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• Possible Data Execution Prevention (DEP) problem in Windows 7

  - by Joel in Gö

  I have a serious problem with my .Net program. It calls a native dll, and then crashes instantly because it can't find a native method. This is behaviour we have seen before, whereby the C# compiler, in its infinite wisdom, sets the flag that the program is DEP compatible, even if it calls a native dll which patently is not. We have the standard workaround for this, where the flag is set to Not DEP Compatible in a post-build step, and this works fine. Everywhere except on my machine. I have Windows 7 32bit, and the program works fine on the Win 7 64bit machines that we have, as well as on Vista and XP; we have not yet been able to check on another Win7 32bit. However, on my machine the DataExecutionPolicy_SupportPolicy is 0, i.e. we have successfully switched DEP off. Does anyone know whether there is some situation in which it can still act? Or any other mechanism which could have the same effect? The dll in question also works fine when called from a native program. We are running out of ideas... any help would be much appreciated!

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• stack overflow on XMLListCollection collectionEvent

  - by reidLinden

  I'm working on a Flex 3 project, and I'm using a pair of XMLListCollection(s) to manage a combobox and a data grid. The combobox piece is working perfectly. The XMLListCollection for this is static. The user picks an item, and, on "change", it fires off an addItem() to the second collection. The second collection's datagrid then displays the updated list, and all is well. The datagrid, however, is editable. A further complication is that I have another event handler bound to the second XMLLIstCollection's "change" event, and in that handler, I do make additional changes to the second list. This essentially causes an infinite loop (a stack overflow :D ), of the second lists "change" handler. I'm not really sure how to handle this. Searching has brought up an idea or two regarding AutoUpdate functionality, but I wasn't able to get much out of them. In particular, the behavior persists, executing the 'updates' as soon as I re-enable, so I imagine I may be doing it wrong. I want the update to run, in general, just not DURING that code block. Thanks for your help!

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• A really smart rails helper needed

  - by Stefan Liebenberg

  In my rails application I have a helper function: def render_page( permalink ) page = Page.find_by_permalink( permalink ) content_tag( :h3, page.title ) + inline_render( page.body ) end If I called the page "home" with: <%= render_page :home %> and "home" page's body was: <h1>Home<h1/> bla bla <%= render_page :about %> <%= render_page :contact %> I would get "home" page, with "about" and "contact", it's nice and simple... right up to where someone goes and changes the "home" page's content to: <h1>Home<h1/> bla bla <%= render_page :home %> <%= render_page :about %> <%= render_page :contact %> which will result in a infinite loop ( a segment fault on webrick )... How would I change the helper function to something that won't fall into this trap? My first attempt was along the lines of: @@list = [] def render_page( permalink ) unless @@list.include?(permalink) @@list += [ permalink ] page = Page.find_by_permalink result = content_tag( :h3, page.title ) + inline_render( page.body ) @@list -= [ permalink ] return result else content_tag :b, "this page is already being rendered" end end which worked on my development environment, but bombed out in production... any suggestions? Thank You Stefan

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• Scala Interpreter scala.tools.nsc.interpreter.IMain Memory leak

  - by Peter

  I need to write a program using the scala interpreter to run scala code on the fly. The interpreter must be able to run an infinite amount of code without being restarted. I know that each time the method interpret() of the class scala.tools.nsc.interpreter.IMain is called, the request is stored, so the memory usage will keep going up forever. Here is the idea of what I would like to do: var interpreter = new IMain while (true) { interpreter.interpret(some code to be run on the fly) } If the method interpret() stores the request each time, is there a way to clear the buffer of stored requests? What I am trying to do now is to count the number of times the method interpret() is called then get a new instance of IMain when the number of times reaches 100, for instance. Here is my code: var interpreter = new IMain var counter = 0 while (true) { interpreter.interpret(some code to be run on the fly) counter = counter + 1 if (counter > 100) { interpreter = new IMain counter = 0 } } However, I still see that the memory usage is going up forever. It seems that the IMain instances are not garbage-collected by the JVM. Could somebody help me solve this issue? I really need to be able to keep my program running for a long time without restarting, but I cannot afford such a memory usage just for the scala interpreter. Thanks in advance, Pet

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• url rewrite & redirect question

