wont block? Is there something like select for pipes? It is possible to set the pipe to unbuffered more or I have to flush it regularly - which one is better? Should I create one more thread, just for flushing the pipe after write? Because flush blocks as well, when the buffer is full, right? I just don't want the 1st and 2nd thread to block.... [Edit] Sorry, I thought the question is platform agnostic but just in case: I'm looking at this from Win32 perspective, possibly MinGW C...

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• how to run TimerTask off main UI thread?

  - by huskyd97

  I am having trouble with a TimerTask Interfering with In App Purchasing (Async Tasks). I am weak with Threads, so I believe it is running on the main UI thread, eating up resources. How can I run this outside the UI thread? I have searched, and tried some suggestions using handlers. but seems like I get the same result, app gets really laggy. when I don't run this task (refreshes every 500mS), the activity runs smoothly, and there are no hangs during In app purchases. Your help is appreciated, code snippet below: public class DummyButtonClickerActivity extends Activity { protected Timer timeTicker = new Timer("Ticker"); private Handler timerHandler = new Handler(); protected int timeTickDown = 20; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.mainhd); // start money earned timer handler TimerTask tick = new TimerTask() { public void run() { myTickTask(); } }; timeTicker.scheduleAtFixedRate(tick, 0, 500); // 500 ms each } // End OnCreate protected void myTickTask() { if (timeTickDown == 0) { /// run my code here //total = total + _Rate; timerHandler.post(doUpdateTimeout); } else if(timeTickDown < 0) { // do nothing } timeTickDown--; } private Runnable doUpdateTimeout = new Runnable() { public void run() { updateTimeout(); } }; private void updateTimeout() { // reset tick timeTickDown = 2; // 2* 500ms == once a second } }

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• Ruby, Python, or PHP?

  - by Gabe

  And so we return to the age old question - but with a few twists. This morning, I searched and read up on which web development language to learn first. I'm thinking Ruby, Python, or perhaps PHP. But I have a few questions before deciding. Background: I'm a year into C++ (through school), but want to get into web development. I have all summer to commit to one language, learn it, do some projects, get up some websites, and so on. Now my questions (and these are assuming that I should choose between Ruby, Python, and PHP - if I should choose a different language, let me know.): I hope to use whichever language I learn for websites/web apps. Some of the threads on stackoverflow suggested Python was the best overall language, but others were unanimous that Ruby was best specifically for web development. For a first language suited towards web development, which language do you recommend, and why? This might tie into the first question, but which language looks most promising for future work, future personal projects, and basically the future in general? I'm just a freshman in college. Ideally, the language I choose would be on the rise, community-wise and opportunity-wise. (One reason I'm leaning towards Ruby is that it seems a lot of the newer tech startups/successes are using it.)

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• Chat app vs REST app - use a thread in an Activity or a thread in a Service?

  - by synic

  In Virgil Dobjanschi's talk, "Developing Android REST client applications" (link here), he said a few things that took me by surprise. Including: Don't run http queries in threads spawned by your activities. Instead, communicate with a service to do them, and store the information in a ContentProvider. Use a ContentObserver to be notified of changes. Always perform long running tasks in a Service, never in your Activity. Stop your Service when you're done with it. I understand that he was talking about a REST API, but I'm trying to make it fit with some other ideas I've had for apps. One of APIs I've been using uses long-polling for their chat interface. There is a loop http queries, most of which will time out. This means that, as long as the app hasn't been killed by the OS, or the user hasn't specifically turned off the chat feature, I'll never be done with the Service, and it will stay open forever. This seems less than optimal. Long question short: For a chat application that uses long polling to simulate push and immediate response, is it still best practice to use a Service to perform the HTTP queries, and store the information in a ContentProvider?

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• C++ gdb GUI

  - by HappyDude

  Briefly: Does anyone know of a GUI for gdb that brings it on par or close to the feature set you get in the more recent version of Visual C++? In detail: As someone who has spent a lot of time programming in Windows, one of the larger stumbling blocks I've found whenever I have to code C++ in Linux is that debugging anything using commandline gdb takes me several times longer than it does in Visual Studio, and it does not seem to be getting better with practice. Some things are just easier or faster to express graphically. Specifically, I'm looking for a GUI that: Handles all the basics like stepping over & into code, watch variables and breakpoints Understands and can display the contents of complex & nested C++ data types Doesn't get confused by and preferably can intelligently step through templated code and data structures while displaying relevant information such as the parameter types Can handle threaded applications and switch between different threads to step through or view the state of Can handle attaching to an already-started process or reading a core dump, in addition to starting the program up in gdb If such a program does not exist, then I'd like to hear about experiences people have had with programs that meet at least some of the bullet points. Does anyone have any recommendations? Edit: Listing out the possibilities is great, and I'll take what I can get, but it would be even more helpful if you could include in your responses: (a) Whether or not you've actually used this GUI and if so, what positive/negative feedback you have about it. (b) If you know, which of the above-mentioned features are/aren't supported Lists are easy to come by, sites like this are great because you can get an idea of people's personal experiences with applications.

