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• Whats a good structure to save and retrieve locations of images?

  - by Goot

  I got a java-ee application, where I collect informations about movies. Im my backend I provide data like the name, description, genre and a random uuid. I also got lots of related files, which are stored on a file server. Including some screenshots, the dvd or bluRay cover and video trailers. My current approach is: When saving the files to the fileserver, I retrieve the movies random uuid (which is the primary key btw.). I then rename the files screenshot_[UUID]_1, screenshot_[UUID]_2 ... etc. Now, there are lots of other ways to handle this, like saving all filenames in a database or creating a dir structure on the fileserver for every uuid and, e.g., return all images in the "[uuid]/screenshots" folder via REST. I expect about 30k requests a day, so the service has to be pretty performant. Whats the best way to solve this?

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• Code Design question, circular reference across classes?

  - by dsollen

  I have no code here, as this is more of a design question (I assume this is still the best place to ask it). I have a very simple server in java which stores a mapping between certain values and UUID which are to be used by many systems across multiple platforms. It accepts a connection from a client and creates a clientSocket which stores the socket and all the other relevant data unique to that connection. Each clientSocket will run in their own thread and will block on the socket waiting for a read. I expect very little strain on this system, it will rarely get called, but when it does get a call it will need to respond quickly and due to the risk of it having a peak time with multiple calls coming in at once threaded is still better. Each thread has a reference to a Mapper class which stores the mapping of UUID which it's reporting to others (with proper synchronization of course). This all works until I have to add a new UUID to the list. When this happens I want to report to all clients that care about that particular UUID that a new one was added. I can't multicast (limitation of the system I'm running on) so I'm having each socket send the message to the client through the established socket. However, since each thread only knows about the socket it's waiting on I didn't have a clear method of looking up every thread/socket that cares about the data to inform them of the new UUID. Polling is out mostly because it seems a little too convoluted to try to maintain a list of newly added UUID. My solution as of now is to have the 'parent' class which creates the mapper class and spawns all the threads pass itself as an argument to the mapper. Then when the mapper creates a new UUID it can make a call to the parent class telling it to send out updates to all the other sockets that care about the change. I'm concerned that this may be a bad design due to the use of a circular reference; parent has a reference to mapper (to pass it to new ClientSocket threads) and mapper points to parent. It doesn't really feel like a bad design to me but I wanted to check since circular references are suppose to be bad. Note: I realize this means that the thread associated with whatever socket originally received the request that spawned the creation of a UUID is going to pay the 'cost' of outputting to all the other clients that care about the new UUID. I don't care about this; as I said I suspect the client to receive only intermittent messages. It's unlikely for one socket to receive multiple messages at one time, and there won't be that many sockets so it shouldn't take too long to send messages to each of them. Perhaps later I'll fix the fact that I'm saddling higher work load on whatever unfortunate thread gets the first request; but for now I think it's fine.

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• Help with infrequent segmentation fault in accessing boost::unordered_multimap or struct

