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• Clarification on Threads and Run Loops In Cocoa

  - by dubbeat

  I'm trying to learn about threading and I'm thoroughly confused. I'm sure all the answers are there in the apple docs but I just found it really hard to breakdown and digest. Maybe somebody could clear a thing or 2 up for me. 1)performSelectorOnMainThread Does the above simply register an event in the main run loop or is it somehow a new thread even though the method says "mainThread"? If the purpose of threads is to relieve processing on the main thread how does this help? 2) RunLoops Is it true that if I want to create a completely seperate thread I use "detachNewThreadSelector"? Does calling start on this initiate a default run loop for the thread that has been created? If so where do run loops come into it? 3) And Finally , I've seen examples using NSOperationQueue. Is it true to say that If you use performSelectorOnMainThread the threads are in a queue anyway so NSOperation is not needed? 4) Should I forget about all of this and just use the Grand Central Dispatch instead?

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• ReaderWriterLockSlim question.

  - by Kamarey

  There are lots written about the ReaderWriterLockSlim class which allows multiple read and a single write. All of these (at least that I had found) tell how to use it without much explanation why and how it works. The standard code sample is: lock.EnterUpgradeableReadLock(); try { if (test if write is required) { lock.EnterWriteLock(); try { change the resourse here. } finally { lock.ExitWriteLock(); } } } finally { lock.ExitUpgradeableReadLock(); } The question is: if upgradeable lock permits only a single thread to enter its section, why I should call EnterWriteLock method within? What will happen if I don't? Or what will happen if instead of EnterUpgradeableReadLock I will call EnterWriteLock and will write to a resource without using upgradeable lock at all?

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• Various way to stop a thread - which is the correct way

  - by Yan Cheng CHEOK

  I had came across different suggestion of stopping a thread. May I know, which is the correct way? Or it depends? Using Thread Variable http://download.oracle.com/javase/1.4.2/docs/guide/misc/threadPrimitiveDeprecation.html private volatile Thread blinker; public void stop() { blinker = null; } public void run() { Thread thisThread = Thread.currentThread(); while (blinker == thisThread) { try { thisThread.sleep(interval); } catch (InterruptedException e){ } repaint(); } } Using boolean flag private volatile boolean flag; public void stop() { flag = false; } public void run() { while (flag) { try { thisThread.sleep(interval); } catch (InterruptedException e){ } repaint(); } } Using Thread Variable together with interrupt private volatile Thread blinker; public void stop() { blinker.interrupt(); blinker = null; } public void run() { Thread thisThread = Thread.currentThread(); while (!thisThread.isInterrupted() && blinker == thisThread) { try { thisThread.sleep(interval); } catch (InterruptedException e){ } repaint(); } }

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• Swing modal dialog refuses to close - sometimes!

  - by Zarkonnen

  // This is supposed to show a modal dialog and then hide it again. In practice, // this works about 75% of the time, and the other 25% of the time, the dialog // stays visible. // This is on Ubuntu 10.10, running: // OpenJDK Runtime Environment (IcedTea6 1.9) (6b20-1.9-0ubuntu1) // This always prints // setVisible(true) about to happen // setVisible(false) about to happen // setVisible(false) has just happened // even when the dialog stays visible. package modalproblemdemo; import java.awt.Frame; import javax.swing.JDialog; import javax.swing.SwingUtilities; public class Main { public static void main(String[] args) { final Dialogs d = new Dialogs(); new Thread() { @Override public void run() { d.show(); d.hide(); } }.start(); } static class Dialogs { final JDialog dialog; public Dialogs() { dialog = new JDialog((Frame) null, "Hello World", /*modal*/ true); dialog.setSize(400, 200); } public void show() { SwingUtilities.invokeLater(new Runnable() { public void run() { dialog.setLocationRelativeTo(null); System.out.println("setVisible(true) about to happen"); dialog.setVisible(true); }}); } public void hide() { SwingUtilities.invokeLater(new Runnable() { public void run() { System.out.println("setVisible(false) about to happen"); dialog.setVisible(false); System.out.println("setVisible(false) has just happened"); }}); } } }

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• Using the windows api and C++, how could I load an exe from the hard drive and run it in its own thread?

