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• Custom webserver caching

  - by Mark Kinsella

  I'm working with a custom webserver on an embedded system and having some problems correctly setting my HTTP Headers for caching. Our webserver is generating all dynamic content as XML and we're using semi-static XSL files to display it with some dynamic JSON requests thrown in for good measure along with semi-static images. I say "semi-static" because the problems occur when we need to do a firmware update which might change the XSL and image files. Here's what needs to be done: cache the XSL and image files and do not cache the XML and JSON responses. I have full control over the HTTP response and am currently: Using ETags with the XSL and image files, using the modified time and size to generate the ETag Setting Cache-Control: no-cache on the XML and JSON responses As I said, everything works dandy until a firmware update when the XSL and image files are sometimes cached. I've seen it work fine with the latest versions of Firefox and Safari but have had some problems with IE. I know one solution to this problem would be simply rename the XSL and image files after each version (eg. logo-v1.1.png, logo-v1.2.png) and set the Expires header to a date in the future but this would be difficult with the XSL files and I'd like to avoid this. Note: There is a clock on the unit but requires the user to set it and might not be 100% reliable which is what might be causing my caching issues when using ETags. What's the best practice that I should employ? I'd like to avoid as many webserver requests as possible but invalidating old XSL and image files after a software update is the #1 priority.

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• problem configure JBoss to work with JNDI

  - by Spiderman

  I am trying to bind connection to the DB using JNDI in my application that runs on JBoss. I did the following: I created the datasource file oracle-ds.xml filled it with the relevant xml elements: <datasources> <local-tx-datasource> <jndi-name>bilby</jndi-name> ... </local-tx-datasource> </datasources> and put it in the folder \server\default\deploy Added the relevant oracle jar file than in my application I performed: JndiObjectFactoryBean factory = new JndiObjectFactoryBean(); factory.setJndiName("bilby"); try{ factory.afterPropertiesSet(); dataSource = factory.getObject(); } catch(NamingException ne) { ne.printStackTrace(); } and this cause the error: javax.naming.NameNotFoundException: bilby not bound then in the output after this error occured I saw the line: 18:37:56,560 INFO [ConnectionFactoryBindingService] Bound ConnectionManager 'jb oss.jca:service=DataSourceBinding,name=bilby' to JNDI name 'java:bilby' So what is my configuration problem? I think that it may be that JBoss first loads and runs the .war file of my application and only then it loads the oracle-ds.xml that contain my data-source definition. The problem is that they are both located in the same folder. Is there a way to define priority of loading them, or maybe this is not the problem at all. Any idea?

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• Project Euler Problem #11

  - by SoulBeaver

  Source: http://projecteuler.net/index.php?section=problems&id=11 Quick overview: Take a 20x20 grid of numbers and compute the largest product of 4 pairs of numbers in either horizontal, vertical, or diagonal. My current approach is to divide the 20x20 grid up into single rows and single columns and go from there with a much more manageable grid. The code I'm using to divide the rows into rows is void fillRows ( string::const_iterator& fieldIter, list<int>& rowElements, vector<list<int>>& rows ) { int count(0); for( ; fieldIter < field.end(); ++fieldIter ) { if(isdigit(field[*fieldIter])) { rowElements.push_back(toInt(field[*fieldIter])); ++count; } if(count == 40) { rows.push_back(rowElements); count = 0; rowElements.clear(); } } } Short explanation: I have the field set as static const std::string field and I am filling a vector with lists of rows. Why a list? Because the queue doesn't have a clear function. Also practice using STL container lists and not ones I write myself. However, this thing isn't working. Oftentimes I see it omitting a character( function toInt parses the const char as int ) and I end up with 18 rows, two rows short of the 20x20 grid. The length of the rows seem good. Rows: 18 RowElements[0]: 40 (instead of pairs I saved each number individually. Will fix that later) What am I doing wrong?