  - by Sarah

  I'm having trouble debugging a segmentation fault. I'd appreciate tips on how to go about narrowing in on the problem. The error appears when an iterator tries to access an element of a struct Infection, defined as: struct Infection { public: explicit Infection( double it, double rt ) : infT( it ), recT( rt ) {} double infT; // infection start time double recT; // scheduled recovery time }; These structs are kept in a special structure, InfectionMap: typedef boost::unordered_multimap< int, Infection > InfectionMap; Every member of class Host has an InfectionMap carriage. Recovery times and associated host identifiers are kept in a priority queue. When a scheduled recovery event arises in the simulation for a particular strain s in a particular host, the program searches through carriage of that host to find the Infection whose recT matches the recovery time (double recoverTime). (For reasons that aren't worth going into, it's not as expedient for me to use recT as the key to InfectionMap; the strain s is more useful, and coinfections with the same strain are possible.) assert( carriage.size() > 0 ); pair<InfectionMap::iterator,InfectionMap::iterator> ret = carriage.equal_range( s ); InfectionMap::iterator it; for ( it = ret.first; it != ret.second; it++ ) { if ( ((*it).second).recT == recoverTime ) { // produces seg fault carriage.erase( it ); } } I get a "Program received signal EXC_BAD_ACCESS, Could not access memory. Reason: KERN_INVALID_ADDRESS at address..." on the line specified above. The recoverTime is fine, and the assert(...) in the code is not tripped. As I said, this seg fault appears 'randomly' after thousands of successful recovery events. How would you go about figuring out what's going on? I'd love ideas about what could be wrong and how I can further investigate the problem. Update I added a new assert and a check just inside the for loop: assert( carriage.size() > 0 ); assert( carriage.count( s ) > 0 ); pair<InfectionMap::iterator,InfectionMap::iterator> ret = carriage.equal_range( s ); InfectionMap::iterator it; cout << "carriage.count(" << s << ")=" << carriage.count(s) << endl; for ( it = ret.first; it != ret.second; it++ ) { cout << "(*it).first=" << (*it).first << endl; // error here if ( ((*it).second).recT == recoverTime ) { carriage.erase( it ); } } The EXC_BAD_ACCESS error now appears at the (*it).first call, again after many thousands of successful recoveries. Can anyone give me tips on how to figure out how this problem arises? I'm trying to use gdb. Frame 0 from the backtrace reads "#0 0x0000000100001d50 in Host::recover (this=0x100530d80, s=0, recoverTime=635.91148029170529) at Host.cpp:317" I'm not sure what useful information I can extract here. Update 2 I added a break; after the carriage.erase(it). This works, but I have no idea why (e.g., why it would remove the seg fault at (*it).first.

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• Extracting pair member in lambda expressions and template typedef idiom

  - by Nazgob

  Hi, I have some complex types here so I decided to use nifty trick to have typedef on templated types. Then I have a class some_container that has a container as a member. Container is a vector of pairs composed of element and vector. I want to write std::find_if algorithm with lambda expression to find element that have certain value. To get the value I have to call first on pair and then get value from element. Below my std::find_if there is normal loop that does the trick. My lambda fails to compile. How to access value inside element which is inside pair? I use g++ 4.4+ and VS 2010 and I want to stick to boost lambda for now. #include <vector> #include <algorithm> #include <boost\lambda\lambda.hpp> #include <boost\lambda\bind.hpp> template<typename T> class element { public: T value; }; template<typename T> class element_vector_pair // idiom to have templated typedef { public: typedef std::pair<element<T>, std::vector<T> > type; }; template<typename T> class vector_containter // idiom to have templated typedef { public: typedef std::vector<typename element_vector_pair<T>::type > type; }; template<typename T> bool operator==(const typename element_vector_pair<T>::type & lhs, const typename element_vector_pair<T>::type & rhs) { return lhs.first.value == rhs.first.value; } template<typename T> class some_container { public: element<T> get_element(const T& value) const { std::find_if(container.begin(), container.end(), bind(&typename vector_containter<T>::type::value_type::first::value, boost::lambda::_1) == value); /*for(size_t i = 0; i < container.size(); ++i) { if(container.at(i).first.value == value) { return container.at(i); } }*/ return element<T>(); //whatever } protected: typename vector_containter<T>::type container; }; int main() { some_container<int> s; s.get_element(5); return 0; }

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• Dataflow Programming - Patterns and Frameworks

  - by Styrac

  I just came across the proposed Boost::Dataflow library. It seems like an interesting approach and I was wondering if there are other such alternative frameworks for C++, and if there are any related design patterns. I have not ruled out Boost::Dataflow, I am just looking into any available alternatives so I can understand the domain and my options better (or roll my own if necessary).

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• How does a portable Thread Specific Storage Mechanism's Naming Scheme Generate Thread Relative Uniqu

  - by Hassan Syed

  A portable thread specific storage reference/identity mechanism, of which boost/thread/tss.hpp is an instance, needs a way to generate a unique keys for itself. This key is unique in the scope of a thread, and is subsequently used to retrieve the object it references. This mechanism is used in code written in a thread neutral manner. Since boost is a portable example of this concept, how specifically does such a mechanism work ?

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• What's your convention for typedef'ing shared_ptr?

  - by Emile Cormier

  I'm flip-flopping between naming conventions for typedef'ing the boost::shared_ptr template. For example: typedef boost::shared_ptr<Foo> FooPtr; Before settling on a convention, I'd like to see what others use. What is your convention?

