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  • Why doesn't Perl threading work when I call readdir beforehand?

    - by Kevin
    Whenever I call readdir before I create a thread, I get an error that looks like this: perl(2820,0x7fff70c33ca0) malloc: * error for object 0x10082e600: pointer being freed was not allocated * set a breakpoint in malloc_error_break to debug Abort trap What's strange is that it happens when I call readdir before I create a thread (i.e. readdir is not called in any concurrent code). I don't even use the results from readdir, just making the call to it seems to screw things up. When I get rid of it, things seem to work fine. Some example code is below: opendir(DIR, $someDir); my @allFiles = readdir(DIR); close(DIR); my $thread = threads-create(\&sub1); $thread-join(); sub sub1 { print "in thread\n" }

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  • PHP Possible Memory Leak

    - by dropson
    I have a script that loops through a database for images to convert with gd & imagick. I unset or replace all variables and objects in between each loop. For each loop, get_memory_usage(1) reveals a concurrent amount of memory used by that script. Which is expected. But, when I run "top", the %MEM column reports that this script, (same PID), increments with several percentages for each loop. I destroy all images when I'm done with them, and when I run get_defined_vars(); only the standard globals and a few variables I have is set. Why is "top" % Memory Usage different than what PHP reports? After 10 loops, PHP has taken 20% percetage of the system memory. I run php 5.2.6 on Debian 5

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  • ASP.NET Thread Safety in aspx.cs code behind file

    - by Tim Michalski
    I am thinking of adding a DataContext as a member variable to my aspx.cs code-behind class for executing LinqToSql queries. Is this thread safe? I am not sure if a new instance of this code-behind class is created for each HTTP request, or if the instance is shared amongst all request threads? My fear is that I will get 10 simultaneous concurrent http requests that will be using the same database session. public partial class MyPage : System.Web.UI.Page { private DataContext myDB = new DataContext(); protected void MyAction_Click(object sender, EventArgs e) { myDB.DoWork(); } }

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  • Replacing objects, handling clones, dealing with write logs