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• How to perform undirected graph processing from SQL data

  - by recipriversexclusion

  I ran into the following problem in dynamically creating topics for our ActiveMQ system: I have a number of processes (M_1, ..., M_n), where n is not large, typically 5-10. Some of the processes will listen to the output of others, through a message queue; these edges are specified in an XML file, e.g. <link from="M1" to="M3"</link> <link from="M2" to="M4"</link> <link from="M3" to="M4"</link> etc. The edges are sparse, so there won't be many of them. I will parse this XML and store this information in an SQL DB, one table for nodes and another for edges. Now, I need to dynamically create strings of the form M1.exe --output_topic=T1 M2.exe --output_topic=T2 M3.exe --input_topic=T1 --output_topic=T3 M4.exe --input_topic=T2 --input_topic=T3 where the tags are sequentially generated. What is the best way to go about querying SQL to obtain these relationships? Are there any tools or other tutorials you can point me to? I've never done graps with SQL. Using SQL is imperative, because we use it for other stuff, too. Thanks!

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• Serial: write() throttling?

  - by damian

  Hi everyone, I'm working on a project sending serial data to control animation of LED lights, which need to stay in sync with a sound engine. There seems to be a large serial write buffer (OSX (POSIX) + FTDI chipset usb serial device), so without manually restricting the transmission rate, the animation system can get several seconds ahead of the serial transmission. Currently I'm manually restricting the serial write speed to the baudrate (8N1 = 10 bytes serial frame per 8 bytes data, 19200 bps serial - 1920 bytes per second max), but I am having a problem with the sound drifting out of sync over time - it starts fine, but after 10 minutes there's a noticeable (100ms+) lag between the sound and the lights. This is the code that's restricting the serial write speed (called once per animation frame, 'elapsed' is the duration of the current frame, 'baudrate' is the bps (19200)): void BufferedSerial::update( float elapsed ) { baud_timer += elapsed; if ( bytes_written > 1024 ) { // maintain baudrate float time_should_have_taken = (float(bytes_written)*10)/float(baudrate); float time_actually_took = baud_timer; // sleep if we have > 20ms lag between serial transmit and our write calls if ( time_should_have_taken-time_actually_took > 0.02f ) { float sleep_time = time_should_have_taken - time_actually_took; int sleep_time_us = sleep_time*1000.0f*1000.0f; //printf("BufferedSerial::update sleeping %i ms\n", sleep_time_us/1000 ); delayUs( sleep_time_us ); // subtract 128 bytes bytes_written -= 128; // subtract the time it should have taken to write 128 bytes baud_timer -= (float(128)*10)/float(baudrate); } } } Clearly there's something wrong, somewhere. A much better approach would be to be able to determine the number of bytes currently in the transmit queue, and try and keep that below a fixed threshold. Any advice appreciated.

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• Is JavaEE really portable?

  - by Bozho

  I'm just implementing a JavaEE assignment I was given on an interview. I have some prior experience with EJB, but nothing related to JMS and MDBs. So here's what I find through the numerous examples: application servers bind their topics and queues to different JNDI names - for example topic/queue, jms the activationConfig property is required on JBoss, while in the Sun tutorial it is not. after starting my application, jboss warns me that my topic isn't bound (it isn't actually - I haven't bound it, but I expect it to be bound automatically - in fact, in an example for JBoss 4.0 automatic binding does seem to happen). A suggested solution is to map it in some jboss files or even use jboss-specific annotations. This might be just JBoss, but since it is certified to implement to spec, it appears the spec doesn't specify these these things. And there all the alleged portability vanishes. So I wonder - how come it is claimed that JavaEE is portable and you can take an ear and deploy it on another application server and it magically runs, if such extremely basic things don't appear to be portable at all. P.S. sorry for the rant, but I'm assume I might be doing/getting something wrong, so state your opinions.

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• Call to a member function num_rows() on a non-object

  - by Patrick

  I need to get the number of rows of a query (so I can paginate results). As I'm learning codeigniter (and OO php) I wanted to try and chain a -num_rows() to the query, but it doesn't work: //this works: $data['count'] = count($this->events->findEvents($data['date'], $data['keyword'])); //the following doesn't work and generates // Fatal Error: Call to a member function num_rows() on a non-object $data['count2'] = $this->events->findEvents($data['date'], $data['keyword'])->num_rows(); the model returns an array of objects, and I think this is the reason why I can't use a method on it. function findEvents($date, $keyword, $limit = NULL, $offset = NULL) { $data = array(); $this->db->select('events.*, venues.*, events.venue AS venue_id'); $this->db->join('venues', 'events.venue = venues.id'); if ($date) { $this->db->where('date', $date); } if ($keyword) { $this->db->like('events.description', $keyword); $this->db->or_like('venues.description', $keyword); $this->db->or_like('band', $keyword); $this->db->or_like('venues.venue', $keyword); $this->db->or_like('genre', $keyword); } $this->db->order_by('date', 'DESC'); $this->db->order_by('events.priority', 'DESC'); $this->db->limit($limit, $offset); //for pagination purposes $Q = $this->db->get('events'); if ($Q->num_rows() > 0) { foreach ($Q->result() as $row) { $data[] = $row; } } $Q->free_result(); return $data; } Is there anything that i can do to be able to use it? EG, instead of $data[] = $row; I should use another (OO) syntax?