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• Apples new section 3.3.1

  - by ML

  With Apple making changes to section 3.3.1 on the iPhone dev agreement can one stillness libraries like boost in their apps? I want to use Boost in my iPad app...

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• python object to native c++ pointer

  - by Lodle

  Im toying around with the idea to use python as an embedded scripting language for a project im working on and have got most things working. However i cant seem to be able to convert a python extended object back into a native c++ pointer. So this is my class: class CGEGameModeBase { public: virtual void FunctionCall()=0; virtual const char* StringReturn()=0; }; class CGEPYGameMode : public CGEGameModeBase, public boost::python::wrapper<CGEPYGameMode> { public: virtual void FunctionCall() { if (override f = this->get_override("FunctionCall")) f(); } virtual const char* StringReturn() { if (override f = this->get_override("StringReturn")) return f(); return "FAILED TO CALL"; } }; Boost wrapping: BOOST_PYTHON_MODULE(GEGameMode) { class_<CGEGameModeBase, boost::noncopyable>("CGEGameModeBase", no_init); class_<CGEPYGameMode, bases<CGEGameModeBase> >("CGEPYGameMode", no_init) .def("FunctionCall", &CGEPYGameMode::FunctionCall) .def("StringReturn", &CGEPYGameMode::StringReturn); } and the python code: import GEGameMode def Ident(): return "Alpha" def NewGamePlay(): return "NewAlpha" def NewAlpha(): import GEGameMode import GEUtil class Alpha(GEGameMode.CGEPYGameMode): def __init__(self): print "Made new Alpha!" def FunctionCall(self): GEUtil.Msg("This is function test Alpha!") def StringReturn(self): return "This is return test Alpha!" return Alpha() Now i can call the first to functions fine by doing this: const char* ident = extract< const char* >( GetLocalDict()["Ident"]() ); const char* newgameplay = extract< const char* >( GetLocalDict()["NewGamePlay"]() ); printf("Loading Script: %s\n", ident); CGEPYGameMode* m_pGameMode = extract< CGEPYGameMode* >( GetLocalDict()[newgameplay]() ); However when i try and convert the Alpha class back to its base class (last line above) i get an boost error: TypeError: No registered converter was able to extract a C++ pointer to type class CGEPYGameMode from this Python object of type Alpha I have done alot of searching on the net but cant work out how to convert the Alpha object into its base class pointer. I could leave it as an object but rather have it as a pointer so some non python aware code can use it. Any ideas?

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• How to pass function reference into arguments

  - by Ockonal

  Hi, I'm using boost::function for making function-references: typedef boost::function<void (SomeClass &handle)> Ref; someFunc(Ref &pointer) {/*...*/} void Foo(SomeClass &handle) {/*...*/} What is the best way to pass Foo into the someFunc? I tried something like: someFunc(Ref(Foo));

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• intrusive_ptr: Why isn't a common base class provided?

  - by Jon

  intrusive_ptr requires intrusive_ptr_add_ref and intrusive_ptr_release to be defined. Why isn't a base class provided which will do this? There is an example here: http://lists.boost.org/Archives/boost/2004/06/66957.php, but the poster says "I don't necessarily think this is a good idea". Why not?

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• As a programmer what single discovery has given you the greatest boost in productivity?

  - by ChrisInCambo

  This question has been inspired by my recent discovery/adoption of distributed version control. I started using it (mercurial) just because I liked the idea of still being able to make commits at times when I couldn't connect to the central server. I never expected it would give me a large boost in general productivity, but a pleasant side effect I discovered was that making a new clone every time I started a new task and giving that clone a descriptive folder name is extremely effective at keeping me on task resulting is a noticeable productivity increase. So as a programmer what single discovery has given you the greatest boost in productivity? Extra respect for answers which involve tools or practices that aren't so obvious from the outside!

