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• How are a session identifiers generated?

  - by Asaf R

  Most web applications depend on some kind of session with the user (for instance, to retain login status). The session id is kept as a cookie in the user's browser and sent with every request. To make it hard to guess the next user's session these session-ids need to be sparse and somewhat random. The also have to be unique. The question is - how to efficiently generate session ids that are sparse and unique? This question has a good answer for unique random numbers, but it seems not scalable for a large range of numbers, simply because the array will end up taking a lot of memory.

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• Garbage Collection Java

  - by simion

  On the slides i am revising from it says the following; Live objects can be identified either by maintaining a count of the number of references to each object, or by tracing chains of references from the roots. Reference counting is expensive – it needs action every time a reference changes and it doesn’t spot cyclical structures, but it can reclaim space incrementally. Tracing involves identifying live objects only when you need to reclaim space – moving the cost from general access to the time at which the GC runs, typically only when you are out of memory. I understand the principles of why reference counting is expensive but do not understand what "doesn’t spot cyclical structures, but it can reclaim space incrementally." means. Could anyone help me out a little bit please? Thanks

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• In C, as free() knows an array size, why isn't there a function that gets the array size? [closed]

  - by user354959

  Possible Duplicate: If free() knows the length of my array, why can’t I ask for it in my own code? Searching around (including here at stackoverflow), I got that malloc() allocates an array and also creates a header to control the array info. In this header, there's also the array size. free() use such information to know how to deallocate that array. So, if the array size info is "there" (somewhere in the memory), why there isn't a function that returns an array size, looking for this at the array header? Or am I missing something?

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• running Hadoop software on office computers (when they are idle)

  - by Shahbaz

  Is there a project which helps setup a Hadoop cluster on office desktops, when they are idle? I'd like to experiment with Hadoop/MR/hbase but don't have acces to 5-10 computers. The computers at work are idle after hours and are connected to each other through a very high speed connection. What's more, data on these computers stays within our network so there is no privacy issue. In order for this to work I need a fairly light weight monitor running on each machine. When the computer has been idle for X hours, it will join the cluster. If the user logs on, it has to drop out of the cluster and return all CPU/memory back. Does something like this exist?

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• How to create a new IDA project based on an existing one with different offsets?

  - by tbergelt

  I have an existing IDA Pro project for a C166 processor embedded application. This project already has many functions, variables, etc defined. There are different versions of the embedded application I am looking at. The different versions of the application are 99% the same, but with slight variations in code and data that cause functions and variables to be at different memory offsets. I want to create a new IDA project for a different version of the application. I would like to somehow import all of my function and variable definitions from my existing IDA project. I would like IDA to recognize the signatures of the existing function definitions and define them at there new location in the new project. How can I do this? Are there certain plugins for IDA I can chain together?

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• Java Web App: Passing form parameters across multiple pages

  - by digiarnie

  Hi, what is the best practice or best way of passing form parameters from page to page in a flow? If I have a flow where a user enters data in a form and hits next and repeats this process until they get to an approval page, what ways could I approach this problem to make the retention of data as simple as possible over the flow? I guess you could put all the information as you go in the session but could you get into memory issues if a lot of people are using your app and going through the flow at the same time?

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• Application disappears after starting - C++ linking managed code dll

  - by Axarydax

  Hello, we have a problem with our applications. We have recently upgraded our toolchain to Visual Studio 2010 and friends, it all works fine on Windows Vista + Windows 7 developer machines and Windows 7 customer machines. (all x86) Today we did a first installation to a client PC running Windows XP SP3 x86, and all applications linking managed code dll do not work. After starting the .exe seemingly nothing happens, no window pops up, no memory gets eaten, no task shows in task manager, nothing, nada. Event log is empty, everything is just as if the application returned to the OS right after the start. I am at loss about this, I really don't know how should I start even debugging this problem. I don't want to install VS2010 and everything onto a Windows XP box, I'd like to try something less time intensive first. This all is happening just to native C++ apps that link managed c++ DLL. Pure managed C# apps do work. Pure native C++ apps do work.