    - by Alix
    Hi everyone, I'm dealing with a problem I can't figure out how to solve, and I'd love to hear some suggestions. [NOTE: I realise I'm asking several questions; however, answers need to take into account all of the issues, so I cannot split this into several questions] Here's the deal: I'm implementing a system that underlies user applications and that protect shared objects from concurrent accesses. The application programmer (whose application will run on top of my system) defines such shared objects like this: public class MyAtomicObject { // These are just examples of fields you may want to have in your class. public virtual int x { get; set; } public virtual List<int> list { get; set; } public virtual MyClassA objA { get; set; } public virtual MyClassB objB { get; set; } } As you can see they declare the fields of their class as auto-generated properties (auto-generated means they don't need to implement get and set). This is so that I can go in and extend their class and implement each get and set myself in order to handle possible concurrent accesses, etc. This is all well and good, but now it starts to get ugly: the application threads run transactions, like this: The thread signals it's starting a transaction. This means we now need to monitor its accesses to the fields of the atomic objects. The thread runs its code, possibly accessing fields for reading or writing. If there are accesses for writing, we'll hide them from the other transactions (other threads), and only make them visible in step 3. This is because the transaction may fail and have to roll back (undo) its updates, and in that case we don't want other threads to see its "dirty" data. The thread signals it wants to commit the transaction. If the commit is successful, the updates it made will now become visible to everyone else. Otherwise, the transaction will abort, the updates will remain invisible, and no one will ever know the transaction was there. So basically the concept of transaction is a series of accesses that appear to have happened atomically, that is, all at the same time, in the same instant, which would be the moment of successful commit. (This is as opposed to its updates becoming visible as it makes them) In order to hide the write accesses in step 2, I clone the accessed field (let's say it's the field list) and put it in the transaction's write log. After that, any time the transaction accesses list, it will actually be accessing the clone in its write log, and not the global copy everyone else sees. Like this, any changes it makes will be done to the (invisible) clone, not to the global copy. If in step 3 the commit is successful, the transaction should replace the global copy with the updated list it has in its write log, and then the changes become visible for everyone else at once. It would be something like this: myAtomicObject.list = updatedCloneOfListInTheWriteLog; Problem #1: possible references to the list. Let's say someone puts a reference to the global list in a dictionary. When I do... myAtomicObject.list = updatedCloneOfListInTheWriteLog; ...I'm just replacing the reference in the field list, but not the real object (I'm not overwriting the data), so in the dictionary we'll still have a reference to the old version of the list. A possible solution would be to overwrite the data (in the case of a list, empty the global list and add all the elements of the clone). More generically, I would need to copy the fields of one list to the other. I can do this with reflection, but that's not very pretty. Is there any other way to do it? Problem #2: even if problem #1 is solved, I still have a similar problem with the clone: the application programmer doesn't know I'm giving him a clone and not the global copy. What if he puts the clone in a dictionary? Then at commit there will be some references to the global copy and some to the clone, when in truth they should all point to the same object. I thought about providing a wrapper object that contains both the cloned list and a pointer to the global copy, but the programmer doesn't know about this wrapper, so they're not going to use the pointer at all. The wrapper would be like this: public class Wrapper<T> : T { // This would be the pointer to the global copy. The local data is contained in whatever fields the wrapper inherits from T. private T thisPtr; } I do need this wrapper for comparisons: if I have a dictionary that has an entry with the global copy as key, if I look it up with the clone, like this: dictionary[updatedCloneOfListInTheWriteLog] I need it to return the entry, that is, to think that updatedCloneOfListInTheWriteLog and the global copy are the same thing. For this, I can just override Equals, GetHashCode, operator== and operator!=, no problem. However I still don't know how to solve the case in which the programmer unknowingly inserts a reference to the clone in a dictionary. Problem #3: the wrapper must extend the class of the object it wraps (if it's wrapping MyClassA, it must extend MyClassA) so that it's accepted wherever an object of that class (MyClass) would be accepted. However, that class (MyClassA) may be final. This is pretty horrible :$. Any suggestions? I don't need to use a wrapper, anything you can think of is fine. What I cannot change is the write log (I need to have a write log) and the fact that the programmer doesn't know about the clone. I hope I've made some sense. Feel free to ask for more info if something needs some clearing up. Thanks so much!

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  • Threads to make video out of images