  - by returneax

  For the sake of learning I'm trying to do what the OS does when launching a program ie. parsing a PE file and giving it a thread of execution. If I have two exe's one called foo.exe and the other bar.exe, how could I have foo.exe load the contents of bar.exe into memory then have it execute from there in its own thread? I know how to get it into memory using MapViewOfFile or by simple loading the contents on the hard drive into a buffer. I'm assuming simply copying the contents of bar.exe on disk into its own suspended thread and running it wouldn't work. I am semi-familiar with PE file internals. All help is very much appreciated, of course :)

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• Java Thread - Memory consistency errors

  - by Yatendra Goel

  I was reading a Sun's tutorial on Concurrency. But I couldn't understand exactly what memory consistency errors are? I googled about that but didn't find any helpful tutorial or article about that. I know that this question is a subjective one, so you can provide me links to articles on the above topic. It would be great if you explain it with a simple example.

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• How to prevent the other threads from accessing a method when one thread is accessing a method?

  - by geeta

  I want to search for a string in 10 files and write the matching lines to a single file. I wrote the matching lines from each file to 10 output files(o/p file1,o/p file2...) and then copied those to a single file using 10 threads. But the output single file has mixed output(one line from o/p file1,another line from o/p file 2 etc...) because its accessed simultaneously by many threads. If I wait for all threads to complete and then write the single file it will be much slower. I want the output file to be written by one thread at a time. What should i do? My source code:(only writing to single file method) public void WriteSingle(File output_file,File final_output) throws IOException { synchronized(output_file){ System.out.println("Writing Single file"); FileOutputStream fo = new FileOutputStream(final_output,true); FileChannel fi = fo.getChannel(); FileInputStream fs = new FileInputStream(output_file); FileChannel fc = fs.getChannel(); int maxCount = (64 * 1024 * 1024) - (32 * 1024); long size = fc.size(); long position = 0; while (position < size) { position += fc.transferTo(position, maxCount, fi); } } }

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• Background thread in C#

  - by Xodarap

  When the user saves some data, I want to spin off a background thread to update my indexes and do some other random stuff. Even if there is an error in this indexing the user can't do anything about it, so there is no point in forcing the main thread to wait until the background thread finishes. I'm doing this from a ASP.NET process, so I think I should be able to do this (as the main thread exiting won't kill the process). When I set a breakpoint in the background thread's method though, the main thread also appears to stop. Is this just an artifact of visual studio's debugger, or is the main thread really not going to return until the background thread stops?

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• Parallel software?

  - by mavric

  What is the meaning of "parallel software" and what are the differences between "parallel software" and "regular software"? What are its advantages and disadvantages? Does writing "parallel software" require a specific hardware or programming language ?

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• Why flex doesn't supports threads ?

  - by Silent Warrior

  Why flex/action script is not supporting threads currently ?

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• Queue access to the database to avoid multiple cache items

  - by MikeJ

  I have a music related ASP.NET web site which caches a lot of static information from the database on the first request. Sometimes, the application is reset and cache is cleared while the application is on heavy load and then all http requests go to the database to retrieve that static data and cache it for other requests. How can I ensure that only one request go to the database and cache the results, so that other request simply read that info from cache and not needlessly retrieve the same info over and over again. Can I use thread locking? For example, can I do something like lock(this) { db access here }?

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• Boost threading/mutexs, why does this work?

