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  • prevent race condition without using locks C++

    - by Hristo
    How do I prevent a race condition with locking or using mutexes/semaphors in C++? I'm dealing with a nested for loop in which I will be setting a value in an array: for (int i = 0; i < m; ++i) for (int j = 0; j < n; ++j) for (int k = 0; k < o; ++k) array[k] += foo(...); More or less, I want to deal with this so that I can ensure different threads running at the same time don't write to array[k] at the same time. Any suggestions on how to approach this? Thanks, Hristo

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  • Rails - How do I write this string condition as an array condition

    - by adam
    named_scope :correct, :include => :correction, :conditions => "checked_at IS NOT NULL AND corrections.id IS NULL" On a side note I have googled loads and looked through books but i cant seem to find a list of all the various types of conditions you can use and how they differ when implenting them as strings, arrays or hashes. Is there a list of the syntax anywhere?

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  • What about race condition in multithreaded reading?

    - by themoob
    Hi, According to an article on IBM.com, "a race condition is a situation in which two or more threads or processes are reading or writing some shared data, and the final result depends on the timing of how the threads are scheduled. Race conditions can lead to unpredictable results and subtle program bugs." . Although the article concerns Java, I have in general been taught the same definition. As far as I know, simple operation of reading from RAM is composed of setting the states of specific input lines (address, read etc.) and reading the states of output lines. This is an operation that obviously cannot be executed simultaneously by two devices and has to be serialized. Now let's suppose we have a situation when a couple of threads access an object in memory. In theory, this access should be serialized in order to prevent race conditions. But e.g. the readers/writers algorithm assumes that an arbitrary number of readers can use the shared memory at the same time. So, the question is: does one have to implement an exclusive lock for read when using multithreading (in WinAPI e.g.)? If not, why? Where is this control implemented - OS, hardware? Best regards, Kuba

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  • Why do condition variables sometimes erroneously wake up?

    - by aspo
    I've known for eons that the way you use a condition variable is lock while not task_done wait on condition variable unlock Because sometimes condition variables will spontaneously wake. But I've never understood why that's the case. In the past I've read it's expensive to make a condition variable that doesn't have that behavior, but nothing more than that. So... why do you need to worry about falsely being woken up when waiting on a condition variable?

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  • Elusive race condition in Java