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• One nginx rules for lots of subdomain

  - by komase

  I have lots of subdomain in a server. Every subdomain has its own Drupal boost rules, like in below codes: server { server_name subdomain1.website.com; location / { root /var/www/html/subdomain/subdomain1.website.com; index index.php; set $boost ""; set $boost_query "_"; if ( $request_method = GET ) { set $boost G; } if ($http_cookie !~ "DRUPAL_UID") { set $boost "${boost}D"; } if ($query_string = "") { set $boost "${boost}Q"; } if ( -f $document_root/cache/normal/$host$request_uri$boost_query.html ) { set $boost "${boost}F"; } if ($boost = GDQF){ rewrite ^.*$ /cache/normal/$host/$request_uri$boost_query.html break; } if (!-e $request_filename) { rewrite ^/(.*)$ /index.php?q=$1 last; break; } } location ~ \.php$ { fastcgi_pass 127.0.0.1:9000; fastcgi_index index.php; fastcgi_param SCRIPT_FILENAME /var/www/html/subdomain/subdomain1.website.com$fastcgi_script_name; include fastcgi_params; } } I adding all subdomain rules manually from time to time. The size of ngin.conf has become too big. So, I need one nginx rules which do: subdomain1.website.com pointing to /var/www/html/subdomain/subdomain1.website.com subdomain2.website.com pointing to /var/www/html/subdomain/subdomain2.website.com subdomain3.website.com pointing to /var/www/html/subdomain/subdomain3.website.com ...and so on (So that no more adding rules for subdomain .website.com I need in the future.)

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• Template with constant expression: error C2975 with VC++2008

  - by Arman

  Hello, I am trying to use elements of meta programming, but hit the wall with the first trial. I would like to have a comparator structure which can be used as following: intersect_by<ID>(L1.data, L2.data, "By ID: "); intersect_by<IDf>(L1.data, L2.data, "By IDf: "); Where: struct ID{};// Tag used for original IDs struct IDf{};// Tag used for the file position //following Boost.MultiIndex examples template<typename Tag,typename MultiIndexContainer> void intersect_by( const MultiIndexContainer& L1,const MultiIndexContainer& L2,std::string msg, Tag* =0 /* fixes a MSVC++ 6.0 bug with implicit template function parms / ) { / obtain a reference to the index tagged by Tag */ const typename boost::multi_index::index<MultiIndexContainer,Tag>::type& L1_ID_index= get<Tag>(L1); const typename boost::multi_index::index<MultiIndexContainer,Tag>::type& L2_ID_index= get<Tag>(L2); std::set_intersection( L1_ID_index.begin(), L1_ID_index.end(), L2_ID_index.begin(), L2_ID_index.end(), std::inserter(s, s.begin()), strComparator() // Here I get the C2975 error ); } template<int N> struct strComparator; template<> struct strComparator<0>{ bool operator () (const particleID& id1, const particleID& id2) const { return id1.ID struct strComparator<1{ bool operator () (const particleID& id1, const particleID& id2) const { return id1.IDf }; What I am missing? kind regards Arman.

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• C++ Serialization Clean XML Similar to XSTREAM

  - by disown

  I need to write a linux c++ app which saves it settings in XML format (for easy hand editing) and also communicates with existing apps through XML messages over sockets and HTTP. Problem is that I haven't been able to find any intelligent libs to help me, I don't particular feel like writing DOM or SAX code just to write and read some very simple messages. Boost Serialization was almost a match, but it adds a lot of boost-specific data to the xml it generates. This obviously doesn't work well for interchange formats. I'm wondering if it is possible to make Boost Serialization or some other c++ serialization library generate clean xml. I don't mind if there are some required extra attributes - like a version attribute, but I'd really like to be able to control their naming and also get rid of 'features' that I don't use - tracking_level and class_id for instance. Ideally I would just like to have something similar to xstream in Java. I am aware of the fact that c++ lacks introspection and that it is therefore necessary to do some manual coding - but it would be nice if there was a clean solution to just read and write simple XML without kludges! If this cannot be done I am also interested in tools where the XML schema is the canonical resource (contract first) - a good JAXB alternative to C++. So far I have only found commercial solutions like CodeSynthesis XSD. I would prefer open source solutions. I have tried gSoap - but it generates really ugly code and it is also SOAP-specific. In desperation I also started looking at alternative serialization formats for protobuffers. This exists - but only for Java! It really surprises me that protocol buffers seems to be a better supported data interchange format than XML. I'm going mad just finding libs for this app and I really need some new ideas. Anyone?