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• CakePHP: Custom Function in bootstrap that uses $ajax->link not working

  - by nekko

  Hello I have two questions: (1) Is it best practice to create global custom functions in the bootstrap file? Is there a better place to store them? (2) I am unable use the following line of code in my custom function located in my bootstrap.php file: $url = $ajax->link ( 'Delete', array ('controller' => 'events', 'action' => 'delete', 22 ), array ('update' => 'event' ), 'Do you want to delete this event?' ); echo $url; I receive the following error: Notice (8): Undefined variable: ajax [APP\config\bootstrap.php, line 271] Code } function testAjax () { $url = $ajax->link ( 'Delete', array ('controller' => 'events', 'action' => 'delete', 22 ), array ('update' => 'event' ), 'Do you want to delete this event?' ); testAjax - APP\config\bootstrap.php, line 271 include - APP\views\event\queue.ctp, line 19 View::_render() - CORE\cake\libs\view\view.php, line 649 View::render() - CORE\cake\libs\view\view.php, line 372 Controller::render() - CORE\cake\libs\controller\controller.php, line 766 Dispatcher::_invoke() - CORE\cake\dispatcher.php, line 211 Dispatcher::dispatch() - CORE\cake\dispatcher.php, line 181 [main] - APP\webroot\index.php, line 91 However it works as intended if I place that same code in my view: <a onclick=" event.returnValue = false; return false;" id="link1656170149" href="/shout/events/delete/22">Delete</a> Please help :) Thanks in advance!!

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• Rails running multiple delayed_job - lock tables

  - by pepernik

  Hey. I use delayed_job for background processing. I have 8 CPU server, MySQL and I start 7 delayed_job processes RAILS_ENV=production script/delayed_job -n 7 start Q1: I'm wondering is it possible that 2 or more delayed_job processes start processing the same process (the same record-row in the database delayed_jobs). I checked the code of the delayed_job plugin but can not find the lock directive in a way it should be. I think each process should lock the database table before executing an UPDATE on lock_by column. They lock the record simply by updating the locked_by field (UPDATE delayed_jobs SET locked_by...). Is that really enough? No locking needed? Why? I know that UPDATE has higher priority than SELECT but I think this does not have the effect in this case. My understanding of the multy-threaded situation is: Process1: Get waiting job X. [OK] Process2: Get waiting jobs X. [OK] Process1: Update locked_by field. [OK] Process2: Update locked_by field. [OK] Process1: Get waiting job X. [Already processed] Process2: Get waiting jobs X. [Already processed] I think in some cases more jobs can get the same information and can start processing the same process. Q2: Is 7 delayed_jobs a good number for 8CPU server? Why yes/not. Thx 10x!

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• Optimizing processing and management of large Java data arrays

  - by mikera

  I'm writing some pretty CPU-intensive, concurrent numerical code that will process large amounts of data stored in Java arrays (e.g. lots of double[100000]s). Some of the algorithms might run millions of times over several days so getting maximum steady-state performance is a high priority. In essence, each algorithm is a Java object that has an method API something like: public double[] runMyAlgorithm(double[] inputData); or alternatively a reference could be passed to the array to store the output data: public runMyAlgorithm(double[] inputData, double[] outputData); Given this requirement, I'm trying to determine the optimal strategy for allocating / managing array space. Frequently the algorithms will need large amounts of temporary storage space. They will also take large arrays as input and create large arrays as output. Among the options I am considering are: Always allocate new arrays as local variables whenever they are needed (e.g. new double[100000]). Probably the simplest approach, but will produce a lot of garbage. Pre-allocate temporary arrays and store them as final fields in the algorithm object - big downside would be that this would mean that only one thread could run the algorithm at any one time. Keep pre-allocated temporary arrays in ThreadLocal storage, so that a thread can use a fixed amount of temporary array space whenever it needs it. ThreadLocal would be required since multiple threads will be running the same algorithm simultaneously. Pass around lots of arrays as parameters (including the temporary arrays for the algorithm to use). Not good since it will make the algorithm API extremely ugly if the caller has to be responsible for providing temporary array space.... Allocate extremely large arrays (e.g. double[10000000]) but also provide the algorithm with offsets into the array so that different threads will use a different area of the array independently. Will obviously require some code to manage the offsets and allocation of the array ranges. Any thoughts on which approach would be best (and why)?