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• Objective-C: Initializing char with char at index of string.

  - by Mr. McPepperNuts

  unichar myChar = [myString characterAtIndex:0]; [myNSMutableArray addObject:myChar]; I am trying to insert the first char of a string into an array, to create an array of chars. the first line does not give me an error. The second line however, provides the following error: warning: passing argument 1 of 'addObject:' makes pointer from integer without a cast This also crashes the application with a "bad address" error. I thought this error was due to a problem with memory allocation. Can someone shed some light on this.

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• string manipulations in C

  - by Vivek27

  Following are some basic questions that I have with respect to strings in C. If string literals are stored in read-only data segment and cannot be changed after initialisation, then what is the difference between the following two initialisations. char *string = "Hello world"; const char *string = "Hello world"; When we dynamically allocate memory for strings, I see the following allocation is capable enough to hold a string of arbitary length.Though this allocation work, I undersand/beleive that it is always good practice to allocate the actual size of actual string rather than the size of data type.Please guide on proper usage of dynamic allocation for strings. char *string = (char *)malloc(sizeof(char));

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• Quick way to do data lookup in PHP

  - by Ghostrider

  I have a data table with 600,000 records that is around 25 megabytes large. It is indexed by a 4 byte key. Is there a way to find a row in such dataset quickly with PHP without resorting to MySQL? The website in question is mostly static with minor PHP code and no database dependencies and therefore fast. I would like to add this data without having to use MySQL if possible. In C++ I would memory map the file and do a binary search in it. Is there a way to do something similar in PHP?

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• Could someone give me their two cents on this optimization strategy

  - by jimstandard

  Background: I am writing a matching script in python that will match records of a transaction in one database to names of customers in another database. The complexity is that names are not unique and can be represented multiple different ways from transaction to transaction. Rather than doing multiple queries on the database (which is pretty slow) would it be faster to get all of the records where the last name (which in this case we will say never changes) is "Smith" and then have all of those records loaded into memory as you go though each looking for matches for a specific "John Smith" using various data points. Would this be faster, is it feasible in python, and if so does anyone have any recommendations for how to do it?

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• Is it faster to use a complicated boolean to limit a ResultSet at the MySQL end or at the Java end?

  - by javanix

  Lets say I have a really big table filled with lots of data (say, enough not to fit comfortably in memory), and I want to analyze a subset of the rows. Is it generally faster to do: SELECT (column1, column2, ... , columnN) FROM table WHERE (some complicated boolean clause); and then use the ResultSet, or is it faster to do: SELECT (column1, column2, ... , columnN) FROM table; and then iterate over the ResultSet, accepting different rows based on a java version of your boolean condition? I think it comes down to whether the Java iterator/boolean evaluator is faster than the MySQL boolean evaluator.

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• Excel workbooks produced by POI don't work when linked

  - by Eric Nicolas

  Here is what I'm doing : Create a workbook in memory (book = new HSSFWorkbook(), ...) Save it to disk (book.write(...)) Open in Excel (ok) Create another workbook in Excel, which links to the first one (=PoiWorkbook?xls!A1) Close Excel Then everytime I open the second workbook again, all the links are #N/A, unless I also open the POI-generated workbook at the same time. I never saw this behaviour with standard workbooks created in Excel. Anyone has seen this and found a workaround ? Thanks.

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• usage of 2 charectors in single qoutes in c

  - by user1632141

  #include<stdio.h> int main() { char ch = 'A'; printf("%d\n",'ag'); printf("%d\n",'a'); printf("%d, %d, %d, %d", sizeof(ch), sizeof('a'), sizeof('Ag'), sizeof(3.14f)); return 0; } I used to have many doubts on the output of this question while running on g++ and gcc. But I have cleared almost all the doubts by referring these links: Single and double quotes in C/C++ Single quotes vs. double quotes in C I still need to understand one thing about the output of this question. Can someone please explain the output of printf("%d\n",'ag'); mentioned above in the program. How is it actually stored in the memory? The output for the program on the Linux/GCC platform is: 24935 97 1, 4, 4, 4

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• OpenGL texture not showing up on some computers, problem with auxDIBImageLoad?