    - by masood
    updates: I think/ suspect the imageIO is not thread safe. shared by all threads. the read() call might use resources that are also shared. Thus it will give the performance of a single thread no matter how many threads used. ? if its correct . what is the solution (in practical code) Single request and response model at one time do not utilizes full network/internet bandwidth, thus resulting in low performance. (benchmark is of half speed utilization or even lower) This is to make a video out of an IP cam that gives a new image on each request. http://149.5.43.10:8001/snapshot.jpg It makes a delay of 3 - 8 seconds no matter what I do. Changed thread no. and thread time intervals, debugged the code by System.out.println statements to see if threads work. All seems normal. Any help? Please show some practical code. You may modify mine. This code works (javascript) with much smoother frame rate and max bandwidth usage. but the later code (java) dont. same 3 to 8 seconds gap. <!DOCTYPE html> <html> <head> <script type="text/javascript"> (function(){ var img="/*url*/"; var interval=50; var pointer=0; function showImg(image,idx) { if(idx<=pointer) return; document.body.replaceChild(image,document.getElementsByTagName("img")[0]); pointer=idx; preload(); } function preload() { var cache=null,idx=0;; for(var i=0;i<5;i++) { idx=Date.now()+interval*(i+1); cache=new Image(); cache.onload=(function(ele,idx){return function(){showImg(ele,idx);};})(cache,idx); cache.src=img+"?"+idx; } } window.onload=function(){ document.getElementsByTagName("img")[0].onload=preload; document.getElementsByTagName("img")[0].src="/*initial url*/"; }; })(); </script> </head> <body> <img /> </body> </html> and of java (with problem) : package camba; import java.applet.Applet; import java.awt.Button; import java.awt.Graphics; import java.awt.Image; import java.awt.Label; import java.awt.Panel; import java.awt.TextField; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import java.net.URL; import java.security.Timestamp; import java.util.Date; import java.util.concurrent.TimeUnit; import java.util.concurrent.atomic.AtomicBoolean; import javax.imageio.ImageIO; public class Camba extends Applet implements ActionListener{ Image img; TextField textField; Label label; Button start,stop; boolean terminate = false; long viewTime; public void init(){ label = new Label("please enter camera URL "); add(label); textField = new TextField(30); add(textField); start = new Button("Start"); add(start); start.addActionListener(this); stop = new Button("Stop"); add(stop); stop.addActionListener(this); } public void actionPerformed(ActionEvent e){ Button source = (Button)e.getSource(); if(source.getLabel() == "Start"){ for (int i = 0; i < 7; i++) { myThread(50*i); } System.out.println("start..."); } if(source.getLabel() == "Stop"){ terminate = true; System.out.println("stop..."); } } public void paint(Graphics g) { update(g); } public void update(Graphics g){ try{ viewTime = System.currentTimeMillis(); g.drawImage(img, 100, 100, this); } catch(Exception e) { e.printStackTrace(); } } public void myThread(final int sleepTime){ new Thread(new Runnable() { public void run() { while(!terminate){ try { TimeUnit.MILLISECONDS.sleep(sleepTime); } catch (InterruptedException ex) { ex.printStackTrace(); } long requestTime= 0; Image tempImage = null; try { URL pic = null; requestTime= System.currentTimeMillis(); pic = new URL(getDocumentBase(), textField.getText()); tempImage = ImageIO.read(pic); } catch(Exception e) { e.printStackTrace(); } if(requestTime >= /*last view time*/viewTime){ img = tempImage; Camba.this.repaint(); } } }}).start(); System.out.println("thread started..."); } }

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  • MySQL : incrementing text id in DB

    - by BarsMonster
    I need to have text IDs in my application. For example, we have acceptable charset azAZ09, and allowed range of IDs [aaa] - [cZ9]. First generated id would be aaa, then aab, aac, aad e.t.c. How one can return ID & increment lower bound in transaction-fashion? (provided that there are hundreds of concurrent requests and all should have correct result) To lower the load I guess it's possible to define say 20 separate ranges, and return id from random range - this should reduce contention, but it's not clear how to do single operation in the first place. Also, please note that number of IDs in range might exceed 2^32. Another idea is having ranges of 64-bit integers, and converting integer-char id in software code, where it could be done asyncroniously. Any ideas?

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  • Multiple calculations on the same set of data: ruby or database?

    - by Pierre
    Hi, I have a model Transaction for which I need to display the results of many calculations on many fields for a subset of transactions. I've seen 2 ways to do it, but am not sure which is the best. I'm after the one that will have the least impact in terms of performance when data set grows and number of concurrent users increases. data[:total_before] = Transaction.where(xxx).sum(:amount_before) data[:total_after] = Transaction.where(xxx).sum(:amount_after) ... or transactions = Transaction.where(xxx) data[:total_before]= transactions.inject(0) {|s, e| s + e.amount_before } data[:total_after]= transactions.inject(0) {|s, e| s + e.amount_after } ... Which one should I choose? (or is there a 3rd, better way?) Thanks, P.

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  • What technologies are appropriate for a human workflow system?

    - by CCw
    I'm researching various workflow architectures and it is overwhelming. The workflow system I am creating will be almost completely human-driven. Very little, if any, asynchronous activity will be taking place. One possibility is to simply use a RDBMS and have a task table, from which stored procedures would be used to enforce synchronous access to each task. This seems very simple, but I'm having a hard time coming up with reasons why I might need to involve a heavier solution. If my system has ~500 concurrent users, and there is very little in the way of automated or asynchronous tasks, should I even consider the various workflow patterns/packages out there like Mule, BPEL/SOA, Spring Work Flow, etc?