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• Database for Python Twisted

  - by Will

  There's an API for Twisted apps to talk to a database in a scalable way: twisted.enterprise.dbapi The confusing thing is, which database to pick? The database will have a Twisted app that is mostly making inserts and updates and relatively few selects, and then other strictly-read-only clients that are accessing the database directly making selects. (The read-only users are not necessarily selecting the data that the Twisted app is inserting; its not as though the database is being used as a message-queue) My understanding - which I'd like corrected/adviced - is that: Postgres is a great DB, but all the Python bindings - and there is a confusing maze of them - are abandonware There is psycopg2, but that makes a lot of noise about doing its own connection-pooling and things; does this co-exist gracefully/usefully/transparently with the Twisted async database connection pooling and such? SQLLite is a great database for little things but if used in a multi-user way it does whole-database locking, so performance would suck in the usage pattern I envisage MySQL - after the Oracle takeover, who'd want to adopt it now or adopt a fork? Is there anything else out there?

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• How to use an Audio Unit on the iPhone

  - by CodeToaster

  I'm looking for a way to change the pitch of recorded audio as it is saved to disk, or played back (in real time). I understand Audio Units can be used for this. The iPhone offers limited support for Audio Units (for example it's not possible to create/use custom audio units, as far as I can tell), but several out-of-the-box audio units are available, one of which is AUPitch. How exactly would I use an audio unit (specifically AUPitch)? Do you hook it into an audio queue somehow? Is it possible to chain audio units together (for example, to simultaneously add an echo effect and a change in pitch)? EDIT: After inspecting the iPhone SDK headers (I think AudioUnit.h, I'm not in front of a Mac at the moment), I noticed that AUPitch is commented out. So it doesn't look like AUPitch is available on the iPhone after all. weep weep Apple seems to have better organized their iPhone SDK documentation at developer.apple.com of late - now its more difficult to find references to AUPitch, etc. That said, I'm still interested in quality answers on using Audio Units (in general) on the iPhone.

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• libav/ffmpeg: avcodec_decode_video2() returns -1 when separating demultiplexing and decoding

  - by unbekannt

  I'm using libav (from a C++ program on Linux and Windows) to decode video streams from a file, which works fine (decoding various formats like H264 and MPEG2) using avformat_open_input(), av_read_frame() and avcodec_decode_video2(). Now I have to separate demultiplexing and decoding. One class will call avformat_open_input() and av_read_frame() and then pass the AVPackets into a queue that is read by another class. There I use avcodec_alloc_context3() to get the AVCodecContext needed for avcodec_decode_video2(). I've tested that with a MPEG2 video stream and it works. Problems arise if I try to decode a H264 stream: avcodec_decode_video2() always returns -1 and outputs "no frame". I understand that additional data (SPS/PPS) is needed to decode this stream, so I've tried to replicate the original AVCodecContext from the demultiplexer in the decoder, but it won't work: Copying the content of the extradata field and setting all other values that differ from the default ones in the decoder: -1 is returned Using the same context (i.e. passing along the pointer) results in a crash I also tried to set CODEC_FLAG2_CHUNKS. avcodec_decode_video2() then always returns packet.size - 3 (??) and frameFinished is never set to 1. In my opinion I have a general problem here that will arise whenever settings from the original CodecContext are needed to decode the AVPackets. I'd be grateful for any hints on how to solve that problem!