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• std::locale breakage on MacOS 10.6 with LANG=en_US.UTF-8

  - by fixermark

  I have a C++ application that I am porting to MacOSX (specifically, 10.6). The app makes heavy use of the C++ standard library and boost. I recently observed some breakage in the app that I'm having difficulty understanding. Basically, the boost filesystem library throws a runtime exception when the program runs. With a bit of debugging and googling, I've reduced the offending call to the following minimal program: #include <locale> int main ( int argc, char *argv [] ) { std::locale::global(std::locale("")); return 0; } This program fails when I run this through g++ and execute the resulting program in an environment where LANG=en_US.UTF-8 is set (which on my computer is part of the default bash session when I create a new console window). Clearing the environment variable (setenv LANG=) allows the program to run without issues. But I'm surprised I'm seeing this breakage in the default configuration. My questions are: Is this expected behavior for this code on MacOS 10.6? What would a proper workaround be? I can't really re-write the function because the version of the boost libraries we are using executes this statement internally as part of the filesystem library. For completeness, I should point out that the program from which this code was synthesized crashes when launched via the 'open' command (or from the Finder) but not when Xcode runs the program in Debug mode. edit The error given by the above code on 10.6.1 is: $ ./locale terminate called after throwing an instance of 'std::runtime_error' what(): locale::facet::_S_create_c_locale name not valid Abort trap

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• How to get at contents of placeholder::_1

  - by sheepsimulator

  I currently have the following code: using boost::bind; typedef boost::signal<void(EventDataItem&)> EventDataItemSignal; class EventDataItem { ... EventDataItemSignal OnTrigger; ... } typedef std::list< shared_ptr<EventDataItem> > DataItemList; typedef std::list<boost::signals::connection> ConnectionList; class MyClass { void OnStart() { DataItemList dilItems; ConnectionList clConns; DataItemList::iterator iterDataItems; for(iterDataItems = dilItems.begin(); iterDataItems != dilItems.end(); iterDataItems++) { // Create Connections from Triggers clConns.push_back((*iterDataItems)->OnTrigger.connect( bind(&MyClass::OnEventTrigger, this))); } } void OnEventTrigger() { // ... Do stuff on Trigger... } } I'd like to change MyClass::OnStart to use std::transform to achieve the same thing: void MyClass::OnStart() { DataItemList dilItems; ConnectionList clConns; // Resize connection list to match number of data items clConns.resize(dilItems.size()); // Build connection list from Items // note: errors on the placeholder _1->OnTrigger std::transform(dilItems.begin(), dilItems.end(), clConns.begin(), bind(&EventDataItemSignal::connect, _1->OnTrigger, bind(&MyClass::Stuff, this))); } However, my hiccup is _1-OnTrigger. How can I reference OnTrigger from placeholder _1?

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• Why is creating a ring buffer shared by different processes so hard (in C++), what I am doing wrong?

  - by recipriversexclusion

  I am being especially dense about this but it seems I'm missing an important, basic point or something, since what I want to do should be common: I need to create a fixed-size ring buffer object from a manager process (Process M). This object has write() and read() methods to write/read from the buffer. The read/write methods will be called by independent processes (Process R and W) I have implemented the buffer, SharedBuffer<T&>, it allocates buffer slots in SHM using boost::interprocess and works perfectly within a single process. I have read the answers to this question and that one on SO, as well as asked my own, but I'm still in the dark about how to have different processes access methods from a common object. The Boost doc has an example of creating a vector in SHM, which is very similar to what I want, but I want to instantiate my own class. My current options are: Use placement new, as suggested by Charles B. to my question; however, he cautions that it's not a good idea to put non-POD objects in SHM. But my class needs the read/write methods, how can I handle those? Add an allocator to my class definition, e.g. have SharedBuffer<T&, Alloc> and proceed similarly to the vector example given in boost. This sounds really complicated. Change SharedBuffer to a POD class, i.e. get rid of all the methods. But then how to synchronize reading and writing between processes? What am I missing? Fixed-length ring buffers are very common, so either this problem has a solution or else I'm doing something wrong.