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• Peculiar JRE behaviour running RMI server under load, should I worry?

  - by darri

  I've been developing a minimalistic Java rich client CRUD application framework for the past few years, mostly as a hobby but also actively using it to write applications for my current employer. The framework provides database access to clients either via a local JDBC based connection or a lightweight RMI server. Last night I started a load testing application, which ran 100 headless clients, bombarding the server with requests, each client waiting only 1 - 2 seconds between running simple use cases, consisting of selecting records along with associated detail records from a simple e-store database (Chinook). This morning when I looked at the telemetry results from the server profiling session I noticed something which to me seemed strange (and made me keep the setup running for the remainder of the day), I don't really know what conclusions to draw from it. Here are the results: Memory GC activity Threads CPU load Interesting, right? So the question is, is this normal or erratic? Is this simply the JRE (1.6.0_03 on Windows XP) doing it's thing (perhaps related to the JRE configuration) or is my framework design somehow causing this? Running the server against MySQL as opposed to an embedded H2 database does not affect the pattern. I am leaving out the details of my server design, but I'll be happy to elaborate if this behaviour is deemed erratic.

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• .NET Best Way to move many files to and from various directories??

  - by Dan

  I've created a program that moves files to and from various directories. An issue I've come across is when you're trying to move a file and some other program is still using it. And you get an error. Leaving it there isn't an option, so I can only think of having to keep trying to move it over and over again. This though slows the entire program down, so I create a new thread and let it deal with the problem file and move on to the next. The bigger problem is when you have too many of these problem files and the program now has so many threads trying to move these files, that it just crashes with some kernel.dll error. Here's a sample of the code I use to move the files: Public Sub MoveIt() Try File.Move(_FileName, _CopyToFileName) Catch ex As Exception Threading.Thread.Sleep(5000) MoveIt() End Try End Sub As you can see.. I try to move the file, and if it errors, I wait and move it again.. over and over again.. I've tried using FileInfo as well, but that crashes WAY sooner than just using the File object. So has anyone found a fool proof way of moving files without it ever erroring? Note: it takes a lot of files to make it crash. It'll be fine on the weekend, but by the end of the day on monday, it's done.

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• Python - How to wake up a sleeping process- multiprocessing?

  - by user1162512

  I need to wake up a sleeping process ? The time (t) for which it sleeps is calculated as t = D/S . Now since s is varying, can increase or decrease, I need to increase/decrease the sleeping time as well. The speed is received over a UDP procotol. So, how do I change the sleeping time of a process, keeping in mind the following:- If as per the previous speed `S1`, the time to sleep is `(D/S1)` . Now the speed is changed, it should now sleep for the new time,ie (D/S2). Since, it has already slept for D/S1 time, now it should sleep for D/S2 - D/S1. How would I do it? As of right now, I'm just assuming that the speed will remain constant all throughout the program, hence not notifying the process. But how would I do that according to the above condition? def process2(): p = multiprocessing.current_process() time.sleep(secs1) # send some packet1 via UDP time.sleep(secs2) # send some packet2 via UDP time.sleep(secs3) # send some packet3 via UDP Also, as in threads, 1) threading.activeCount(): Returns the number of thread objects that are active. 2) threading.currentThread(): Returns the number of thread objects in the caller's thread control. 3) threading.enumerate(): Returns a list of all thread objects that are currently active. What are the similar functions for getting activecount, enumerate in multiprocessing?

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• Why doesn't Win Forms application update label immediately?

  - by rosscj2533

  I am doing some experimenting with threads, and made a 'control' method to compare against where all the processing happens in the UI thread. It should run a method, which will update a label at the end. This method runs four times, but the labels are not updated until all 4 have completed. I expected one label to get updated about every 2 seconds. Here's the code: private void button1_Click(object sender, EventArgs e) { Stopwatch watch = new Stopwatch(); watch.Start(); UIThreadMethod(lblOne); UIThreadMethod(lblTwo); UIThreadMethod(lblThree); UIThreadMethod(lblFour); watch.Stop(); lblTotal.Text = "Total Time (ms): " + watch.ElapsedMilliseconds.ToString(); } private void UIThreadMethod(Label label) { Stopwatch watch = new Stopwatch(); watch.Start(); for (int i = 0; i < 10; i++) { Thread.Sleep(200); } watch.Stop(); // this doesn't set text right away label.Text = "Done, Time taken (ms): " + watch.ElapsedMilliseconds; } Maybe I'm just missing something basic, but I'm stuck. Any ideas? Thanks.