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• bad_alloc occuring when allocating small structs

  - by SalamiArmi

  A bad_alloc has started showing up in some code which looks perfectly valid to me and has worked very well in the past. The bad alloc only occurs once every 50-3000 iterations of the code, which is also confusing. The code itself is from a singly linked list, simply adding a new element to the queue: template<typename T> struct container { inline container() : next(0) {} container *next; T data; }; void push(const T &data) { container<T> *newQueueMember = new container<T>; //... unrelated to crash } Where T is: struct test { int m[256]; }; Changing the size of the array allocated array to anything but very small values (1-8 ints) still results in a bad_alloc occasionally. A few extra notes about my program: - I used Poco::ThreadPool to thread my program. I've only recently added this functionality, before I had it running with Win32 threads. However, only the main thread ever calls push(). - I am also occasionally getting other crashes which could be related. However, when I try to debug with visual studio 2008, I can't navigate back to the call stack, or the crash happens deep within new(). Thanks in advance.

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• Distributed Message Ordering

  - by sbanwart

  I have an architectural question on handling message ordering. For purposes of this question, the transport is irrelevant, so I'm not going to specify one. Say we have three systems, a website, a CRM and an ERP. For this example, the ERP will be the "master" system in terms of data ownership. The website and the CRM can both send a new customer message to the ERP system. The ERP system then adds a customer and publishes the customer with the newly assigned account number so that the website and CRM can add the account number to their local customer records. This is a pretty straight forward process. Next we move on to placing orders. The account number is required in order for the CRM or website to place an order with the ERP system. However the CRM will permit the user to place an order even if the customer lacks an account number. (For this example assume we can't modify the CRM behavior) This creates the possibility that a user could create a new customer, and place an order before the account number gets updated in the CRM. What is the best way to handle this scenario? Would it be best to send the order message sans account number and let it go to an error queue? Would it be better to have the CRM endpoint hold the message and wait until the account number is updated in the CRM? Maybe something completely different that I haven't thought of? Thanks in advance for any help.

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• How to increase the performance of a loop which runs for every 'n' minutes.

  - by GustlyWind

  Hi Giving small background to my requirement and what i had accomplished so far: There are 18 Scheduler tasks run at regular intervals (least being 30 mins) takes input of nearly 5000 eligible employees run into a static method for iteration and generates a mail content for that employee and mails. An average task takes about 9 min multiplied by 18 will be roughly 162 mins meanwhile there would be next tasks which will be in queue (I assume). So my plan is something like the below loop try { // Handle Arraylist of alerts eligible employees Iterator employee = empIDs.iterator(); while (employee.hasNext()) { ScheduledImplementation.getInstance().sendAlertInfoToEmpForGivenAlertType((Long)employee.next(), configType,schedType); } } catch (Exception vEx) { _log.error("Exception Caught During sending " + configType + " messages:" + configType, vEx); } Since I know how many employees would come to my method I will divide the while loop into two and perform simultaneous operations on two or three employees at a time. Is this possible. Or is there any other ways I can improve the performance. Some of the things I had implemented so far 1.Wherever possible made methods static and variables too Didn't bother to catch exceptions and send back because these are background tasks. (And I assume this improves performance) Get the DB values in one query instead of multiple hits. If am successful in optimizing the while loop I think i can save couple of mins. Thanks

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• What techniques can be used to detect so called "black holes" (a spider trap) when creating a web crawler?

  - by Tom

  When creating a web crawler, you have to design somekind of system that gathers links and add them to a queue. Some, if not most, of these links will be dynamic, which appear to be different, but do not add any value as they are specifically created to fool crawlers. An example: We tell our crawler to crawl the domain evil.com by entering an initial lookup URL. Lets assume we let it crawl the front page initially, evil.com/index The returned HTML will contain several "unique" links: evil.com/somePageOne evil.com/somePageTwo evil.com/somePageThree The crawler will add these to the buffer of uncrawled URLs. When somePageOne is being crawled, the crawler receives more URLs: evil.com/someSubPageOne evil.com/someSubPageTwo These appear to be unique, and so they are. They are unique in the sense that the returned content is different from previous pages and that the URL is new to the crawler, however it appears that this is only because the developer has made a "loop trap" or "black hole". The crawler will add this new sub page, and the sub page will have another sub page, which will also be added. This process can go on infinitely. The content of each page is unique, but totally useless (it is randomly generated text, or text pulled from a random source). Our crawler will keep finding new pages, which we actually are not interested in. These loop traps are very difficult to find, and if your crawler does not have anything to prevent them in place, it will get stuck on a certain domain for infinity. My question is, what techniques can be used to detect so called black holes? One of the most common answers I have heard is the introduction of a limit on the amount of pages to be crawled. However, I cannot see how this can be a reliable technique when you do not know what kind of site is to be crawled. A legit site, like Wikipedia, can have hundreds of thousands of pages. Such limit could return a false positive for these kind of sites. Any feedback is appreciated. Thanks.

