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  • Lock free multiple readers single writer

    - by dummzeuch
    I have got an in memory data structure that is read by multiple threads and written by only one thread. Currently I am using a critical section to make this access threadsafe. Unfortunately this has the effect of blocking readers even though only another reader is accessing it. There are two options to remedy this: use TMultiReadExclusiveWriteSynchronizer do away with any blocking by using a lock free approach For 2. I have got the following so far (any code that doesn't matter has been left out): type TDataManager = class private FAccessCount: integer; FData: TDataClass; public procedure Read(out _Some: integer; out _Data: double); procedure Write(_Some: integer; _Data: double); end; procedure TDataManager.Read(out _Some: integer; out _Data: double); var Data: TDAtaClass; begin InterlockedIncrement(FAccessCount); try // make sure we get both values from the same TDataClass instance Data := FData; // read the actual data _Some := Data.Some; _Data := Data.Data; finally InterlockedDecrement(FAccessCount); end; end; procedure TDataManager.Write(_Some: integer; _Data: double); var NewData: TDataClass; OldData: TDataClass; ReaderCount: integer; begin NewData := TDataClass.Create(_Some, _Data); InterlockedIncrement(FAccessCount); OldData := TDataClass(InterlockedExchange(integer(FData), integer(NewData)); // now FData points to the new instance but there might still be // readers that got the old one before we exchanged it. ReaderCount := InterlockedDecrement(FAccessCount); if ReaderCount = 0 then // no active readers, so we can safely free the old instance FreeAndNil(OldData) else begin /// here is the problem end; end; Unfortunately there is the small problem of getting rid of the OldData instance after it has been replaced. If no other thread is currently within the Read method (ReaderCount=0), it can safely be disposed and that's it. But what can I do if that's not the case? I could just store it until the next call and dispose it there, but Windows scheduling could in theory let a reader thread sleep while it is within the Read method and still has got a reference to OldData. If you see any other problem with the above code, please tell me about it. This is to be run on computers with multiple cores and the above methods are to be called very frequently. In case this matters: I am using Delphi 2007 with the builtin memory manager. I am aware that the memory manager probably enforces some lock anyway when creating a new class but I want to ignore that for the moment. Edit: It may not have been clear from the above: For the full lifetime of the TDataManager object there is only one thread that writes to the data, not several that might compete for write access. So this is a special case of MREW.

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  • free() on stack memory

    - by vidicon
    I'm supporting some c code on Solaris, and I've seen something weird at least I think it is: char new_login[64]; ... strcpy(new_login, (char *)login); ... free(new_login); My understanding is that since the variable is a local array the memory comes from the stack and does not need to be freed, and moreover since no malloc/calloc/realloc was used the behaviour is undefined. This is a real-time system so I think it is a waste of cycles. Am I missing something obvious?

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  • free RSS feed caching

    - by cherouvim
    Hello I've got an application which serves an rss feed of headlines and I need to provide this rss feed to other consumers. I don't want to provide the rss directly from my server though, due to limited server resources, so I need to proxy (cache) it through some service which will handle the load. Assuming the rss feed URL of my application is http://example.com/rss I initially provided my consumers with the url http://ajax.googleapis.com/ajax/services/feed/load?v=1.0&q=http%3A%2F%2Fexample.com%2Frss which solved my server load problem but introduced a liveness problem. The headlines are minutes to hours late from the actual feed (haven't exactly measured how much late). I've also tried distributing through feedburner so the url became something like http://feeds.feedburner.com/example123?format=xml but the liveness problem still exists. Is there a public and free solution for this problem? Anything below 5 minutes of liveness delay would be totally acceptable. thanks

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  • Free US sales-tax lookup (per zip etc.)?

    - by Shimmy
    I am creating a pricing program. I need to calculate the amounts according to the current tax list in the US (in various places). I want to have a button 'Update taxes' in the administrative settings of the application, so when the user clicks it, it should download from somewhere the active tax amounts. So I actually want to have a function decimal GetTax(string zip). Does anyone knows about a free downloadable xml, or RSS accessible or even a website that I can crawle in and get this info from?

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  • In WPF: Children.Remove or Children.Clear doesn't free objects

    - by Bart Roozendaal
    I create some UIElements from code behind and was anticipating the garbage collection to clear up stuff. However, the objects are not free-ed at the time I expected it. I was expecting them to be freeed at RemoveAt(0), but they are only freed at the end of the program. How can I make the objects be freed when removed from the Children collection of the Canvas? <Window x:Class="WpfApplication1.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="Window1" Height="300" Width="300" MouseDown="Window_MouseDown"> <Grid> <Canvas x:Name="main" /> </Grid> </Window> The code behind is: public partial class MainWindow : Window { public MainWindow() { InitializeComponent(); } private void Window_MouseDown(object sender, MouseButtonEventArgs e) { if (main.Children.Count == 0) main.Children.Add(new MyControl() { Background = Brushes.Yellow, Width = 100, Height = 50 }); else main.Children.RemoveAt(0); } } public class MyControl : UserControl { ~MyControl() { Debug.WriteLine("Goodbye"); } }