  - by Flamewires

  Code: #include <iostream> #include "stdafx.h" #include <boost/thread.hpp> #include <boost/thread/mutex.hpp> using namespace std; boost::mutex mut; double results[10]; void doubler(int x) { //boost::mutex::scoped_lock lck(mut); results[x] = x*2; } int _tmain(int argc, _TCHAR* argv[]) { boost::thread_group thds; for (int x = 10; x>0; x--) { boost::thread *Thread = new boost::thread(&doubler, x); thds.add_thread(Thread); } thds.join_all(); for (int x = 0; x<10; x++) { cout << results[x] << endl; } return 0; } Output: 0 2 4 6 8 10 12 14 16 18 Press any key to continue . . . So...my question is why does this work(as far as i can tell, i ran it about 20 times), producing the above output, even with the locking commented out? I thought the general idea was: in each thread: calculate 2*x copy results to CPU register(s) store calculation in correct part of array copy results back to main(shared) memory I would think that under all but perfect conditions this would result in some part of the results array having 0 values. Is it only copying the required double of the array to a cpu register? Or is it just too short of a calculation to get preempted before it writes the result back to ram? Thanks.

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• Are there any tools to optimize the number of consumer and producer threads on a JMS queue?

  - by lindelof

  I'm working on an application that is distributed over two JBoss instances and that produces/consumes JMS messages on several JMS queues. When we configured the application we had to determine which threading model we would use, in particular the number of producing and consuming threads per queue. We have done this in a rather ad-hoc fashion but after reading the most recent columns by Herb Sutter in Dr Dobbs (in particular this one) I would like to size our threads in a more rigorous manner. Are there any methods/tools to measure the throughput of JMS queues (in particular JBoss Messaging queues) as a function of the number of producing/consuming threads?

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• Explicit call of Runnable.run

  - by klaudio

  Hi, I have a question. Somebody, who was working on my code before me, created some method and passed Runnable as parameter, more likely: void myMethod(Runnable runnable){ runnable.run(); } Then calling myMethod out of main looks like: public static void main(String args[]) { try { myMethod(new Runnable(){ public void run() { //do something...; }}); } catch (Throwable t) { } } So, to supply parameter to myMethod I need to instantiate object of (in this case anonymous) class implementing Runnable. My question is: is it necessary to use Runnable in this example? Can I use any different interface? I mean I can create new interface with single method i.e. interface MyInterface{ void doThis(); } then change look of myMethod: void myMethod(MyInterface myObject){ myObject.doThis(); } And of course client too: public static void main(String args[]) { try { myMethod(new MyInterface (){ public void doThis() { //do something...; }}); } catch (Throwable t) { } } Or maybe something is about Runnable?!

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• C++ - how does Sleep() and cin work?

  - by quano

  Just curious. How does actually the function Sleep() work (declared in windows.h)? Maybe not just that implementation, but anyone. With that I mean - how is it implemented? How can it make the code "stop" for a specific time? Also curious about how cin and those actually work. What do they do exactly? The only way I know how to "block" something from continuing to run is with a while loop, but considering that that takes a huge amount of processing power in comparison to what's happening when you're invoking methods to read from stdin (just compare a while (true) to a read from stdin), I'm guessing that isn't what they do.

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• What are some good ways to do intermachine locking?

  - by mike

  Our server cluster consists of 20 machines, each with 10 pids of 5 threads. We'd like some way to prevent any two threads, in any pid, on any machine, from modifying the same object at the same time. Our code's written in Python and runs on Linux, if that helps narrow things down. Also, it's a pretty rare case that two such threads want to do this, so we'd prefer something that optimizes the "only one thread needs this object" case to be really fast, even if it means that the "one thread has locked this object and another one needs it" case isn't great. What are some of the best practices?

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• Why does this threading approach not work?

  - by Tomas Lycken

  I have a wierd problem with threading in an ASP.NET application. For some reason, when I run the code in the request thread, everything works as expected. But when I run it in a separate thread, nothing happens. This is verified by calling the below handler with the three flags "on", "off" and "larma" respectively - in the two first cases everything works, but in the latter nothing happens. What am I doing wrong here? In the web project I have a generic handler with the following code: If task = "on" Then Alarm.StartaLarm(personId) context.Response.Write("Larmet är PÅ") ElseIf task = "off" Then Alarm.StoppaLarm(personId) context.Response.Write("Larmet är AV") ElseIf task = "larma" Then Alarm.Larma(personId) context.Response.Write("Larmar... (stängs av automagiskt)") Else context.Response.Write("inget hände - task: " & task) End If The Alarm class has the following methods: Private Shared Sub Larma_Thread(ByVal personId As Integer) StartaLarm(personId) Thread.Sleep(1000 * 30) StoppaLarm(personId) End Sub Public Shared Sub StartaLarm(ByVal personId As Integer) SandSMS(True, personId) End Sub Public Shared Sub StoppaLarm(ByVal personId As Integer) SandSMS(False, personId) End Sub Public Shared Sub SandSMS(ByVal setOn As Boolean, ByVal personId As Integer) ... End Sub

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• Can two threads of the same process produce the same GUID?