  - by Shawn

  Say currently I have url like : http://mydomain.com/showpost.php?p=123 Now I want to make it prettier : http://mydomain.com/123/post-title I'm using apache rewrite which grabs segment '123' and put the url back to http://mydomain.com/showpost.php?p=123 OK. Here is the problem. I want to redirect the original non-pretty urls which were indexed by Google to the pretty versions, I want this because I heard that Google may punish me if he sees multiple urls pointing to identical content. So I need to redirect /showpost.php?p=123 to /123/post-title This I have to do in my php code coz there's no way Apache to be able to figure out the 'post-title', but if I put the redirect code in php code, then it will be a infinite loop, such as : Request : /showpost.php?p=123 redirected to : /123/post-title rewritten to: /showpost.php?p=123 redirected again to : /123/post-title ... So on and so forth. Sorry I should Google the solution first but I really don't know how to describe my situation in English to make Google return reasonable results. Please help me. Thanks.

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• How can I change the CSS visibility style for elements that are not on the screen?

  - by RenderIn

  I have a lot of data being placed into a <DIV> with the overflow: auto style. Firefox handles this gracefully but IE becomes very sluggish both when scrolling the div and when executing any Javascript on the page. At first I thought IE just couldn't handle that much data in its DOM, but then I did a simple test where I applied the visibility: hidden style to every element past the first 100. They still take up space and cause the scrollbars to appear. IE no longer had a problem with the data when I did this. So, I'd like to have a "smart" div that hides all the nested div elements which are not currently visible on the screen. Is there a simple solution to this or will I need to have an infinite loop which calculates the location of the scrollbar? If not, is there a particular event that I can hook into where I could do this? Is there a jQuery selector or plugin that will allow me to select all elements not currently visible on the screen?

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• Can Haskell's Parsec library be used to implement a recursive descent parser with backup?

  - by Thor Thurn

  I've been considering using Haskell's Parsec parsing library to parse a subset of Java as a recursive descent parser as an alternative to more traditional parser-generator solutions like Happy. Parsec seems very easy to use, and parse speed is definitely not a factor for me. I'm wondering, though, if it's possible to implement "backup" with Parsec, a technique which finds the correct production to use by trying each one in turn. For a simple example, consider the very start of the JLS Java grammar: Literal: IntegerLiteral FloatingPointLiteral I'd like a way to not have to figure out how I should order these two rules to get the parse to succeed. As it stands, a naive implementation like this: literal = do { x <- try (do { v <- integer; return (IntLiteral v)}) <|> (do { v <- float; return (FPLiteral v)}); return(Literal x) } Will not work... inputs like "15.2" will cause the integer parser to succeed first, and then the whole thing will choke on the "." symbol. In this case, of course, it's obvious that you can solve the problem by re-ordering the two productions. In the general case, though, finding things like this is going to be a nightmare, and it's very likely that I'll miss some cases. Ideally, I'd like a way to have Parsec figure out stuff like this for me. Is this possible, or am I simply trying to do too much with the library? The Parsec documentation claims that it can "parse context-sensitive, infinite look-ahead grammars", so it seems like something like I should be able to do something here.

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• Can I ignore a SIGFPE resulting from division by zero?

  - by Mikeage

  I have a program which deliberately performs a divide by zero (and stores the result in a volatile variable) in order to halt in certain circumstances. However, I'd like to be able to disable this halting, without changing the macro that performs the division by zero. Is there any way to ignore it? I've tried using #include <signal.h> ... int main(void) { signal(SIGFPE, SIG_IGN); ... } but it still dies with the message "Floating point exception (core dumped)". I don't actually use the value, so I don't really care what's assigned to the variable; 0, random, undefined... EDIT: I know this is not the most portable, but it's intended for an embedded device which runs on many different OSes. The default halt action is to divide by zero; other platforms require different tricks to force a watchdog induced reboot (such as an infinite loop with interrupts disabled). For a PC (linux) test environment, I wanted to disable the halt on division by zero without relying on things like assert.