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  • PHP - "Fat Free Framework" Find Methods and Showing Results in Template

    - by user1672808
    Just started trying the "Fat Free Framework" I'm building a site using a MySQL DB with 265 fields, and 5000+ rows in the DB; I can load() a specific record easily, no problems. When using find(), afind(), and even "select()", template will show blank lines or lines with "filler" text, with the correct number of rows for the query results, but no text/data from the DB itself; Same problem whether using objects or simply arrays from result (afind() and find()). I've copied/pasted the code verbatim from examples and from documentation, with only the DB specific items changed. Still, no luck. CODE IN PHP FILE (function from CLASS): static function home() { $featured=new Axon('boats'); $F3::set('boatlist',$featured->afind('D_CustomerID=173')); F3::set('content',TEMPLATE_DIR .'/home.html'); echo Template::serve(TEMPLATE_DIR .'/layout.html'); } TEMPLATE home.html: <div class="span8"> <h3> Featured Boats </h3> <F3:repeat group="{{@boatlist}}" value="{{@boat}}"> <div style="margin-left: 2em" class="thumbnails"> <p> <a href="boat/{{@boat['D_BoatNum']}}">{{trim(@boat['D_Description'])}}</a> by {{@boat['D_CustomerID']}} </p> <p> {{@boat['D_Price']}} </p> </div> </F3:repeat> </div> The number of rows this produces coincides with the correct number of rows in the DB. However, the actual data from each field does not show. Any ideas?

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  • Lock-Free Data Structures in C++ Compare and Swap Routine

    - by slf
    In this paper: Lock-Free Data Structures (pdf) the following "Compare and Swap" fundamental is shown: template <class T> bool CAS(T* addr, T exp, T val) { if (*addr == exp) { *addr = val; return true; } return false; } And then says The entire procedure is atomic But how is that so? Is it not possible that some other actor could change the value of addr between the if and the assignment? In which case, assuming all code is using this CAS fundamental, it would be found the next time something "expected" it to be a certain way, and it wasn't. However, that doesn't change the fact that it could happen, in which case, is it still atomic? What about the other actor returning true, even when it's changes were overwritten by this actor? If that can't possibly happen, then why? I want to believe the author, so what am I missing here? I am thinking it must be obvious. My apologies in advance if this seems trivial.

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  • Recommendations for a free GIS library supporting raster images

    - by gspr
    Hi. I'm quite new to the whole field of GIS, and I'm about to make a small program that essentially overlays GPS tracks on a map together with some other annotations. I primarily need to allow scanned (thus raster) maps (although it would be nice to support proper map formats and something like OpenStreetmap in the long run). My first exploratory program uses Qt's graphics view framework and overlays the GPS points by simply projecting them onto the tangent plane to the WGS84 ellipsoid at a calibration point. This gives half-decent accuracy, and actually looks good. But then I started wondering. To get the accuracy I need (i.e. remove the "half" in "half-decent"), I have to correct for the map projection. While the math is not a problem in itself, supporting many map projection feels like needless work. Even though a few projections would probably be enough, I started thinking about just using something like the PROJ.4 library to do my projections. But then, why not take it all the way? Perhaps I might aswell use a full-blown map library such as Mapnik (edit: Quantum GIS also looks very nice), which will probably pay off when I start to want even more fancy annotations or some other symptom of featuritis. So, finally, to the question: What would you do? Would you use a full-blown map library? If so, which one? Again, it's important that it supports using (and zooming in and out with) raster maps and has pretty overlay features. Or would you just keep it simple, and go with Qt's own graphics view framework together with something like PROJ.4 to handle the map projections? I appreciate any feedback! Some technicalities: I'm writing in C++ with a Qt-based GUI, so I'd prefer something that plays relatively nicely with those. Also, the library must be free software (as in FOSS), and at least decently cross-platform (GNU/Linux, Windows and Mac, at least). Edit: OK, it seems I didn't do quite enough research before asking this question. Both Quantum GIS and Mapnik seem very well suited for my purpose. The former especially so since it's based on Qt.

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  • BlackBerry 10 : RIM prévoit de lancer 4 modèles de smartphones et une tablette en 2013, la RoadMap du constructeur dévoilée

    BlackBerry 10 : RIM prévoit de lancer 4 modèles de smartphones et une tablette en 2013 la RoadMap du constructeur dévoilée La dernière carte dont dispose probablement RIM pour sortir du gouffre dans lequel il se trouve est BlackBerry 10, l'OS sur lequel le constructeur canadien repose tous ses espoirs. Le site spécialisé BlackberryOS.com a obtenu des informations d'une RoadMap de RIM, qui prévoit de lancer plusieurs dispositifs en 2013. [IMG]http://rdonfack.developpez.com/rim-roadmap2013.jpg[/IMG] Le constructeur canadien doit marquer un gros coup, et pour cela, la firme envisage de commercialiser au moins quatre nouveaux modèles de smartphones BlackBerry au...