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• Handling file uploads with JavaScript and Google Gears, is there a better solution?

  - by gnarf

  So - I've been using this method of file uploading for a bit, but it seems that Google Gears has poor support for the newer browsers that implement the HTML5 specs. I've heard the word deprecated floating around a few channels, so I'm looking for a replacement that can accomplish the following tasks, and support the new browsers. I can always fall back to gears / standard file POST's but these following items make my process much simpler: Users MUST to be able to select multiple files for uploading in the dialog. I MUST be able to receive status updates on the transmission of a file. (progress bars) I would like to be able to use PUT requests instead of POST I would like to be able to easily attach these events to existing HTML elements using JavaScript. I.E. the File Selection should be triggered on a <button> click. I would like to be able to control response/request parameters easily using JavaScript. I'm not sure if the new HTML5 browsers have support for the desktop/request objects gears uses, or if there is a flash uploader that has these features that I am missing in my google searches. An example of uploading code using gears: // select some files: var desktop = google.gears.factory.create('beta.desktop'); desktop.openFiles(selectFilesCallback); function selectFilesCallback(files) { $.each(files,function(k,file) { // this code actually goes through a queue, and creates some status bars // but it is unimportant to show here... sendFile(file); }); } function sendFile(file) { google.gears.factory.create('beta.httprequest'); request.open('PUT', upl.url); request.setRequestHeader('filename', file.name); request.upload.onprogress = function(e) { // gives me % status updates... allows e.loaded/e.total }; request.onreadystatechange = function() { if (request.readyState == 4) { // completed the upload! } }; request.send(file.blob); return request; } Edit: apparently flash isn't capable of using PUT requests, so I have changed it to a "like" instead of a "must".

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• Optimizing a shared buffer in a producer/consumer multithreaded environment

  - by Etan

  I have some project where I have a single producer thread which writes events into a buffer, and an additional single consumer thread which takes events from the buffer. My goal is to optimize this thing for a single machine to achieve maximum throughput. Currently, I am using some simple lock-free ring buffer (lock-free is possible since I have only one consumer and one producer thread and therefore the pointers are only updated by a single thread). #define BUF_SIZE 32768 struct buf_t { volatile int writepos; volatile void * buffer[BUF_SIZE]; volatile int readpos;) }; void produce (buf_t *b, void * e) { int next = (b->writepos+1) % BUF_SIZE; while (b->readpos == next); // queue is full. wait b->buffer[b->writepos] = e; b->writepos = next; } void * consume (buf_t *b) { while (b->readpos == b->writepos); // nothing to consume. wait int next = (b->readpos+1) % BUF_SIZE; void * res = b->buffer[b->readpos]; b->readpos = next; return res; } buf_t *alloc () { buf_t *b = (buf_t *)malloc(sizeof(buf_t)); b->writepos = 0; b->readpos = 0; return b; } However, this implementation is not yet fast enough and should be optimized further. I've tried with different BUF_SIZE values and got some speed-up. Additionaly, I've moved writepos before the buffer and readpos after the buffer to ensure that both variables are on different cache lines which resulted also in some speed. What I need is a speedup of about 400 %. Do you have any ideas how I could achieve this using things like padding etc?

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• Does SetFileBandwidthReservation affect memory-mapped file performance?