  - by mark

  Dear ladies and sirs. If two threads in a process generate a new GUID concurrently using .NET API (Guid.NewGuid()) is it possible that the two GUIDs will be identical? Thanks. UPDATE I want to get practical. I know that it is widely assumed that GUIDs are unique for all practical purposes. I am wondering if I can treat GUIDS produced by the different threads of the same process in the same manner.

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• passing parameters to a thread

  - by assassin

  I want to pass a function that takes a parameter to the ThreadStart Constructor in C#. But, it seems that this is not possible, as I get a syntax error it I try to do something like this Thread t1 = new Thread(new Thread Start(func1(obj1)); where obj1 is an object of type List<string> (say). If I want a thread to execute this function that takes in an object as a parameter, and I plan to create 2 such threads simultaneously with different parameter values what is the best method to achieve this?

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• Performing time consuming operation on STL container within a lock

  - by Ashley

  I have an unordered_map of an unordered_map which stores a pointer of objects. The unordered map is being shared by multiple threads. I need to iterate through each object and perform some time consuming operation (like sending it through network etc) . How could I lock the multiple unordered_map so that it won't blocked for too long? typedef std::unordered_map<string, classA*>MAP1; typedef std::unordered_map<int, MAP1*>MAP2; MAP2 map2; pthread_mutex_lock(&mutexA) //how could I lock the maps? Could I reduce the lock granularity? for(MAP2::iterator it2 = map2.begin; it2 != map2.end; it2++) { for(MAP1::iterator it1 = *(it2->second).begin(); it1 != *(it2->second).end(); it1++) { //perform some time consuming operation on it1->second eg sendToNetwork(*(it1->second)); } } pthread_mutex_unlock(&mutexA)

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• VB.NET Two different approaches to generic cross-threaded operations; which is better?

  - by BASnappl

  VB.NET 2010, .NET 4 Hello, I recently read about using SynchronizationContext objects to control the execution thread for some code. I have been using a generic subroutine to handle (possibly) cross-thread calls for things like updating UI controls that utilizes Invoke. I'm an amateur and have a hard time understanding the pros and cons of any particular approach. I am looking for some insight on which approach might be preferable and why. Update: This question is motivated, in part, by statements such as the following from the MSDN page on Control.InvokeRequired. An even better solution is to use the SynchronizationContext returned by SynchronizationContext rather than a control for cross-thread marshaling. Method 1: Public Sub InvokeControl(Of T As Control)(ByVal Control As T, ByVal Action As Action(Of T)) If Control.InvokeRequired Then Control.Invoke(New Action(Of T, Action(Of T))(AddressOf InvokeControl), New Object() {Control, Action}) Else Action(Control) End If End Sub Method 2: Public Sub UIAction(Of T As Control)(ByVal Control As T, ByVal Action As Action(Of Control)) SyncContext.Send(New Threading.SendOrPostCallback(Sub() Action(Control)), Nothing) End Sub Where SyncContext is a Threading.SynchronizationContext object defined in the constructor of my UI form: Public Sub New() InitializeComponent() SyncContext = WindowsFormsSynchronizationContext.Current End Sub Then, if I wanted to update a control (e.g., Label1) on the UI form, I would do: InvokeControl(Label1, Sub(x) x.Text = "hello") or UIAction(Label1, Sub(x) x.Text = "hello") So, what do y'all think? Is one way preferred or does it depend on the context? If you have the time, verbosity would be appreciated! Thanks in advance, Brian

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• REST WCF service locks thread when called using AJAX in an ASP.Net site