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• jQuery .attr() crashing Internet Explorer

  - by Sunyatasattva

  Hello everyone, This is my first time posting a question here, as I usually try to find solutions myself. This one, though, being an IE issue, just drives me crazy. I use jQuery cycle plug-in on a website I made and, to populate a caption div, I use a little function that is called after the image is loaded, which uses the "alt" attribute of the image. This seems to exasperate Internet Explorer, which, doesn't have the time to fulfill this apparently so-complicated task, and, as the slideshow cycles, it enters in an infinite loop and eventually crashes – the newer the version, the worse the crash: the older IEs just display an error message saying “The webpage cannot be displayed”, while the newer (7 and 8) completely crash the system. I have no idea on how to solve or work around this. Here is the problematic code. function changeCaption() { var caption = $("img", this).attr("alt"); $('#caption').fadeIn("slow").html(caption); } Thanks in advance for any pointer: I am amazed as how something so simple and globally recognized (didn't encounter any other browser who had problem with this), can cause a problem so big. I also read somewhere that being able to crash a browser remotely is a serious issue :) Lucio

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• Switch statement usage - C

  - by Jamie Keeling

  Hello, I have a thread function on Process B that contains a switch to perform certain operations based on the results of an event sent from Process A, these are stored as two elements in an array. I set the first element to the event which signals when Process A has data to send and I have the second element set to the event which indicates when Process A has closed. I have began to implement the functionality for the switch statement but I'm not getting the results as I expect. Consider the following: // //Thread function DWORD WINAPI ThreadFunc(LPVOID passedHandle) { for(i = 0; i < 2; i++) { ghEvents[i] = OpenEvent(EVENT_ALL_ACCESS, FALSE, TEXT("Global\\ProducerEvents")); if(ghEvents[i] == NULL) { getlasterror = GetLastError(); } } dwProducerEventResult = WaitForMultipleObjects( 2, ghEvents, FALSE, INFINITE); switch (dwProducerEventResult) { case WAIT_OBJECT_0 + 0: { //Producer sent data //unpackedHandle = *((HWND*)passedHandle); MessageBox(NULL,L"Test",L"Test",MB_OK); break; } case WAIT_OBJECT_0 + 1: { //Producer closed ExitProcess(1); break; } default: return; } } As you can see if the event in the first array is signalled Process B should display a simple message box, if the second array is signalled the application should close. When I actually close Process A, Process B displays the message box instead. If I leave the first case blank (Do nothing) both applications close as they should. Furthermore Process B sends data an error is thrown (When I comment out the unpacking): Have I implemented my switch statement incorrectly? I though I handled the unpacking of the HWND correctly too, any suggestions? Thanks for your time.

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• python 'with' statement

  - by Stephen

  Hi, I'm using Python 2.5. I'm trying to use this 'with' statement. from __future__ import with_statement a = [] with open('exampletxt.txt','r') as f: while True: a.append(f.next().strip().split()) print a The contents of 'exampletxt.txt' are simple: a b In this case, I get the error: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/tmp/python-7036sVf.py", line 5, in <module> a.append(f.next().strip().split()) StopIteration And if I replace f.next() with f.read(), it seems to be caught in an infinite loop. I wonder if I have to write a decorator class that accepts the iterator object as an argument, and define an __exit__ method for it? I know it's more pythonic to use a for-loop for iterators, but I wanted to implement a while loop within a generator that's called by a for-loop... something like def g(f): while True: x = f.next() if test(x): a = x elif test(x): b = f.next() yield [a,x,b] a = [] with open(filename) as f: for x in g(f): a.append(x)

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• How close can I get C# to the performance of C++ for small intensive tasks?

  - by SLC

  I was thinking about the speed difference of C++ to C# being mostly about C# compiling to byte-code that is taken in by the JIT compiler (is that correct?) and all the checks C# does. I notice that it is possible to turn a lot of these functions off, both in the compile options, and possibly through using the unsafe keyword as unsafe code is not verifiable by the common language runtime. Therefore if you were to write a simple console application in both languages, that flipped an imaginary coin an infinite number of times and displayed the results to the screen every 10,000 or so iterations, how much speed difference would there be? I chose this because it's a very simple program. I'd like to test this but I don't know C++ or have the tools to compile it. This is my C# version though: static void Main(string[] args) { unsafe { Random rnd = new Random(); int heads = 0, tails = 0; while (true) { if (rnd.NextDouble() > 0.5) heads++; else tails++; if ((heads + tails) % 1000000 == 0) Console.WriteLine("Heads: {0} Tails: {1}", heads, tails); } } } Is the difference enough to warrant deliberately compiling sections of code "unsafe" or into DLLs that do not have some of the compile options like overflow checking enabled? Or does it go the other way, where it would be beneficial to compile sections in C++? I'm sure interop speed comes into play too then. To avoid subjectivity, I reiterate the specific parts of this question as: Does C# have a performance boost from using unsafe code? Do the compile options such as disabling overflow checking boost performance, and do they affect unsafe code? Would the program above be faster in C++ or negligably different? Is it worth compiling long intensive number-crunching tasks in a language such as C++ or using /unsafe for a bonus? Less subjectively, could I complete an intensive operation faster by doing this?