  - by Ghostrider

  Does this function affect Memory-mapped file performance? Here's the problem I need to solve: I have two applications competing for disk access: "reader" and "updater". Whole system runs on Windows Server 2008 R2 x64 "Updater" constantly accesses disk in a linear manner, updating data. They system is set up in such a way that updater always has infinite data to update. Consider that it is constantly approximating a solution of a huge set of equations that takes up entire 2TB disk drive. Updater uses ReadFile and WriteFile to process data in a linear fashion. "Reader" is occasionally invoked by user to get some pieces of data. Usually user would read several 4kb blocks from the drive and stop. Occasionally user needs to read up to 100mb sequentially. In exceptional cases up to several gigabytes. Reader maps files to memory to get data it needs. What I would like to achieve is for "reader" to have absolute priority so that "updater" would completely stop if needed so that "reader" could get the data user needs ASAP. Is this problem solvable by using SetPriorityClass and SetFileBandwidthReservation calls? I would really hate to put synchronization login in "reader" and "updater" and rather have the OS take care of priorities.

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• In C++, what is the scope resolution ("order of precedence") for shadowed variable names?

  - by Emile Cormier

  In C++, what is the scope resolution ("order of precedence") for shadowed variable names? I can't seem to find a concise answer online. For example: #include <iostream> int shadowed = 1; struct Foo { Foo() : shadowed(2) {} void bar(int shadowed = 3) { std::cout << shadowed << std::endl; // What does this output? { int shadowed = 4; std::cout << shadowed << std::endl; // What does this output? } } int shadowed; }; int main() { Foo().bar(); } I can't think of any other scopes where a variable might conflict. Please let me know if I missed one. What is the order of priority for all four shadow variables when inside the bar member function?

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• Returning the same type the function was passed

  - by Ken Bloom

  I have the following code implementation of Breadth-First search. trait State{ def successors:Seq[State] def isSuccess:Boolean = false def admissableHeuristic:Double } def breadthFirstSearch(initial:State):Option[List[State]] = { val open= new scala.collection.mutable.Queue[List[State]] val closed = new scala.collection.mutable.HashSet[State] open.enqueue(initial::Nil) while (!open.isEmpty){ val path:List[State]=open.dequeue() if(path.head.isSuccess) return Some(path.reverse) closed += path.head for (x <- path.head.successors) if (!closed.contains(x)) open.enqueue(x::path) } return None } If I define a subtype of State for my particular problem class CannibalsState extends State { //... } What's the best way to make breadthFirstSearch return the same subtype as it was passed? Supposing I change this so that there are 3 different state classes for my particular problem and they share a common supertype: abstract class CannibalsState extends State { //... } class LeftSideOfRiver extends CannibalsState { //... } class InTransit extends CannibalsState { //... } class RightSideOfRiver extends CannibalsState { //... } How can I make the types work out so that breadthFirstSearch infers that the correct return type is CannibalsState when it's passed an instance of LeftSideOfRiver? Can this be done with an abstract type member, or must it be done with generics?

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• Does Android AsyncTaskQueue or similar exist?

  - by Ben L.

  I read somewhere (and have observed) that starting threads is slow. I always assumed that AsyncTask created and reused a single thread because it required being started inside the UI thread. The following (anonymized) code is called from a ListAdapter's getView method to load images asynchronously. It works well until the user moves the list quickly, and then it becomes "janky". final File imageFile = new File(getCacheDir().getPath() + "/img/" + p.image); image.setVisibility(View.GONE); view.findViewById(R.id.imageLoading).setVisibility(View.VISIBLE); (new AsyncTask<Void, Void, Bitmap>() { @Override protected Bitmap doInBackground(Void... params) { try { Bitmap image; if (!imageFile.exists() || imageFile.length() == 0) { image = BitmapFactory.decodeStream(new URL( "http://example.com/images/" + p.image).openStream()); image.compress(Bitmap.CompressFormat.JPEG, 85, new FileOutputStream(imageFile)); image.recycle(); } image = BitmapFactory.decodeFile(imageFile.getPath(), bitmapOptions); return image; } catch (MalformedURLException ex) { // TODO Auto-generated catch block ex.printStackTrace(); return null; } catch (IOException ex) { // TODO Auto-generated catch block ex.printStackTrace(); return null; } } @Override protected void onPostExecute(Bitmap image) { if (view.getTag() != p) // The view was recycled. return; view.findViewById(R.id.imageLoading).setVisibility( View.GONE); view.findViewById(R.id.image) .setVisibility(View.VISIBLE); ((ImageView) view.findViewById(R.id.image)) .setImageBitmap(image); } }).execute(); I'm thinking that a queue-based method would work better, but I'm wondering if there is one or if I should attempt to create my own implementation.