  - by Jupaol

  I have a WCF REST service consumed in an ASP.Net site, from a page, using AJAX. I want to be able to call methods from my service async, which means I will have callback handlers in my javascript code and when the methods finish, the output will be updated. The methods should run in different threads, because each method will take different time to complete their task I have the code semi-working, but something strange is happening because the first time I execute the code after compiling, it works, running each call in a different threads but subsequent calls blocs the service, in such a way that each method call has to wait until the last call ends in order to execute the next one. And they are running on the same thread. I have had the same problem before when I was using Page Methods, and I solved it by disabling the session in the page but I have not figured it out how to do the same when consuming WCF REST services Note: Methods complete time (running them async should take only 7 sec and the result should be: Execute1 - Execute3 - Execute2) Execute1 -- 2 sec Execute2 -- 7 sec Execute3 -- 4 sec Output After compiling Output subsequent calls (this is the problem) I will post the code...I'll try to simplify it as much as I can Service Contract [ServiceContract( SessionMode = SessionMode.NotAllowed )] public interface IMyService { // I have other 3 methods like these: Execute2 and Execute3 [OperationContract] [WebInvoke( RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json, UriTemplate = "/Execute1", Method = "POST")] string Execute1(string param); } [AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)] [ServiceBehavior( InstanceContextMode = InstanceContextMode.PerCall )] public class MyService : IMyService { // I have other 3 methods like these: Execute2 (7 sec) and Execute3(4 sec) public string Execute1(string param) { var t = Observable.Start(() => Thread.Sleep(2000), Scheduler.NewThread); t.First(); return string.Format("Execute1 on: {0} count: {1} at: {2} thread: {3}", param, "0", DateTime.Now.ToString(), Thread.CurrentThread.ManagedThreadId.ToString()); } } ASPX page <%@ Page EnableSessionState="False" Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="RestService._Default" %> <asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent"> <script type="text/javascript"> function callMethodAsync(url, data) { $("#message").append("<br/>" + new Date()); $.ajax({ cache: false, type: "POST", async: true, url: url, data: '"de"', contentType: "application/json", dataType: "json", success: function (msg) { $("#message").append("<br/>&nbsp;&nbsp;&nbsp;" + msg); }, error: function (xhr) { alert(xhr.responseText); } }); } $(function () { $("#callMany").click(function () { $("#message").html(""); callMethodAsync("/Execute1", "hello"); callMethodAsync("/Execute2", "crazy"); callMethodAsync("/Execute3", "world"); }); }); </script> </asp:Content> <asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent"> <input type="button" id="callMany" value="Post Many" /> <div id="message"> </div> </asp:Content> Web.config (relevant) <system.webServer> <modules runAllManagedModulesForAllRequests="true" /> </system.webServer> <system.serviceModel> <serviceHostingEnvironment aspNetCompatibilityEnabled="true" multipleSiteBindingsEnabled="true" /> <standardEndpoints> <webHttpEndpoint> <standardEndpoint name="" helpEnabled="true" automaticFormatSelectionEnabled="true" /> </webHttpEndpoint> </standardEndpoints> </system.serviceModel> Global.asax void Application_Start(object sender, EventArgs e) { RouteTable.Routes.Ignore("{resource}.axd/{*pathInfo}"); RouteTable.Routes.Add(new ServiceRoute("", new WebServiceHostFactory(), typeof(MyService))); }

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• Best way to implement game loop without freezing UI thread