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  • Servlet 3 spec and ThreadLocal

    - by mindas
    As far as I know, Servlet 3 spec introduces asynchronous processing feature. Among other things, this will mean that the same thread can and will be reused for processing another, concurrent, HTTP request(s). This isn't revolutionary, at least for people who worked with NIO before. Anyway, this leads to another important thing: no ThreadLocal variables as a temporary storage for the request data. Because if the same thread suddenly becomes the carrier thread to a different HTTP request, request-local data will be exposed to another request. All of that is my pure speculation based on reading articles, I haven't got time to play with any Servlet 3 implementations (Tomcat 7, GlassFish 3.0.X, etc.). So, the questions: Am I correct to assume that ThreadLocal will cease to be a convenient hack to keep the request data? Has anybody played with any of Servlet 3 implementations and tried using ThreadLocals to prove the above? Apart from storing data inside HTTP Session, are there any other similar easy-to-reach hacks you could possibly advise?

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  • MySQL with Java: Open connection only if possible

    - by emempe
    I'm running a database-heavy Java application on a cluster, using Connector/J 5.1.14. Therefore, I have up to 150 concurrent tasks accessing the same MySQL database. I get the following error: Exception in thread "main" com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Too many connections This happens because the server can't handle so many connections. I can't change anything on the database server. So my question is: Can I check if a connection is possible BEFORE I actually connect to the database? Something like this (pseudo code): check database for open connection slots if (slot is free) { Connection cn = DriverManager.getConnection(url, username, password); } else { wait ... } Cheers

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  • Which one is better apporach comet or ajax?

    - by Rajesh Rolen- DotNet Developer
    i need to update client on any changes occurred on server. for that i found 2 approach. 1. using ajax which is also known as reverse ajax for this purpose. 2. using COMET. but i don't know exact difference in both. my site contains news content and i want that news to be automatically updates when new news is entered by my CMS application. i have got hundreds of concurrent users on my web application. please suggest me which approach should i use to get best solution. also please provide me good example's like for that so that i can implement it. NOTE: i am using .net framework 2.0 but if its not possible in 2.0 then can also move to 3.5 Thanks.

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  • JPA in distributed Java EE configuration

    - by sof
    Hello, I'm developing a JEE application to run on Glassfish: Database (javaDB, MS SQL, MySQL or Oracle) EJB layer with JPA (Toplink essentials - from Glassfish) for database access JSF/Icefaces based web UI accessing the EJB layer The application will have a lot of concurrent web client, so I want to run it on different physical servers and use a load-balancer. My problem is now how to keep the applications synchronized. I intend to set up multiple servers, each running Glassfish with my EAR app installed. Whenever on one of the servers data is added to or removed from the database (via JPA, no direct SQL queries), this change should be reflected in the JPA layer on the other servers. I've been looking around for solutions to this, but couldn't find anything I really like (the full Toplink from Oracle claims to have a solution, but don't know). Doing a refresh before every access to a JPA entity could work, but is far from efficient. Are there any patterns, libraries, ... that could help here? Thanks a lot!

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  • Good code visualization / refactoring tools for C++?

    - by Paul D.
    I've found myself coming across a lot of reasonably large, complicated codebases at work recently which I've been asked to either review or refactor or both. This can be extremely time consuming when the code is highly concurrent, makes heavy use of templates (particularly static polymorphism) and has logic that depends on callbacks/signals/condition variables/etc. Are there any good visualization tools for C++ period, and of those are there any that actually play well with "advanced" C++ features? Anything would probably be better than my approach now, which is basically pen+paper or stepping through the debugger. The debugger method can be good for following a particular code path, but isn't great for seeing the big picture you really need when doing serious refactoring. EDIT: I should mention that Visual Studio plugins aren't going to be a lot of help to me, since our stuff is mostly Linux-only.

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  • Understanding Clojure concurrency example

    - by dusha
    Hello, I just go through various documentation on Clojure concurrency and came accross the example on the website (http://clojure.org/concurrent_programming). (import '(java.util.concurrent Executors)) (defn test-stm [nitems nthreads niters] (let [refs (map ref (replicate nitems 0)) pool (Executors/newFixedThreadPool nthreads) tasks (map (fn [t] (fn [] (dotimes [n niters] (dosync (doseq [r refs] (alter r + 1 t)))))) (range nthreads))] (doseq [future (.invokeAll pool tasks)] (.get future)) (.shutdown pool) (map deref refs))) I understand what it does and how it works, but I don't get why the second anonymous function fn[] is needed? Many thanks, dusha. P.S. Without this second fn [] I get NullPointerException.