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• Immutable classes in C++

  - by ereOn

  Hi, In one of my projects, I have some classes that represent entities that cannot change once created, aka. immutable classes. Example : A class RSAKey that represent a RSA key which only has const methods. There is no point changing the existing instance: if you need another one, you just create one. My objects sometimes are heavy and I enforced the use of smart pointers to avoid copy. So far, I have the following pattern for my classes: class RSAKey : public boost::noncopyable, public boost::enable_shared_from_this<RSAKey> { public: /** * \brief Some factory. * \param member A member value. * \return An instance. */ static boost::shared_ptr<const RSAKey> createFromMember(int member); /** * \brief Get a member. * \return The member. */ int getMember() const; private: /** * \brief Constructor. * \param member A member. */ RSAKey(int member); /** * \brief Member. */ const int m_member; }; So you can only get a pointer (well, a smart pointer) to a const RSAKey. To me, it makes sense, because having a non-const reference to the instance is useless (it only has const methods). Do you guys see any issue regarding this pattern ? Are immutable classes something common in C++ or did I just created a monster ? Thank you for your advices !

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• Macro to improve callback registration readability

  - by Warren Seine

  I'm trying to write a macro to make a specific usage of callbacks in C++ easier. All my callbacks are member functions and will take this as first argument and a second one whose type inherits from a common base class. The usual way to go is: register_callback(boost::bind(&my_class::member_function, this, _1)); I'd love to write: register_callback(HANDLER(member_function)); Note that it will always be used within the same class. Even if typeof is considered as a bad practice, it sounds like a pretty solution to the lack of __class__ macro to get the current class name. The following code works: typedef typeof(*this) CLASS; boost::bind(& CLASS :: member_function, this, _1)(my_argument); but I can't use this code in a macro which will be given as argument to register_callback. I've tried: #define HANDLER(FUN) \ boost::bind(& typeof(*this) :: member_function, this, _1); which doesn't work for reasons I don't understand. Quoting GCC documentation: A typeof-construct can be used anywhere a typedef name could be used. My compiler is GCC 4.4, and even if I'd prefer something standard, GCC-specific solutions are accepted.

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• Is there a way to increase the efficiency of shared_ptr by storing the reference count inside the co

  - by BillyONeal

  Hello everyone :) This is becoming a common pattern in my code, for when I need to manage an object that needs to be noncopyable because either A. it is "heavy" or B. it is an operating system resource, such as a critical section: class Resource; class Implementation : public boost::noncopyable { friend class Resource; HANDLE someData; Implementation(HANDLE input) : someData(input) {}; void SomeMethodThatActsOnHandle() { //Do stuff }; public: ~Implementation() { FreeHandle(someData) }; }; class Resource { boost::shared_ptr<Implementation> impl; public: Resource(int argA) explicit { HANDLE handle = SomeLegacyCApiThatMakesSomething(argA); if (handle == INVALID_HANDLE_VALUE) throw SomeTypeOfException(); impl.reset(new Implementation(handle)); }; void SomeMethodThatActsOnTheResource() { impl->SomeMethodThatActsOnTheHandle(); }; }; This way, shared_ptr takes care of the reference counting headaches, allowing Resource to be copyable, even though the underlying handle should only be closed once all references to it are destroyed. However, it seems like we could save the overhead of allocating shared_ptr's reference counts and such separately if we could move that data inside Implementation somehow, like boost's intrusive containers do. If this is making the premature optimization hackles nag some people, I actually agree that I don't need this for my current project. But I'm curious if it is possible.

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• C++ socket protocol design issue (ring inclusion)