  - by Matt H

  I'm trying to make a simple 2D game in Java. So far I have a JFrame, with a menubar, and a class which extends JPanel and overrides it's paint method. Now, I need to get a game loop going, where I will update the position of images and so on. However, I'm stuck at how best to achieve this. Should I use multi-threading, because surely, if you put an infinite loop on the main thread, the UI (and thus my menu bar) will freeze up? Here's my code so far: import java.awt.Color; import java.awt.Graphics; import javax.swing.JPanel; @SuppressWarnings("serial") public class GameCanvas extends JPanel { public void paint(Graphics g) { while (true) { g.setColor(Color.DARK_GRAY); try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); } } } } import javax.swing.JFrame; import javax.swing.JMenu; import javax.swing.JMenuBar; import javax.swing.JMenuItem; @SuppressWarnings("serial") public class Main extends JFrame { GameCanvas canvas = new GameCanvas(); final int FRAME_HEIGHT = 400; final int FRAME_WIDTH = 400; public static void main(String args[]) { new Main(); } public Main() { super("Game"); JMenuBar menuBar = new JMenuBar(); JMenu fileMenu = new JMenu("File"); JMenuItem startMenuItem = new JMenuItem("Pause"); menuBar.add(fileMenu); fileMenu.add(startMenuItem); super.add(canvas); super.setVisible(true); super.setSize(FRAME_WIDTH, FRAME_WIDTH); super.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); super.setJMenuBar(menuBar); } } Any pointers/tips? Also, where should I put my loop? In my main class, or my GameCanvas class? Any help is appreciated, thanks.

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• Does an asynchronous call always create/call a new thread?

  - by Ted Smith

  Does asynchronous call always create a new thread? Example: If Javascript is single threaded then how can it do an async postback? Is it actually blocking until it gets a callback? If so, is this really an async call?

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• multi-thread access MySQL error

  - by user188916

  I have written a simple multi-threaded C program to access MySQL,it works fine except when i add usleep() or sleep() function in each thread function. i created two pthreads in the main method, int main(){ mysql_library_init(0,NULL,NULL); printf("Hello world!\n"); init_pool(&p,100); pthread_t producer; pthread_t consumer_1; pthread_t consumer_2; pthread_create(&producer,NULL,produce_fun,NULL); pthread_create(&consumer_1,NULL,consume_fun,NULL); pthread_create(&consumer_2,NULL,consume_fun,NULL); mysql_library_end(); } void * produce_fun(void *arg){ pthread_detach(pthread_self()); //procedure while(1){ usleep(500000); printf("producer...\n"); produce(&p,cnt++); } pthread_exit(NULL); } void * consume_fun(void *arg){ pthread_detach(pthread_self()); MYSQL db; MYSQL *ptr_db=mysql_init(&db); mysql_real_connect(); //procedure while(1){ usleep(1000000); printf("consumer..."); int item=consume(&p); addRecord_d(ptr_db,"test",item); } mysql_thread_end(); pthread_exit(NULL); } void addRecord_d(MYSQL *ptr_db,const char *t_name,int item){ char query_buffer[100]; sprintf(query_buffer,"insert into %s values(0,%d)",t_name,item); //pthread_mutex_lock(&db_t_lock); int ret=mysql_query(ptr_db,query_buffer); if(ret){ fprintf(stderr,"%s%s\n","cannot add record to ",t_name); return; } unsigned long long update_id=mysql_insert_id(ptr_db); // pthread_mutex_unlock(&db_t_lock); printf("add record (%llu,%d) ok.",update_id,item); } the program output errors like: [Thread debugging using libthread_db enabled] [New Thread 0xb7ae3b70 (LWP 7712)] Hello world! [New Thread 0xb72d6b70 (LWP 7713)] [New Thread 0xb6ad5b70 (LWP 7714)] [New Thread 0xb62d4b70 (LWP 7715)] [Thread 0xb7ae3b70 (LWP 7712) exited] producer... producer... consumer...consumer...add record (31441,0) ok.add record (31442,1) ok.producer... producer... consumer...consumer...add record (31443,2) ok.add record (31444,3) ok.producer... producer... consumer...consumer...add record (31445,4) ok.add record (31446,5) ok.producer... producer... consumer...consumer...add record (31447,6) ok.add record (31448,7) ok.producer... Error in my_thread_global_end(): 2 threads didn't exit [Thread 0xb72d6b70 (LWP 7713) exited] [Thread 0xb6ad5b70 (LWP 7714) exited] [Thread 0xb62d4b70 (LWP 7715) exited] Program exited normally. and when i add pthread_mutex_lock in function addRecord_d,the error still exists. So what exactly the problem is?

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