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  • how to pass session variable to model in RoR?

    - by siulamvictor
    I used a global variable in my app for passing information before. But I got a problem and thanks everyone here suggested me to store those data in session with database. I tried, but I found that I can't access the session variable in Model. I googled and knew this is the Model normal behavior, RoR won't pass the session variable to Model. So how to pass the value of the session variable into Models? or is there any other method for my use case? I need a variable storing a value, which is required in all MVCs, and should be independent between different concurrent users. Thanks everyone. :)

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  • Java - JPA - @Version annotation

    - by Yatendra Goel
    I am new to JPA. I am cofused about the @Version annotation. How it works? I have googled it and found various answers whose extract is as follows: JPA uses a version field in your entities to detect concurrent modifications to the same datastore record. When the JPA runtime detects an attempt to concurrently modify the same record, it throws an exception to the transaction attempting to commit last. But still I am not sure how it works? ================================================================================== Also as from the following lines: You should consider version fields immutable. Changing the field value has undefined results. Does it mean that we should declare our version field as final

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  • Find maximum number of logged on users in SQL

    - by lleto
    Hi, I want to keep tabs on the number of concurrent users of my application. I therefore log a time_start and a time_stop. If I now want to query the database for the maximum number of logged on users and return the start date, how would I do that. The table looks like this: id | time_start | time_stop ----+---------------------+--------------------- 1 | 2010-03-07 05:40:59 | 2010-03-07 05:41:33 2 | 2010-03-07 06:50:51 | 2010-03-07 10:50:51 3 | 2010-02-21 05:20:00 | 2010-03-07 12:23:44 4 | 2010-02-19 08:21:12 | 2010-03-07 12:37:28 5 | 2010-02-13 05:52:13 | Where time_stop is empty the user is still logged on. In this case I would expect to see 2010-03-07 returned, since all users (5) were logged on at that moment. However if I would run the query with 'where time_start BETWEEN '2010-02-17' AND '2010-02-23' I would expect to see 2010-02-21 with a maximum of 2. Is this possible direct in SQL (using postgres) or do I need to parse the results in PHP? Thanks, lleto

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  • Database suggestion

    - by superartsy
    I am working on an enterprise level product that is designed around SQL Express Database ( views,concurrent users,stored procedures,case,IF).Though we dont use any SQL Server advanced features, if the database size of 4Gb in express ends up being a limitation customers can move to advanced versions of SQL Server. However my problem is, SQL Express deployment is not easy and the installer size is huge. This is a major drawback for someone looking to try our product.You dont want end users to not buy a product becos the download is huge. Does anyone have any recommendations of a database that has a smaller footprint but all the features of Express and which can be migrated to express?

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  • How do I choose a database?

    - by liamzebedee
    I need a comparison table of some sort for database varieties (MySQL, SQLite etc.). I can't find one. My use case is, I am implementing storage of objects in a distributed hash table. I need a database solution that is: Fast for sorting Simplistic (no users, preferably no additional structures like multiple tables etc.) Concurrent (if possible) Multi-platform File based (not stored in memory primarily) Centralized I will be programming in Go. As I understand, I believe I need what is called a Document Orientated Database, because I am storing objects, identified by keys. EDIT: While I am implementing a DHT, I will also be storing metadata about the objects, such as access counts etc. It would also be preferable to have TLL (time to live)

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  • Java 7 New Features

    - by John W.
    I have done some good reading on the new java.util.concurrent features being introduced with the java 7 release. For instance, Phaser, TransferQueue and the more exciting Fork Join Framework. I recently saw a power point made by Josh Bloch about even more features that are going to be introduced however that link has been lost. For example I remember one change is being able to build a Map the same way you can build an array for: Map myMap = {"1,Dog","2,Cat"}; and so forth (this may not be 100% correct but the idea is there) Does anyone know of a list or just can name some new things to look forward to? Note: I did see a question asked http://stackoverflow.com/questions/213958/new-features-in-java-7 however it was asked ~2 years ago and I am sure the list of updates are more concrete. Thanks!