    - by nasufara
    I am creating a graphing calculator. In an attempt to squeeze some more performance out of it, I added some multithreaded to the line calculator. Essentially what my current implementation does is construct a thread-safe Queue of X values, then start however many threads it needs, each one calculating a point on the line using the queue to get its values, and then ordering the points using a HashMap when the calculations are done. This implementation works great, and that's not where my race condition is (merely some background info). In examining the performance results from this, I found that the HashMap is a performance bottleneck, since I do that synchronously on one thread. So I figured that ordering each point as its calculated would work best. I tried a PriorityQueue, but that was slower than the HashMap. I ended up creating an algorithm that essentially works like this: I construct a list of X values to calculate, like in my current algorithm. I then copy that list of values into another class, unimaginatively and temporarily named BlockingList, which is responsible for ordering the points as they are calculated. BlockingList contains a put() method, which takes in two BigDecimals as parameters, the first the X value, the second the calculated Y value. put() will only accept a value if the X value is the next one on the list to be accepted in the list of X values, and will block until another thread gives it the next excepted value. For example, since that can be confusing, say I have two threads, Thread-1 and Thread-2. Thread-2 gets the X value 10.0 from the values queue, and Thread-1 gets 9.0. However, Thread-1 completes its calculations first, and calls put() before Thread-2 does. Because BlockingList is expecting to get 10.0 first, and not 9.0, it will block on Thread-1 until Thread-2 finishes and calls put(). Once Thread-2 gives BlockingList 10.0, it notify()s all waiting threads, and expects 9.0 next. This continues until BlockingList gets all of its expected values. (I apologise if that was hard to follow, if you need more clarification, just ask.) As expected by the question title, there is a race condition in here. If I run it without any System.out.printlns, it will sometimes lock because of conflicting wait() and notifyAll()s, but if I put a println in, it will run great. A small implementation of this is included below, and exhibits the same behavior: import java.math.BigDecimal; import java.util.concurrent.ConcurrentLinkedQueue; public class Example { public static void main(String[] args) throws InterruptedException { // Various scaling values, determined based on the graph size // in the real implementation BigDecimal xMax = new BigDecimal(10); BigDecimal xStep = new BigDecimal(0.05); // Construct the values list, from -10 to 10 final ConcurrentLinkedQueue<BigDecimal> values = new ConcurrentLinkedQueue<BigDecimal>(); for (BigDecimal i = new BigDecimal(-10); i.compareTo(xMax) <= 0; i = i.add(xStep)) { values.add(i); } // Contains the calculated values final BlockingList list = new BlockingList(values); for (int i = 0; i < 4; i++) { new Thread() { public void run() { BigDecimal x; // Keep looping until there are no more values while ((x = values.poll()) != null) { PointPair pair = new PointPair(); pair.realX = x; try { list.put(pair); } catch (Exception ex) { ex.printStackTrace(); } } } }.start(); } } private static class PointPair { public BigDecimal realX; } private static class BlockingList { private final ConcurrentLinkedQueue<BigDecimal> _values; private final ConcurrentLinkedQueue<PointPair> _list = new ConcurrentLinkedQueue<PointPair>(); public BlockingList(ConcurrentLinkedQueue<BigDecimal> expectedValues) throws InterruptedException { // Copy the values into a new queue BigDecimal[] arr = expectedValues.toArray(new BigDecimal[0]); _values = new ConcurrentLinkedQueue<BigDecimal>(); for (BigDecimal dec : arr) { _values.add(dec); } } public void put(PointPair item) throws InterruptedException { while (item.realX.compareTo(_values.peek()) != 0) { synchronized (this) { // Block until someone enters the next desired value wait(); } } _list.add(item); _values.poll(); synchronized (this) { notifyAll(); } } } } My question is can anybody help me find the threading error? Thanks!

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  • SQL Server Process Queue Race Condition

    - by William Edmondson
    I have an order queue that is accessed by multiple order processors through a stored procedure. Each processor passes in a unique ID which is used to lock the next 20 orders for its own use. The stored procedure then returns these records to the order processor to be acted upon. There are cases where multiple processors are able to retrieve the same 'OrderTable' record at which point they try to simultaneously operate on it. This ultimately results in errors being thrown later in the process. My next course of action is to allow each processor grab all available orders and just round robin the processors but I was hoping to simply make this section of code thread safe and allow the processors to grab records whenever they like. So Explicitly - Any idea why I am experiencing this race condition and how I can solve the problem. BEGIN TRAN UPDATE OrderTable WITH ( ROWLOCK ) SET ProcessorID = @PROCID WHERE OrderID IN ( SELECT TOP ( 20 ) OrderID FROM OrderTable WITH ( ROWLOCK ) WHERE ProcessorID = 0) COMMIT TRAN SELECT OrderID, ProcessorID, etc... FROM OrderTable WHERE ProcessorID = @PROCID

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  • Race condition for thread startup