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• Treetop basic parsing and regular expression usage

  - by ucint

  I'm developing a script using the ruby Treetop library and having issues working with its syntax for regex's. First off, many regular expressions that work in other settings dont work the same in treetop. This is my grammar: (myline.treetop) grammar MyLine rule line string whitespace condition end rule string [\S]* end rule whitespace [\s]* end rule condition "new" / "old" / "used" end end This is my usage: (usage.rb) require 'rubygems' require 'treetop' require 'polyglot' require 'myline' parser = MyLineParser.new p parser.parse("randomstring new") This should find the word new for sure and it does! Now I wont to extend it so that it can find new if the input string becomes "randomstring anotherstring new yetanother andanother" and possibly have any number of strings followed by whitespace (tab included) before and after the regex for rule condition. In other words, if I pass it any sentence with the word "new" etc in it, it should be able to match it. So let's say I change my grammar to: rule line string whitespace condition whitespace string end Then, it should be able to find a match for: p parser.parse("randomstring new anotherstring") So, what do I have to do to allow the string whitespace to be repeated before and after condition? If I try to write this: rule line (string whitespace)* condition (whitespace string)* end , it goes in an infinite loop. If i replace the above () with [], it returns nil In general, regex's return a match when i use the above, but treetop regex's dont. Does anyone have any tips/points on how to go about this? Plus, since there isn't much documentation for treetop and the examples are either too trivial or too complex, is there anyone who knows a more thorough documentation/guide for treetop?

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• Is it possible to unstick a remote IIS ASP server after an exception hangs the session?

  - by user89691

  I have been coding an app in classic ASP that accesses 2 Access databases. I had a page I was working on throw an exception, which is normal during development and causes no lasting problems. This time however, after the exception any attempt to open either of the databases would freeze the session with an infinite script timeout. If I delete the session cookie I an able to access ASP pages again until I try to open the database again. The database that was open when the exception was thrown is left open. There is a LDB lock file and I can't rename or delete either the LDB or MDB file, though I can download the MDB file with FTP. The 2nd access database is not open but any attempt to read this also hangs the session. Accessing HTML pages is fine. The site is hosted with Hostway and they are not interested ("Coding problem = Your problem" even though it leaves my site dead in the water, I suspect until the next reboot, whenever that might be). Here is the dump from the relevant ASP page that threw the exception: Active Server Pages error 'ASP 0115' Unexpected error /translatestats.asp A trappable error (C0000005) occurred in an external object. The script cannot continue running. Active Server Pages error 'ASP 0240' Script Engine Exception /translatestats.asp A ScriptEngine threw exception 'C0000005' in 'IActiveScript::Close()' from 'CActiveScriptEngine::FinalRelease()'. Is there any way I can unstick the site / force close the database remotely ?

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• Multi-level shop, xml or sql. best practice?

  - by danrichardson

  Hello, i have a general "best practice" question regarding building a multi-level shop, which i hope doesn't get marked down/deleted as i personally think it's quite a good "subjective" question. I am a developer in charge (in most part) of maintaining and evolving a cms system and associated front-end functionality. Over the past half year i have developed a multiple level shop system so that an infinite level of categories may exist down into a product level and all works fine. However over the last week or so i have questioned by own methods in front-end development and the best way to show the multi-level data structure. I currently use a sql server database (2000) and pull out all the shop levels and then process them into an enumerable typed list with child enumerable typed lists, so that all levels are sorted. This in my head seems quite process heavy, but we're not talking about thousands of rows, generally only 1-500 rows maybe. I have been toying with the idea recently of storing the structure in an XML document (as well as the database) and then sending last modified headers when serving/requesting the document for, which would then be processed as/when nessecary with an xsl(t) document - which would be processed server side. This is quite a handy, reusable method of storing the data but does it have more overheads in the fact im opening and closing files? And also the xml will require a bit of processing to pull out blocks of xml if for instance i wanted to show two level mid way through the tree for a side menu. I use the above method for sitemap purposes so there is currently already code i have built which does what i require, but unsure what the best process is to go about. Maybe a hybrid method which pulls out the data, sorts it and then makes an xml document/stream (XDocument/XmlDocument) for xsl processing is a good way? - This is the way i currently make the cms work for the shop. So really (and thanks for sticking with me on this), i am just wandering which methods other people use or recommend as being the best/most logical way of doing things. Thanks Dan