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  • PHP no wait sem_acquire?

    - by SerEnder
    Not a specific code question, but more of a general coding question. I'm trying to use a semaphore in a work project to limit the number of users that can access certain processes at a concurrent time. From my understanding the following: $iKey = ftock($sSomeFileLocation,'sOneCharacterString'); //Generate the key if($sem_id = sem_get($iKey){ //1 user allowed if(sem_acquire($sem_id)){ //Do the limited process here sem_release($sem_id); } } The problem that I see here is that if there is already one user who has the semaphore key, then the next user just waits until the first user is done rather than just faulting out. Anyone know of a way that if the max_acquire number has been reached, sem_acquire (or similar) will just return false? Thanks

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  • Why are C, C++, and LISP so prevalent in embedded devices and robots?

    - by David
    It seems that the software language skills most sought for embedded devices and robots are C, C++, and LISP. Why haven't more recent languages made inroads into these applications? For example, Erlang would seem particularly well-suited to robotic applications, since it makes concurrent programming easier and allows hot swapping of code. Python would seem to be useful, if for no other reason than its support of multiple programming paradigms. I'm even surprised that Java hasn't made a foray into general robotic programming. I'm sure one argument would be, "Some newer languages are interpreted, not compiled" - implying that compiled languages are quicker and use fewer computational resources. Is this still the case, in a time when we can put a Java Virtual Machine on a cell phone or a SunSpot? (and isn't LISP interpreted anyway?)

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  • C# similar technologies and books in Java

    - by MigNix
    Hi, I know this can look like dublicate of other discussions and I have already reviewed them, but they are different. I want to learn Java and I want to know are there any books similar to those that I have seen for C#. Here is the list: C# 4.0 in a Nutshell: The Definitive Reference Probably Java in a Nutshell :) Professional C# 4.0 and .NET 4 Pro C# 2010 and the .NET 4 Platform Accelerated C# 2010 Programming WCF Services Essential Windows Communication Foundation (WCF): For .NET Framework 3.5 C# in Depth: What you need to master C# 2 and 3 CLR via C# Professional ADO.NET 3.5 with LINQ and the Entity Framework Professional Enterprise .NET Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries Mainly I am interested in Java 6 and want to know about JVM , Cuncurency, Data access and distributed application technologies. I have seen there are some books on JVM like Inside the Java 2 Virtual Machine also some free books on official web site. Also I found concurrent programming in Java. But may be you can suggest any others. Thank you

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  • Chat Server with sockets in C

    - by Andrew
    I'm trying to write a chat server in C that allows communication between two clients using POSIX sockets. I'm not sure I have a good grasp on this concept or how I should set up the communication protocol between the clients and the server. I know I need one socket to bind() the server port to so I can accept incoming connections from clients, but in order to have two clients connected at the same time do I need to create a socket for each of these clients when I accept() or should I accept() a client and then fork() so I can have another client accept? I'm not worried about concurrent chatting yet, it's more of a ping-pong approach where the clients need to wait for a recv() after they send() before they can type a new message.

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  • Suspend TimerTask until the next execution

    - by user1052518
    I am using a TimerTask to run some periodic tasks, the task being processing a set of files. I have a requirement where if the number of files to be processed exceeds a pre-determined limit, the thread suspends execution and waits till the next cycle to start processing the files again. Is there a way to suspend the TimerTask until the next execution period or do I have to extend the TimerTask class to achieve this functionality? I saw there is a TimerTask.cancel method, but this will cancel all further executions of this thread. I don't want this to happen. I just want the thread to be suspended until the next execution period. I don't have the luxury of moving to any of the other concurrent classes in Java as our framework uses TimerTask, and I have to stick with it. Any suggestions, pointers or tips are greatly appreciated. thanks, Asha

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