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• How can I work out what events are being waited for with WinDBG in a kernel debug session

  - by Benj

  I'm a complete WinDbg newbie and I've been trying to debug a WindowsXP problem that a customer has sent me where our software and some third party software prevent windows from logging off. I've reproduced the problem and have verified that only when our software and the customers software are both installed (although not necessarily running at logoff) does the log off problem occur. I've observed that WM_ENDSESSION messages are not reaching the running windows when the user tries to log off and I know that the third party software uses a kernel driver. I've been looking at the processes in WinDbg and I know that csrss.exe would normally send all the windows a WM_ENDSESSION message. When I ran: !process 82356020 6 To look at csrss.exe's stack I can see: WARNING: Frame IP not in any known module. Following frames may be wrong. 00000000 00000000 00000000 00000000 00000000 0x7c90e514 THREAD 8246d998 Cid 0248.02a0 Teb: 7ffd7000 Win32Thread: e1627008 WAIT: (WrUserRequest) UserMode Non-Alertable 8243d9f0 SynchronizationEvent 81fe0390 SynchronizationEvent Not impersonating DeviceMap e1004450 Owning Process 82356020 Image: csrss.exe Attached Process N/A Image: N/A Wait Start TickCount 1813 Ticks: 20748 (0:00:05:24.187) Context Switch Count 3 LargeStack UserTime 00:00:00.000 KernelTime 00:00:00.000 Start Address 0x75b67cdf Stack Init f80bd000 Current f80bc9c8 Base f80bd000 Limit f80ba000 Call 0 Priority 14 BasePriority 13 PriorityDecrement 0 DecrementCount 0 Kernel stack not resident. ChildEBP RetAddr Args to Child f80bc9e0 80500ce6 00000000 8246d998 804f9af2 nt!KiSwapContext+0x2e (FPO: [Uses EBP] [0,0,4]) f80bc9ec 804f9af2 804f986e e1627008 00000000 nt!KiSwapThread+0x46 (FPO: [0,0,0]) f80bca24 bf80a4a3 00000002 82475218 00000001 nt!KeWaitForMultipleObjects+0x284 (FPO: [Non-Fpo]) f80bca5c bf88c0a6 00000001 82475218 00000000 win32k!xxxMsgWaitForMultipleObjects+0xb0 (FPO: [Non-Fpo]) f80bcd30 bf87507d bf9ac0a0 00000001 f80bcd54 win32k!xxxDesktopThread+0x339 (FPO: [Non-Fpo]) f80bcd40 bf8010fd bf9ac0a0 f80bcd64 00bcfff4 win32k!xxxCreateSystemThreads+0x6a (FPO: [Non-Fpo]) f80bcd54 8053d648 00000000 00000022 00000000 win32k!NtUserCallOneParam+0x23 (FPO: [Non-Fpo]) f80bcd54 7c90e514 00000000 00000022 00000000 nt!KiFastCallEntry+0xf8 (FPO: [0,0] TrapFrame @ f80bcd64) This waitForMultipleObjects looks interesting because I'm wondering if csrss.exe is waiting on some event which isn't arriving to allow the logoff. Can anyone tell me how I might find out what event it's waiting for anything else I might do to further investigate the problem?

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• Winsock WSAAsyncSelect sending without an infinite buffer

  - by Xexr

  Hi, This is more of a design question than a specific code question, I'm sure I am missing the obvious, I just need another set of eyes. I am writing a multi-client server based on WSAAsyncSelect, each connection is made into an object of a connection class I have written which contains associated settings and buffers etc. My question concerns FD_WRITE, I understand how it operates: One FD_WRITE is sent immediately after a connection is established. Thereafter, you should send until WSAEWOULDBLOCK is received at which point you store what is left to send in a buffer, and wait to be told that it is ok to send again. This is where I have a problem, how large do I make this holding buffer within each connections object? The amount of time until a new FD_WRITE is received is unknown, I could be attempting to send a lot of stuff during this period, all the time adding to my outgoing buffer. If I make the buffer dynamic, memory usage could spiral out of control if for whatever reason, I am unable to send() and reduce the buffer. So my question is how do you generally handle this situation? Note I am not talking about the network buffer itself which winsock uses, but one of my own creation used to "queue" up sends. Hope I explained that well enough, thanks all!

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