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• Check if files in a directory are still being written using Windows Batch Script

  - by FMFF

  Hello. Here's my batch file to parse a directory, and zip files of certain type REM Begin ------------------------ tasklist /FI "IMAGENAME eq 7za.exe" /FO CSV > search.log FOR /F %%A IN (search.log) DO IF %%~zA EQU 0 GOTO end for /f "delims=" %%A in ('dir C:\Temp\*.ps /b') do ( "C:\Program Files\7-Zip\cmdline\7za.exe" a -tzip -mx9 "C:\temp\Zip\%%A.zip" "C:\temp\%%A" Move "C:\temp\%%A" "C:\Temp\Archive" ) :end del search.log REM pause exit REM End --------------------------- This code works just fine for 90% of my needs. It will be deployed as a scheduled task. However, the *.ps files are rather large (minimum of 1GB) in real time cases. So the code is supposed to check if the incoming file is completely written and is not locked by the application that is writing it. I saw another example elsewhere, that suggested the following approach :TestFile ren c:\file.txt c:\file.txt if errorlevel 0 goto docopy sleep 5 goto TestFile :docopy However this example is good for a fixed file. How can I use that many labels and GoTo's inside a for loop without causing an infinite loop? Or is this code safe to be used in the For Loop? Thank you for any help.

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• How can I make a Maybe-Transformer MaybeT into an instance of MonadWriter?

  - by martingw

  I am trying to build a MaybeT-Transformer Monad, based on the example in the Real World Haskell, Chapter Monad Transformers: data MaybeT m a = MaybeT { runMT :: m (Maybe a) } instance (Monad m) => Monad (MaybeT m) where m >>= f = MaybeT $ do a <- runMT m case a of Just x -> runMT (f x) Nothing -> return Nothing return a = MaybeT $ return (Just a) instance MonadTrans MaybeT where lift m = MaybeT $ do a <- m return (Just a) This works fine, but now I want to make MaybeT an instance of MonadWriter: instance (MonadWriter w m) => MonadWriter w (MaybeT m) where tell = lift . tell listen m = MaybeT $ do unwrapped <- listen (runMT m) return (Just unwrapped) The tell is ok, but I can't get the listen function right. The best I could come up with after 1 1/2 days of constructor origami is the one you see above: unwrapped is supposed to be a tuple of (Maybe a, w), and that I want to wrap up in a Maybe-Type and put the whole thing in an empty MonadWriter. But the compiler complains with: Occurs check: cannot construct the infinite type: a = Maybe a When generalising the type(s) for `listen' In the instance declaration for `MonadWriter w (MaybeT m)' What am I missing?

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• Time Machine for Windows?

  - by Blub

  Finally got convinced to start using some kind of version control for my code instead of zipping down a copy of the project at the end of each day. Downloaded Tortoise SVN and used it to create a repository localy on my hdd. I've been using it for 2 days now but I have to say that using it is actually more hassle than just copying the project manually in explorer. Sure, you only store incremental changes but with the cheap disks of today I can't really say that's an argument when you only have small projects. I haven't realy found a quick way to browse the older versions of my files eighter. What I want is an infinite undo that is completely transparent while I code, if I save the file I want a backup. I don't want to check out, check in and don't even get me started on moving files. I haven't tried Time Machine for OS X but it looks like it's exactly what I'm looking for. Does such a program exist for windows? Preferably free and with some kind of tagging-system so I can tag a timestamp when the project is working etc. Maybe should add that I mostly work alone on a single computer. Update: Some of you asked why I want backup. Since I work alone it's mostly to allow me to quickly hack up a solution without worrying that something will screw up.

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• ASP.NET: disabling authentication for a single aspx page (custom error page)?