    - by Ozzah
    A similar question was asked here, but the answers generally all seem to relate to the lambda notation. I get a similar result without the lambda so I thought I'd ask for some clarification: Say I have something like this: for (int i = 0; i < 5; i++) (new Thread(new ThreadStart(delegate() { Console.WriteLine("Thread " + i); }))).Start(); One would expect the following output: Thread 0 Thread 1 Thread 2 Thread 3 Thread 4 Now I realise that the threads aren't started in any particular order, so let's just assume that the above lines can come out in any order. But that is not what happens. What instead happens: Thread 3 Thread 4 Thread 4 Thread 4 Thread 4 or something similar, which leads me to believe that rather than passing the value if i, it is passing the reference. (Which is weird, since an int is a value type). Doing something like this: for (int i = 0; i < 5; i++) (new Thread(new ThreadStart(delegate() { int j = i; Console.WriteLine("Thread " + j); }))).Start(); does not help either, even though we have made a copy of i. I am assuming the reason is that it hasn't made a copy of i in time. Doing something like this: for (int i = 0; i < 5; i++) { (new Thread(new ThreadStart(delegate() { Console.WriteLine("Thread " + i); }))).Start(); Thread.Sleep(50); } seems to fix the problem, however it is extremely undesirable as we're wasting 50ms on each iteration, not to mention the fact that if the computer is heavily loaded then maybe 50ms may not be enough. Here is a sample with my current, specific problem: Thread t = new Thread(new ThreadStart(delgate() { threadLogic(param1, param2, param3, param4); })); t.Start(); param1 = param2 = param3 = param4 = null; with: void threadLogic(object param1, object param2, object param3, object param4) { // Do some stuff here... } I want threadLogic() to run in its own thread, however the above code gives a null reference exception. I assume this is because the values are set to null before the thread has had a chance to start. Again, putting a Thread.Sleep(100) works, but it is an awful solution from every aspect. What do you guys recommend for this particular type of race condition?

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  • Dropbox’s Great Space Race Delivers Additional Space to Students

    - by Jason Fitzpatrick
    If you’re a student or faculty member (or still have an active .edu email account) now’s the time to cash in on some free cloud storage courtesy of Dropbox’s Great Space Race. Just by linking your .edu address with your Dropbox account you’ll get an extra 3GB of storage for the next two years. The more people from your school that sign up, the higher the total climbs–up to an extra 25GB for two years. The Space Race lasts for the next eight weeks, you can read more about the details here or just jump to the signup page at the link below. The Great Space Race [Dropbox] Why Enabling “Do Not Track” Doesn’t Stop You From Being Tracked HTG Explains: What is the Windows Page File and Should You Disable It? How To Get a Better Wireless Signal and Reduce Wireless Network Interference

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  • Race condition firing events in AS3

    - by crispclean
    Hello I have some troubles firing and removing events in the right chronicle order. The code below gives the following output: save poster into db, and dispatch event calling service, dispatch event removed = false calling service, dispatch event removed = false calling service, dispatch event removed = true save poster into db, and dispatch event save poster into db, and dispatch event of course this should be more something like: save poster into db, and dispatch event calling service, dispatch event removed = true save poster into db, and dispatch event calling service, dispatch event removed = true save poster into db, and dispatch event calling service, dispatch event removed = true Can someone help me with this? I'm running out of ideas on how to tackle this. thx! for(var i:int = 0;i< 3;i++){ createPoster(); } function createPoster(){ Main.db.savePoster(); Main.db.addEventListener(Config.evt_SAVEPOSTER_READY, callService); } function callService(){ Main.db.removeEventListener(Config.evt_SAVEPOSTER_READY, callService); }

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  • Long running method causing race condition

    - by keeleyt83
    Hi, I'm relatively new with hibernate so please be gentle. I'm having an issue with a long running method (~2 min long) and changing the value of a status field on an object stored in the DB. The pseudo-code below should help explain my issue. public foo(thing) { if (thing.getStatus() == "ready") { thing.setStatus("finished"); doSomethingAndTakeALongTime(); } else { // Thing already has a status of finished. Send the user back a message. } } The pseudo-code shouldn't take much explanation. I want doSomethingAndTakeALongTime() to run, but only if it has a status of "ready". My issue arises whenever it takes 2 minutes for doSomethingAndTakeALongTime() to finish and the change to thing's status field doesn't get persisted to the database until it leaves foo(). So another user can put in a request during those 2 minutes and the if statement will evaluate to true. I've already tried updating the field and flushing the session manually, but it didn't seem to work. I'm not sure what to do from here and would appreciate any help. PS: My hibernate session is managed by spring.