  - by Martin Lauridsen

  So I have these two classes, mpqs_client and client_protocol. The mpqs_client class handles a Boost socket connection to a server (sending and receiving messages with some specific format. Upon receiving a message, it calls a static method, parse_message(..), in the class client_protocol, and this method should analyse the message received and perform some corresponding action. Given some specific input, the parse_message method needs to send some data back to the server. As mentioned, this happens through the class mpqs_client. So I could, from mpqs_client, pass "this" to parse_message(..) in client_protocol. However, this leads to a two-way association relationship between the two classes. Something which I reckon is not desireable. Also, to implement this, I would need to include the other in each one, and this gives me a terrible pain. I am thinking this is more of a design issue. What is the best solution here? Code is posted below. Class mpqs_client: #include "mpqs_client.h" mpqs_client::mpqs_client(boost::asio::io_service& io_service, tcp::resolver::iterator endpoint_iterator) : io_service_(io_service), socket_(io_service) { ... } ... void mpqs_client::write(const network_message& msg) { io_service_.post(boost::bind(&mpqs_client::do_write, this, msg)); } Class client_protocol: #include "../network_message.hpp" #include "../protocol_consts.h" class client_protocol { public: static void parse_message(network_message& msg, mpqs_sieve **instance_, mpqs_client &client_) { ... switch (type) { case MPQS_DATA: ... break; case POLYNOMIAL_DATA: ... break; default: break; } }

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• How to implement a multi-threaded asynchronous operation?

  - by drowneath

  Here's how my current approach looks like: // Somewhere in a UI class // Called when a button called "Start" clicked MyWindow::OnStartClicked(Event &sender) { _thread = new boost::thread(boost::bind(&MyWindow::WorkToDo, this)); } MyWindow::WorkToDo() { for(int i = 1; i < 10000000; i++) { int percentage = (int)((float)i / 100000000.f); _progressBar->SetValue(percentage); _statusText->SetText("Working... %d%%", percentage); printf("Pretend to do something useful...\n"); } } // Called on every frame MyWindow::OnUpdate() { if(_thread != 0 && _thread->timed_join(boost::posix_time::seconds(0)) { _progressBar->SetValue(100); _statusText->SetText("Completed!"); delete _thread; _thread = 0; } } But I'm afraid this is far from safe since I keep getting unhandled exception at the end of the program execution. I basically want to separate a heavy task into another thread without blocking the GUI part.

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• How to synchronize access to many objects

  - by vividos

  I have a thread pool with some threads (e.g. as many as number of cores) that work on many objects, say thousands of objects. Normally I would give each object a mutex to protect access to its internals, lock it when I'm doing work, then release it. When two threads would try to access the same object, one of the threads has to wait. Now I want to save some resources and be scalable, as there may be thousands of objects, and still only a hand full of threads. I'm thinking about a class design where the thread has some sort of mutex or lock object, and assigns the lock to the object when the object should be accessed. This would save resources, as I only have as much lock objects as I have threads. Now comes the programming part, where I want to transfer this design into code, but don't know quite where to start. I'm programming in C++ and want to use Boost classes where possible, but self written classes that handle these special requirements are ok. How would I implement this? My first idea was to have a boost::mutex object per thread, and each object has a boost::shared_ptr that initially is unset (or NULL). Now when I want to access the object, I lock it by creating a scoped_lock object and assign it to the shared_ptr. When the shared_ptr is already set, I wait on the present lock. This idea sounds like a heap full of race conditions, so I sort of abandoned it. Is there another way to accomplish this design? A completely different way?

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• Why is my RAID /dev/md1 showing up as /dev/md126? Is mdadm.conf being ignored?