  - by Richard Collette

  I am using a custom error page: <customErrors redirectMode="ResponseRedirect" mode="On" defaultRedirect="Error2.aspx"/> I want to disable authentication for the custom error page because the error being raised is related to an authentication module and I don't want to get into an infinite loop and I want to display a clean error page to the user. I have been trying the following configuration to do that. <location path="Error2.aspx"> <system.web> <authentication mode="None"/> <authorization> <allow users="?"/> <allow users="*"/> </authorization> </system.web> </location> I am getting a System.Configuration.ConfigurationErrorsException for the line that sets the authentication mode. It is an error to use a section registered as allowDefinition='MachineToApplication' beyond application level. This error can be caused by a virtual directory not being configured as an application in IIS. I have verified that there are no other web.config files in subdirectories under the application's folder. The applications folder is configured as an application in IIS and the error page is at the application's root. File permissions set for the error page in IIS include anonymous and windows authentication (I have tried just anonymous as well).

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• Throttling CPU/Memory usage of a Thread in Java?

  - by Nalandial

  I'm writing an application that will have multiple threads running, and want to throttle the CPU/memory usage of those threads. There is a similar question for C++, but I want to try and avoid using C++ and JNI if possible. I realize this might not be possible using a higher level language, but I'm curious to see if anyone has any ideas. EDIT: Added a bounty; I'd like some really good, well thought out ideas on this. EDIT 2: The situation I need this for is executing other people's code on my server. Basically it is completely arbitrary code, with the only guarantee being that there will be a main method on the class file. Currently, multiple completely disparate classes, which are loaded in at runtime, are executing concurrently as separate threads. I inherited this code (the original author is gone). The way it's written, it would be a pain to refactor to create separate processes for each class that gets executed. If that's the only good way to limit memory usage via the VM arguments, then so be it. But I'd like to know if there's a way to do it with threads. Even as a separate process, I'd like to be able to somehow limit its CPU usage, since as I mentioned earlier, several of these will be executing at once. I don't want an infinite loop to hog up all the resources. EDIT 3: An easy way to approximate object size is with java's Instrumentation classes; specifically, the getObjectSize method. Note that there is some special setup needed to use this tool.

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• Qt - QWebView Problem

  - by user547057

  Hi, I have a PyQt gui script which consists of a QWebView widget. I'm trying to send a GET request, i.e go to a page, fill a form and hit click using the code at the bottom of this question. Since i'm dealing with the documentElement(a QWebElement) of the webview, I need to place all DOM actions in a separate function(which I have named fillForm) and connect the loadFinished() signal into the function. Without connecting the signal, the document will not have loaded and I won't be able to get the elements I want. I'm able to submit the form correctly and get the proper response from the webpage. The problem i'm having is that, the above leads to a sort of infinite loop. This is because the webpage gets reloaded each time a new page is loaded, so the form gets filled each and every single time without stopping. I'd like to know if there's some way of finding out whether the WebView's page has loaded fully, non-asynchronously or maybe pause execution of the script(without freezing the gui) until the whole document has loaded. I'm unable to come up with a satisfactory solution(my idea consisted of keeping a global variable to track clicks) to this problem. I would appreciate it if someone could help me out with a better way to tackle this. Thanks! Here's the code i'm using import sys from PyQt4.QtCore import * from PyQt4.QtGui import * from PyQt4.QtWebKit import * from PyQt4 import QtCore app = QApplication(sys.argv) web = QWebView() web.load(QUrl("http://mywebsite.com")) def fillForm(): doc = web.page().mainFrame().documentElement() searchform = doc.findFirst("input[type=text]") searchform.setAttribute("value", "hello") button = doc.findFirst("input[type=submit]") button.evaluateJavaScript("click()") QtCore.QObject.connect(web, QtCore.SIGNAL("loadFinished"), fillForm) web.show() sys.exit(app.exec_())

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• Why doesn't gcc remove this check of a non-volatile variable?

  - by Thomas

  This question is mostly academic. I ask out of curiosity, not because this poses an actual problem for me. Consider the following incorrect C program. #include <signal.h> #include <stdio.h> static int running = 1; void handler(int u) { running = 0; } int main() { signal(SIGTERM, handler); while (running) ; printf("Bye!\n"); return 0; } This program is incorrect because the handler interrupts the program flow, so running can be modified at any time and should therefore be declared volatile. But let's say the programmer forgot that. gcc 4.3.3, with the -O3 flag, compiles the loop body (after one initial check of the running flag) down to the infinite loop .L7: jmp .L7 which was to be expected. Now we put something trivial inside the while loop, like: while (running) putchar('.'); And suddenly, gcc does not optimize the loop condition anymore! The loop body's assembly now looks like this (again at -O3): .L7: movq stdout(%rip), %rsi movl $46, %edi call _IO_putc movl running(%rip), %eax testl %eax, %eax jne .L7 We see that running is re-loaded from memory each time through the loop; it is not even cached in a register. Apparently gcc now thinks that the value of running could have changed. So why does gcc suddenly decide that it needs to re-check the value of running in this case?