  - by CptanPanic

  I have a simple openGL GUI that has uses a bmp as a texture. The problem is on some computers the texture doesn't show up. But on ones with newer OpenGL drivers it works. How can I make it more compatible. I think the problem may be with auxDIBImageLoad. When I inspect the result, I see the x,y correct, but the data pointer points to memory bank with all zeros. What could be wrong?

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• Forcing a templated object to construct from a pointer

  - by SalamiArmi

  I have a fictional class: template<typename T> class demonstration { public: demonstration(){} ... T *m_data; } At some point in the program's execution, I want to set m_data to a big block of allocated memory and construct an object T there. At the moment, I've been using this code: void construct() { *m_data = T(); } Which I've now realised is probably not the best idea... wont work under certain cirumstances, if T has a private assignment operator for example. Is there a normal/better way to do what I'm attempting here?

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• how can i set mTextView to show on button click

  - by Ajay Pandey

  i have a UITextView object which i have created in interface builder with it's hidden property unmarked.Now i want this textView to be invisible when my application launches and first view appears.Further i want it to be displayed when a particular method is called.Now this is what i have written in view did load [mTextView setHidden:YES]; self.mTextView=[[UITextView alloc] init]; it hides the textView ats the first view appears but when my desired method is called and i write [mTextView setHidden:NO]; it does not show it again.. Is it like we cant change the appearance of a textView once it is assigned because after assigning memory as self.mTextView=[[UITextView alloc] init]; and then writing [mTextView setHidden:YES]; it does not hide textView in viwDidLoad either.....

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• Unit testing with serialization mock objects in C++

  - by lhumongous

  Greetings, I'm fairly new to TDD and ran across a unit test that I'm not entirely sure how to address. Basically, I'm testing a couple of legacy class methods which read/write a binary stream to a file. The class functions take a serializable object as a parameter, which handles the actual reading/writing to the file. For testing this, I was thinking that I would need a serialization mock object that I would pass to this function. My initial thought was to have the mock object hold onto a (char*) which would dynamically allocate memory and memcpy the data. However, it seems like the mock object might be doing too much work, and might be beyond the scope of this particular test. Is my initial approach correct, or can anyone think of another way of correctly testing this? Thanks!

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• Freeing JavaScript object

  - by pion

  I am looking at the example from http://www.javascriptkit.com/javatutors/oopjs.shtml var person = new Object() person.name = "Tim Scarfe" person.height = "6Ft" But there is no mention how to "free" it in order to avoid memory leak. Will the following code free it? person = null; How do you free a JavaScript Object using "new Object()? How do you free a JavaScript Array allocated using "new Array(10)"? How do you free a JavaScript JSON allocated using "var json = {"width": 480, "height": 640}"? Thanks in advance for your help.

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• OpenGL Vertex Array/Buffer Objects

  - by sadanjon

  Question 1 Do vertex buffer objects created under a certain VAO deleted once that VAO is deleted? An example: glGenBuffers(1, &bufferObject); glGenVertexArrays(1, &VAO); glBindVertexArray(VAO); glBindBuffer(GL_ARRAY_BUFFER, bufferObject); glBufferData(GL_ARRAY_BUFFER, sizeof(someVertices), someVertices, GL_STATIC_DRAW); glEnableVertexAttribArray(positionAttrib); glVertexAttribPointer(positionAttrib, 3, GL_FLOAT, GL_FALSE, 0, NULL); When later calling glDeleteVertexArrays(1, &VAO);, will bufferObject be deleted as well? The reason I'm asking is that I saw a few examples over the web that didn't delete those buffer objects. Question 2 What is the maximum amount of memory that I can allocate for buffer objects? It must be system dependent of course, but I can't seem find an estimation for it. What happens when video RAM isn't big enough? How would I know?