  - by mmorris

  I created a RAID with: sudo mdadm --create --verbose /dev/md1 --level=mirror --raid-devices=2 /dev/sdb1 /dev/sdc1 sudo mdadm --create --verbose /dev/md2 --level=mirror --raid-devices=2 /dev/sdb2 /dev/sdc2 sudo mdadm --detail --scan returns: ARRAY /dev/md1 metadata=1.2 name=ion:1 UUID=aa1f85b0:a2391657:cfd38029:772c560e ARRAY /dev/md2 metadata=1.2 name=ion:2 UUID=528e5385:e61eaa4c:1db2dba7:44b556fb Which I appended it to /etc/mdadm/mdadm.conf, see below: # mdadm.conf # # Please refer to mdadm.conf(5) for information about this file. # # by default (built-in), scan all partitions (/proc/partitions) and all # containers for MD superblocks. alternatively, specify devices to scan, using # wildcards if desired. #DEVICE partitions containers # auto-create devices with Debian standard permissions CREATE owner=root group=disk mode=0660 auto=yes # automatically tag new arrays as belonging to the local system HOMEHOST <system> # instruct the monitoring daemon where to send mail alerts MAILADDR root # definitions of existing MD arrays # This file was auto-generated on Mon, 29 Oct 2012 16:06:12 -0500 # by mkconf $Id$ ARRAY /dev/md1 metadata=1.2 name=ion:1 UUID=aa1f85b0:a2391657:cfd38029:772c560e ARRAY /dev/md2 metadata=1.2 name=ion:2 UUID=528e5385:e61eaa4c:1db2dba7:44b556fb cat /proc/mdstat returns: Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] md2 : active raid1 sdb2[0] sdc2[1] 208629632 blocks super 1.2 [2/2] [UU] md1 : active raid1 sdb1[0] sdc1[1] 767868736 blocks super 1.2 [2/2] [UU] unused devices: <none> ls -la /dev | grep md returns: brw-rw---- 1 root disk 9, 1 Oct 30 11:06 md1 brw-rw---- 1 root disk 9, 2 Oct 30 11:06 md2 So I think all is good and I reboot. After the reboot, /dev/md1 is now /dev/md126 and /dev/md2 is now /dev/md127????? sudo mdadm --detail --scan returns: ARRAY /dev/md/ion:1 metadata=1.2 name=ion:1 UUID=aa1f85b0:a2391657:cfd38029:772c560e ARRAY /dev/md/ion:2 metadata=1.2 name=ion:2 UUID=528e5385:e61eaa4c:1db2dba7:44b556fb cat /proc/mdstat returns: Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] md126 : active raid1 sdc2[1] sdb2[0] 208629632 blocks super 1.2 [2/2] [UU] md127 : active (auto-read-only) raid1 sdb1[0] sdc1[1] 767868736 blocks super 1.2 [2/2] [UU] unused devices: <none> ls -la /dev | grep md returns: drwxr-xr-x 2 root root 80 Oct 30 11:18 md brw-rw---- 1 root disk 9, 126 Oct 30 11:18 md126 brw-rw---- 1 root disk 9, 127 Oct 30 11:18 md127 All is not lost, I: sudo mdadm --stop /dev/md126 sudo mdadm --stop /dev/md127 sudo mdadm --assemble --verbose /dev/md1 /dev/sdb1 /dev/sdc1 sudo mdadm --assemble --verbose /dev/md2 /dev/sdb2 /dev/sdc2 and verify everything: sudo mdadm --detail --scan returns: ARRAY /dev/md1 metadata=1.2 name=ion:1 UUID=aa1f85b0:a2391657:cfd38029:772c560e ARRAY /dev/md2 metadata=1.2 name=ion:2 UUID=528e5385:e61eaa4c:1db2dba7:44b556fb cat /proc/mdstat returns: Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] md2 : active raid1 sdb2[0] sdc2[1] 208629632 blocks super 1.2 [2/2] [UU] md1 : active raid1 sdb1[0] sdc1[1] 767868736 blocks super 1.2 [2/2] [UU] unused devices: <none> ls -la /dev | grep md returns: brw-rw---- 1 root disk 9, 1 Oct 30 11:26 md1 brw-rw---- 1 root disk 9, 2 Oct 30 11:26 md2 So once again, I think all is good and I reboot. Again, after the reboot, /dev/md1 is /dev/md126 and /dev/md2 is /dev/md127????? sudo mdadm --detail --scan returns: ARRAY /dev/md/ion:1 metadata=1.2 name=ion:1 UUID=aa1f85b0:a2391657:cfd38029:772c560e ARRAY /dev/md/ion:2 metadata=1.2 name=ion:2 UUID=528e5385:e61eaa4c:1db2dba7:44b556fb cat /proc/mdstat returns: Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] md126 : active raid1 sdc2[1] sdb2[0] 208629632 blocks super 1.2 [2/2] [UU] md127 : active (auto-read-only) raid1 sdb1[0] sdc1[1] 767868736 blocks super 1.2 [2/2] [UU] unused devices: <none> ls -la /dev | grep md returns: drwxr-xr-x 2 root root 80 Oct 30 11:42 md brw-rw---- 1 root disk 9, 126 Oct 30 11:42 md126 brw-rw---- 1 root disk 9, 127 Oct 30 11:42 md127 What am I missing here?

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