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• A really smart rails helper needed

  - by Stefan Liebenberg

  In my rails application I have a helper function: def render_page( permalink ) page = Page.find_by_permalink( permalink ) content_tag( :h3, page.title ) + inline_render( page.body ) end If I called the page "home" with: <%= render_page :home %> and "home" page's body was: <h1>Home<h1/> bla bla <%= render_page :about %> <%= render_page :contact %> I would get "home" page, with "about" and "contact", it's nice and simple... right up to where someone goes and changes the "home" page's content to: <h1>Home<h1/> bla bla <%= render_page :home %> <%= render_page :about %> <%= render_page :contact %> which will result in a infinite loop ( a segment fault on webrick )... How would I change the helper function to something that won't fall into this trap? My first attempt was along the lines of: @@list = [] def render_page( permalink ) unless list.include?(permalink) @@list += [ permalink ] page = Page.find_by_permalink content_tag( :h3, page.title ) + inline_render( page.body ) @@list -= [ permalink ] else content_tag :b, "this page is already being rendered" end end which worked on my development environment, but bombed out in production... any suggestions? Thank You Stefan

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• Recursion Problems in Prolog

  - by Humble_Student

  I'm having some difficulties in prolog, I'm trying to write a predicate that will return all paths between two cities, although at the moment it returns the first path it finds on an infinite loop. Not sure where I'm going wrong but I've been trying to figure this out all day and I'm getting nowhere. Any help that could be offered would be appreciated. go:- repeat, f([],0,lon,spa,OP,OD), write(OP), write(OD), fail. city(lon). city(ath). city(spa). city(kol). path(lon,1,ath). path(ath,3,spa). path(spa,2,kol). path(lon,1,kol). joined(X,Y,D):- path(X,D,Y);path(Y,D,X). f(Ci_Vi,Di,De,De,PaO,Di):- append([De],Ci_Vi,PaO), !. f(Cities_Visited,Distance,Start,Destination,Output_Path,Output_Distance):- repeat, city(X), joined(Start,X,D), not_member(X,Cities_Visited), New_Distance is Distance + D, f([Start|Cities_Visited],New_Distance,X,Destination,Output_Path,Output_Distance). not_member(X,List):- member(X,List), !, fail. not_member(X,List). The output I'm expecting here is [spa,ath,lon]4 [spa,kol,lon]3. Once again, any help would be appreciated. Many thanks in advance.

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• Lightweight alternative to Manual/AutoResetEvent in C#

  - by sweetlilmre

  Hi, I have written what I hope is a lightweight alternative to using the ManualResetEvent and AutoResetEvent classes in C#/.NET. The reasoning behind this was to have Event like functionality without the weight of using a kernel locking object. Although the code seems to work well in both testing and production, getting this kind of thing right for all possibilities can be a fraught undertaking and I would humbly request any constructive comments and or criticism from the StackOverflow crowd on this. Hopefully (after review) this will be useful to others. Usage should be similar to the Manual/AutoResetEvent classes with Notify() used for Set(). Here goes: using System; using System.Threading; public class Signal { private readonly object _lock = new object(); private readonly bool _autoResetSignal; private bool _notified; public Signal() : this(false, false) { } public Signal(bool initialState, bool autoReset) { _autoResetSignal = autoReset; _notified = initialState; } public virtual void Notify() { lock (_lock) { // first time? if (!_notified) { // set the flag _notified = true; // unblock a thread which is waiting on this signal Monitor.Pulse(_lock); } } } public void Wait() { Wait(Timeout.Infinite); } public virtual bool Wait(int milliseconds) { lock (_lock) { bool ret = true; // this check needs to be inside the lock otherwise you can get nailed // with a race condition where the notify thread sets the flag AFTER // the waiting thread has checked it and acquires the lock and does the // pulse before the Monitor.Wait below - when this happens the caller // will wait forever as he "just missed" the only pulse which is ever // going to happen if (!_notified) { ret = Monitor.Wait(_lock, milliseconds); } if (_autoResetSignal) { _notified = false; } return (ret); } } }

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