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  • BlackBerry 10 : RIM prévoit lancer 4 modèles de smartphones et une tablette en 2013, la RoadMap du constructeur dévoilée

    BlackBerry 10 : RIM prévoit lancer 4 modèles de smartphones et une tablette en 2013 la RoadMap du constructeur dévoilée La dernière carte dont dispose probablement RIM pour sortir du gouffre dans lequel il se trouve est BlackBerry 10, l'OS sur lequel le constructeur canadien repose tous ses espoirs. Le site spécialisé BlackberryOS.com a obtenu des informations d'une RoadMap de RIM, qui prévoit lancer plusieurs dispositifs en 2013. [IMG]http://rdonfack.developpez.com/rim-roadmap2013.jpg[/IMG] Le constructeur canadien doit marquer un gros coup, et pour cela, la firme envisage de commercialiser au moins quatre nouveaux modèles de smartphones BlackBerry au cour...

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  • Where can I find free and open data?

    - by kitsune
    Sooner or later, coders will feel the need to have access to "open data" in one of their projects, from knowing a city's zip to a more obscure information such as the axial tilt of Pluto. I know data.un.org which offers access to the UN's extensive array of databases that deal with human development and other socio-economic issues. The other usual suspects are NASA and the USGS for planetary data. There's an article at readwriteweb with more links. infochimps.org seems to stand out. Personally, I need to find historic commodity prices, stock values and other financial data. All these data sets seem to cost money however. Clarification To clarify, I'm interested in all kinds of open data, because sooner or later, I know I will be in a situation where I could need it. I will try to edit this answer and include the suggestions in a structured manners. A link for financial data was hidden in that readwriteweb article, doh! It's called opentick.com. Looks good so far! Update I stumbled over semantic data in another question of mine on here. There is opencyc ('the world's largest and most complete general knowledge base and commonsense reasoning engine'). A project called UMBEL provides a light-weight, distilled version of opencyc. Umbel has semantic data in rdf/owl/skos n3 syntax. The Worldbank also released a very nice API. It offers data from the last 50 years for about 200 countries

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  • Theory of computation - Using the pumping lemma for context free languages

    - by Tony
    I'm reviewing my notes for my course on theory of computation and I'm having trouble understanding how to complete a certain proof. Here is the question: A = {0^n 1^m 0^n | n>=1, m>=1} Prove that A is not regular. It's pretty obvious that the pumping lemma has to be used for this. So, we have |vy| = 1 |vxy| <= p (p being the pumping length, = 1) uv^ixy^iz exists in A for all i = 0 Trying to think of the correct string to choose seems a bit iffy for this. I was thinking 0^p 1^q 0^p, but I don't know if I can obscurely make a q, and since there is no bound on u, this could make things unruly.. So, how would one go about this?

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  • Where can one find free software icons / images?

    - by mmattax
    This may not be directly related to programming, but I always find it hard to get quality icons that can be used for software. I currently have the need for some type of "green checkmark image", and I always seem to be looking for print, save, delete types of icons... Anybody have good resources? Note: I rather not be stealing someone's intellectual property.

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  • Jquery's a FREE grid display

    - by mmcgrail
    I have been using Flesigrid for my CMS and I like it except there are some things I think it could do better like get the column headers from the ajax results rather then in the setup of the flexigrid or having the ability to change the buttons on reload things of that nature. I've been considering a mod of the plugin though I'm not sure i really want to do that. But the question is ... is there something that might be better that I somehow missed when I was looking for grid display ?

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  • Large free block of english non-pronoun text

    - by Tom
    As part of teaching myself python I've written a script which allows a user to play hangman. At the moment, the hangman word to be guessed is simply entered manually at the start of the script's code. I want instead for the script to choose randomly from a large list of english words. This I know how to do - my problem is finding that list of words to work from in the first place. Does anyone know of a source on the net for, say, 1000 common english words where they can be downloaded as a block of text or something similar that I can work with? (My initial thought was grabbing a chunk of a novel from project gutenburg [this project is only for my own amusement and won't be available anywhere else so copyright etc doesn't matter hugely to me btw], but anything like that is likely to contain too many names or non-standard words that wouldn't be suitable for hangman. I need text that only has words legal for use in scrabble, basically). It's a slightly odd question for here I suppose, but actually I thought the answer might be of use not just to me but anyone else working on a project for a wordgame or similar that needs a large seed list of words to work from. Many thanks for any links or suggestions :)

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  • Reminder: Totally Awesome and Totally Free Training SQL Server Training

    - by KKline
    One of the things that I enjoy about working for Quest Software is that we give back copiously to the community. From activities and offerings like SQLServerPedia , to our free posters mailed anywhere in North America (and don't forget the free hi-res PDFs for the rest of the world ), Don't forget that free DVDs of our virtual conferences featuring me, along with Buck Woody ( blog | twitter ) and Brent Ozar ( blog | twitter ) will be mailed anywhere in North America free of charge, now available...(read more)

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