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• Caching issue with Centos forwarding DNS server

  - by Paddington

  I installed a Forwarding DNS server on Centos 5.10 and it is resolving addresses e.g google.com. When I stopped named (service named stop) and tried to dig (dig @localhost A google.com) there was a failure to resolve the address. I checked and see the caching daemon nscd is running. Does this mean the server is not caching at all? How can I get it to cache? named.conf options { // Those options should be used carefully because they disable port // randomization // query-source port 53; // query-source-v6 port 53; // Put files that named is allowed to write in the data/ directory: listen-on port 53 {127.0.0.1; 10.0.0.4;}; directory "/var/named"; // the default dump-file "/var/named/chroot/var/named/data/cache_dump.db"; statistics-file "/var/named/chroot/var/named/data/named_stats.txt"; memstatistics-file "/var/named/chroot/var/named/data/named_mem_stats.txt"; // allow-query {localhost; 192.168.0.0/24; 10.0.0.0/8;}; recursion yes; //allow-query { localhost; 10.0.0.0/8;}; allow-query { localhost; any; }; allow-query-cache { localhost; any; }; forward only; forwarders {8.8.8.8; 8.8.4.4;}; dnssec-enable yes; // dnssec-lookaside auto; /* Path to ISC DLV key */ // bindkeys-file "/etc/named.iscdlv.key"; // managed-keys-directory "/var/named/dynamic"; }; logging { channel default_debug { file "data/named.run"; severity dynamic; }; }; **

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• storing an integer constant other than zero in a pointer variable

  - by benjamin button

  int main() { int *d=0; printf("%d\n",*d); return 0; } this works fine. >cc legal.c > ./a.out 0 if i change the statement int *d=0; to int *d=1; i see the error. cc: "legal.c", line 6: error 1522: Cannot initialize a pointer with an integer constant other than zero. so its obvious that it will allow only zero.i want to know what happens inside the memory when we do this int *d=0 which is making it valid syntax. I am just asking this out of curiosity!

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• How does Photoshop (Or drawing programs) blit?

  - by user146780

  I'm getting ready to make a drawing application in Windows. I'm just wondering, do drawing programs have a memory bitmap which they lock, then set each pixel, then blit? I don't understand how Photoshop can move entire layers without lag or flicker without using hardware acceleration. Also in a program like Expression Design, I could have 200 shapes and move them around all at once with no lag. I'm really wondering how this can be done without GPU help. I don't think super efficient algorithms could justify that? Thanks

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• How do I write raw binary data in Python?

  - by Chris B.

  I've got a Python program that stores and writes data to a file. The data is raw binary data, stored internally as str. I'm writing it out through a utf-8 codec. However, I get UnicodeDecodeError: 'charmap' codec can't decode byte 0x8d in position 25: character maps to <undefined> in the cp1252.py file. This looks to me like Python is trying to interpret the data using the default code page. But it doesn't have a default code page. That's why I'm using str, not unicode. I guess my questions are: How do I represent raw binary data in memory, in Python? When I'm writing raw binary data out through a codec, how do I encode/unencode it?

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• Output of System.out.println(object)

  - by Shaarad Dalvi

  I want to know what exactly the output tells when I do the following : class data { int a=5; } class main { public static void main(String[] args) { data dObj=new data(); System.out.println(dObj); } } I know it gives something related to object as the output in my case is data@1ae73783. I guess the '1ae73783' is a hex number. I also did some work around and printed System.out.println(dObj.hashCode()); I got number 415360643. I got an integer value. I don't know what hashCode() returns, still out of curiosity, when I converted 1ae73783 to decimal, I got 415360643! That's why I am curious that what exactly is this number?? Is this some memory location of Java's sandbox or some other thing? Any light on this matter will be helpful..thanks! :)

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