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  • Preventing Race Conditions

    - by Qua
    I'm using the built-in ajax functionality of MVC2. Basically the user types a search query, and on every key press a set of results for the query is shown. This works fine in almost all cases, but sometimes the server is a bit slow with a single response, and thus the result for the next key stroke is returned before the previous. When the previous key stroke result set is finally returned to the client it will overwrite the results for the newer search query that should actually have been shown. My code follows more or less along these lines: <% using (Ajax.BeginForm("SearchUser", null, new AjaxOptions() { UpdateTargetId = "findUserResults" }, new { id = "findUserAjaxForm" })) {%> Every keystroke submits this form and thus outputs the results in the 'findUserResults' element. How can I prevent older results from being displayed while still making use of the built-in functions provided in MVC2?

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  • Checking for empty variable in PHPTAL condition

    - by kodziek
    In PHPTAL tal:condition can check is variable empty? Something like that: < tag tal:condition="var" >Some text< /tag > but the value of variable is like that: <?php $variable = ''; $Tpl->var = $variable; ?> And it's a problem 'cause PHPTAL that value '' interpreting like not empty value and condition return true. How fix it in PHPTAL side?

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  • Editing files without race conditions?

    - by user2569445
    I have a CSV file that needs to be edited by multiple processes at the same time. My question is, how can I do this without introducing race conditions? It's easy to write to the end of the file without race conditions by open(2)ing it in "a" (O_APPEND) mode and simply write to it. Things get more difficult when removing lines from the file. The easiest solution is to read the file into memory, make changes to it, and overwrite it back to the file. If another process writes to it after it is in memory, however, that new data will be lost upon overwriting. To further complicate matters, my platform does not support POSIX record locks, checking for file existence is a race condition waiting to happen, rename(2) replaces the destination file if it exists instead of failing, and editing files in-place leaves empty bytes in it unless the remaining bytes are shifted towards the beginning of the file. My idea for removing a line is this (in pseudocode): filename = "/home/user/somefile"; file = open(filename, "r"); tmp = open(filename+".tmp", "ax") || die("could not create tmp file"); //"a" is O_APPEND, "x" is O_EXCL|O_CREAT while(write(tmp, read(file)); //copy the $file to $file+".new" close(file); //edit tmp file unlink(filename) || die("could not unlink file"); file = open(filename, "wx") || die("another process must have written to the file after we copied it."); //"w" is overwrite, "x" is force file creation while(write(file, read(tmp))); //copy ".tmp" back to the original file unlink(filename+".tmp") || die("could not unlink tmp file"); Or would I be better off with a simple lock file? Appender process: lock = open(filename+".lock", "wx") || die("could not lock file"); file = open(filename, "a"); write(file, "stuff"); close(file); close(lock); unlink(filename+".lock"); Editor process: lock = open(filename+".lock", "wx") || die("could not lock file"); file = open(filename, "rw"); while(contents += read(file)); //edit "contents" write(file, contents); close(file); close(lock); unlink(filename+".lock"); Both of these rely on an additional file that will be left over if a process terminates before unlinking it, causing other processes to refuse to write to the original file. In my opinion, these problems are brought on by the fact that the OS allows multiple writable file descriptors to be opened on the same file at the same time, instead of failing if a writable file descriptor is already open. It seems that O_CREAT|O_EXCL is the closest thing to a real solution for preventing filesystem race conditions, aside from POSIX record locks. Another possible solution is to separate the file into multiple files and directories, so that more granular control can be gained over components (lines, fields) of the file using O_CREAT|O_EXCL. For example, "file/$id/$field" would contain the value of column $field of the line $id. It wouldn't be a CSV file anymore, but it might just work. Yes, I know I should be using a database for this as databases are built to handle these types of problems, but the program is relatively simple and I was hoping to avoid the overhead. So, would any of these patterns work? Is there a better way? Any insight into these kinds of problems would be appreciated.