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• Making Extension method Generic

  - by Ian

  In this post there's a very interesting way of updating UI threads using a static extension method. public static void InvokeIfRequired(this Control c, Action<Control> action) { if(c.InvokeRequired) { c.Invoke(() => action(c)); } else { action(c); } } What I want to do, is to make a generic version, so I'm not constrained by a control. This would allow me to do the following for example (because I'm no longer constrained to just being a Control) this.progressBar1.InvokeIfRequired(pb => pb.Value = e.Progress); I've tried the following: public static void InvokeIfRequired<T>(this T c, Action<T> action) where T : Control { if (c.InvokeRequired) { c.Invoke(() => action(c)); } else { action(c); } } But I get the following error that I'm not sure how to fix. Anyone any suggestions? Error 5 Cannot convert lambda expression to type 'System.Delegate' because it is not a delegate type

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• The shell dotfile cookbook

  - by Jason Baker

  I constantly hear from other people about how much of the stuff they've used to customize their *nix setup they've shamelessly stolen from other people. So in that spirit, I'd like to start a place to share that stuff here on SO. Here are the rules: DON'T POST YOUR ENTIRE DOTFILE. Instead, just show us the cool stuff. One recipe per answer You may, however, post multiple versions of your recipe in the same answer. For example, you may post a version that works for bash, a version that works for zsh, and a version that works for csh in the same answer. State what shells you know your recipe will work with in the answer. Let's build this cookbook as a team. If you find out that an answer works with other shells other than the one the author posted, edit it in. If you like an idea and rewrite it to work with another shell, edit the modified version in to the original post. Give credit where credit is due. If you got your idea from someone else, give them credit if possible. And for those of you (justifiably) asking "Why do we need another one of these threads?": Most of what I've seen is along the lines of "post your entire dotfile." Personally, I don't want to try to parse through a person's entire dotfile to figure out what I want. I just want to know about all the cool parts of it. It's helpful to have a single dotfile thread. I think most of the stuff that works in bash will work in zsh and it may be adapted to work with csh fairly easily.

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• New to J2EE; architecture suggestions for a service/daemon?

  - by Kate

  I am brand new to the J2EE world. As an exercise to try and familiarize myself with J2EE, I'm trying to create a tiered web-app, but I'm getting a little stuck on what the best way is to spin up a service in the background that does work. Paramters of the service: It must open and hold a socket connection and receive information from the connected server. There is a 1-to-1 correlation between a user and a new socket connection. So the idea is the user presses a button on the web-page, and somewhere on the server a socket connection is opened. For the remainder of the users session (or until the user presses some sort of disconnect button) the socket remains open and pushes received information to some sort of centralized store that servlets can query and return to the user via AJAX. Is there a J2EE type way to handle this situation? Naturally what I would think to do is to just write a Java application that listens on a port that the servlets can connect to and spawns new threads that open these sockets, but that seems very ad-hoc to me. (PS: I am also new to Stack Overflow, so forgive me if it takes me some time to figure the site out!)

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• Core Data Errors vs Exceptions Part 3

  - by John Gallagher

  My question is similar to this one. Background I'm creating a large number of objects in a core data store using NSOperations to speed things up. I've followed all the Core Data multithreading rules - I've got a single persistent store coordinator and a managed object context per thread that on save is merging back to the main managed object context. The Problem When the number of threads running at once is more than 1, I get the exception logged on save of my core data store: NSExceptionHandler has recorded the following exception: NSInternalInconsistencyException -- optimistic locking failure What I've Tried My code that creates new entities is quite complex - it makes entities that have relationships with other entities that could be being created in a separate thread. If I replace my object creation routine with some very simple code just making non-related entries, everything works perfectly. Initially, as well as the exceptions, I was getting a save error saying core data couldn't save due to the merge failing. I read the docs and realised I needed a merge policy on the Managed Object Context I was saving to. I set this up and as this question states, the save error goes away, but the exception remains. My Question Do I need to worry about these exceptions? If I do need to get rid of the exceptions, any ideas on how I do it?

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• "pseudo-atomic" operations in C++

  - by dan

  So I'm aware that nothing is atomic in C++. But I'm trying to figure out if there are any "pseudo-atomic" assumptions I can make. The reason is that I want to avoid using mutexes in some simple situations where I only need very weak guarantees. 1) Suppose I have globally defined volatile bool b, which initially I set true. Then I launch a thread which executes a loop while(b) doSomething(); Meanwhile, in another thread, I execute b=true. Can I assume that the first thread will continue to execute? In other words, if b starts out as true, and the first thread checks the value of b at the same time as the second thread assigns b=true, can I assume that the first thread will read the value of b as true? Or is it possible that at some intermediate point of the assignment b=true, the value of b might be read as false? 2) Now suppose that b is initially false. Then the first thread executes bool b1=b; bool b2=b; if(b1 && !b2) bad(); while the second thread executes b=true. Can I assume that bad() never gets called? 3) What about an int or other builtin types: suppose I have volatile int i, which is initially (say) 7, and then I assign i=7. Can I assume that, at any time during this operation, from any thread, the value of i will be equal to 7? 4) I have volatile int i=7, and then I execute i++ from some thread, and all other threads only read the value of i. Can I assume that i never has any value, in any thread, except for either 7 or 8? 5) I have volatile int i, from one thread I execute i=7, and from another I execute i=8. Afterwards, is i guaranteed to be either 7 or 8 (or whatever two values I have chosen to assign)?

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