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  • Common Lisp condition system for transfer of control

    - by Ken
    I'll admit right up front that the following is a pretty terrible description of what I want to do. Apologies in advance. Please ask questions to help me explain. :-) I've written ETLs in other languages that consist of individual operations that look something like: // in class CountOperation IEnumerable<Row> Execute(IEnumerable<Row> rows) { var count = 0; foreach (var row in rows) { row["record number"] = count++; yield return row; } } Then you string a number of these operations together, and call The Dispatcher, which is responsible for calling Operations and pushing data between them. I'm trying to do something similar in Common Lisp, and I want to use the same basic structure, i.e., each operation is defined like a normal function that inputs a list and outputs a list, but lazily. I can define-condition a condition (have-value) to use for yield-like behavior, and I can run it in a single loop, and it works great. I'm defining the operations the same way, looping through the inputs: (defun count-records (rows) (loop for count from 0 for row in rows do (signal 'have-value :value `(:count ,count @,row)))) The trouble is if I want to string together several operations, and run them. My first attempt at writing a dispatcher for these looks something like: (let ((next-op ...)) ;; pick an op from the set of all ops (loop (handler-bind ((have-value (...))) ;; records output from operation (setq next-op ...) ;; pick a new next-op (call next-op))) But restarts have only dynamic extent: each operation will have the same restart names. The restart isn't a Lisp object I can store, to store the state of a function: it's something you call by name (symbol) inside the handler block, not a continuation you can store for later use. Is it possible to do something like I want here? Or am I better off just making each operation function explicitly look at its input queue, and explicitly place values on the output queue?

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  • Rails Layout Rendering with controller condition

    - by Victor Martins
    I don't know what's the best way to doing this. On my application.html.erb I define a header div. My default controller ( root ) is a homepage controller. And I wish that if I'm at index of the homepage the header is rendering with some content, but all other controllers render another content inside that header. How can I make a condition in that header div to render different content based on the controller that's being rendered?

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  • Rails - Find Condition of two model fields

    - by ChrisWesAllen
    I'm trying to find the results of a model where it queries as the result of two conditions. I have a search tag that looks for Model.find(:all, :conditions => "name LIKE params[search]") but I'd like for the search to find all records where "name LIKE params[search] or description LIKE params[search] . Is there any way to add an OR into a condition in rails? or should I make an if statement?

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  • What is the relaxation condition in graph theory

    - by windopal
    Hi, I'm trying to understand the main concepts of graph theory and the algorithms within it. Most algorithms seem to contain a "Relaxation Condition" I'm unsure about what this is. Could some one explain it to me please. An example of this is dijkstras algorithm, here is the pseudo-code. 1 function Dijkstra(Graph, source): 2 for each vertex v in Graph: // Initializations 3 dist[v] := infinity // Unknown distance function from source to v 4 previous[v] := undefined // Previous node in optimal path from source 5 dist[source] := 0 // Distance from source to source 6 Q := the set of all nodes in Graph // All nodes in the graph are unoptimized - thus are in Q 7 while Q is not empty: // The main loop 8 u := vertex in Q with smallest dist[] 9 if dist[u] = infinity: 10 break // all remaining vertices are inaccessible from source 11 remove u from Q 12 for each neighbor v of u: // where v has not yet been removed from Q. 13 alt := dist[u] + dist_between(u, v) 14 if alt < dist[v]: // Relax (u,v,a) 15 dist[v] := alt 16 previous[v] := u 17 return dist[] Thanks

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  • Problem with writing if condition

    - by Himadri
    I have two decimal numbers. I want those number to be same upto 4 decimal points without rounding. If numbers are different I want 2nd number to be replaced by 1st. What if condition should I write? Eg, 1. num1 = 0.94618976 num2 = 0.94620239 If we round these numbers upto 4 decimal then we get 0.9462 same number, but I don't want to round these numbers. 2. num1 = 0.94620239 num2 = 0.94639125 The one way I found is take absolute difference of both numbers say diff and then check the value. My problem is of checking the range of diff. I am using delphi but you can answer in any language.Thank You.

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  • Condition checking vs. Exception handling

    - by Aidas Bendoraitis
    When is exception handling more preferable than condition checking? There are many situations where I can choose using one or the other. For example, this is a summing function which uses a custom exception: # module mylibrary class WrongSummand(Exception): pass def sum_(a, b): """ returns the sum of two summands of the same type """ if type(a) != type(b): raise WrongSummand("given arguments are not of the same type") return a + b # module application using mylibrary from mylibrary import sum_, WrongSummand try: print sum_("A", 5) except WrongSummand: print "wrong arguments" And this is the same function, which avoids using exceptions # module mylibrary def sum_(a, b): """ returns the sum of two summands if they are both of the same type """ if type(a) == type(b): return a + b # module application using mylibrary from mylibrary import sum_ c = sum_("A", 5) if c is not None: print c else: print "wrong arguments" I think that using conditions is always more readable and manageable. Or am I wrong? What are the proper cases for defining APIs which raise exceptions and why?

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  • Avoid IF statement after condition has been met

    - by greye
    I have a division operation inside a cycle that repeats many times. It so happens that in the first few passes through the loop (more or less first 10 loops) the divisor is zero. Once it gains value, a div by zero error is not longer possible. I have an if condition to test the divisor value in order to avoid the div by zero, but I am wondering that there is a performance impact that evaluating this if will have for each run in subsequent loops, especially since I know it's of no use anymore. How should this be coded? in Python?

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  • how avoids deadlock condition

    - by rima
    Hi I try to write a program like a compiler.In this case I must simulate these codes of SQLPL(This one can be just for example).for example in command prompt i wana do these: c:\> sqlplus .... Enter User-Name:(here we must enter username) xxxx Enter password:(same up)yyyyy ... sql>(now i want to send my sql command to shell)prompt "rima"; "rima" [<-output] sql> prompt "xxxx" sql> exit very simple. I try to use this code: ProcessStartInfo psi = new ProcessStartInfo( @"sqlplus"); psi.UseShellExecute = false; psi.CreateNoWindow = true; psi.RedirectStandardInput = true; psi.RedirectStandardOutput = true; //psi.RedirectStandardError = true; Process process = new Process(); process.StartInfo = psi; bool started = process.Start(); if (started) { process.StandardInput.WriteLine("user/password"); process.StandardInput.Flush(); string ret = process.StandardOutput.ReadLine(); // <-- stalls here System.Console.WriteLine("sqlplus says :" + ret + "\"."); } i find out it form here but as if u read this code has problem!DEADLOCK Condition problem! this is my second time i ask my question,every time my knowledge growing,but i cant get how I can solve my problem at last!!! So I kindly kiss your hand,please help me,these process is not clear for me.any one can give me a code that handle my problem?Already i look at all codes,also I try to use ProcessRunner that in my last post some one offer me,but I cant use it also,i receive an error. I hope by first example u find out what i want,and solve the second code problem... thanks I use C# for implemantation

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  • MySQL where condition but not limited by it

    - by Manny Calavera
    Hello. I would like to run a query on my database like this: SELECT SUM( t1.value ) AS total1, SUM( t2.value ) AS total2, SUM( t3.value ) AS total3, SUM( t4.value ) AS total4 FROM pay1 t1, pay2 t2, pay3 t3, pay4 t4 WHERE t1.date = '2010-04-29' AND t2.date = '2010-04-29' AND t3.date = '2010-04-29' AND t4.date = '2010-04-29' I am generating a report on payments and I would like to see a total of payments from each table based on the matching date. The problem is that some of the tables would not meet the condition of date and I want them to show up with 0 value if not. Currently, if any of the tables does not match the date, I get 0 results. I want to display value of 0 anywhere the date is not met and other fields should appear with the found values. The perfect operand for me would be "ANDOR" so that it won't be limited by any date that doesn't math in any table. Unfortunately, ANDOR does not exist as I am aware of so what should I do ? Can anyone help ? Thanks.

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