C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Using local static variables within concurrent environment.          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Vladimir Frolov Guest Posted: Thu Dec 29, 2005 12:15 pm    Post subject: Using local static variables within concurrent environment. Hello, I have found problem using local static variable within concurrent environment. Undefined behavior can be caused not only by recursive control re-entering the declaration of local static variable (as it is specified in standard, see [stmt.dcl]) but also concurrence control re-entering. int foo(int i) { static int s = foo(2*i); // recursive call cause undefined behavior return i+1; } const std::string& f1() { static const std::string string_data("data"); // undefined behavior if f1() will be invoked concurrently for the first time return string_data; } const std::string string_global_data("global_data"); const std::string& f2() { // Ok. f2() can be invoked concurrently return string_global_data; } One has to note that function f1() unlike f2() cannot be invoked concurrently because construction of string_data object will be done without any synchronization, and constructor of std::string can be invoked concurrently several times before string_data will be marked as constructed. It means that Meyers singleton cannot be used on concurrent environment without additional non-trivial synchronization as well. Thanks. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Sat Dec 31, 2005 1:38 pm    Post subject: Re: Using local static variables within concurrent environme Vladimir Frolov wrote: Quote: I have found problem using local static variable within concurrent environment. Undefined behavior can be caused not only by recursive control re-entering the declaration of local static variable (as it is specified in standard, see [stmt.dcl]) but also concurrence control re-entering. int foo(int i) { static int s = foo(2*i); // recursive call cause undefined behavior return i+1; } const std::string& f1() { static const std::string string_data("data"); // undefined behavior if f1() will be invoked concurrently for the first time return string_data; } const std::string string_global_data("global_data"); const std::string& f2() { // Ok. f2() can be invoked concurrently return string_global_data; } One has to note that function f1() unlike f2() cannot be invoked concurrently because construction of string_data object will be done without any synchronization, and constructor of std::string can be invoked concurrently several times before string_data will be marked as constructed. The standard doesn't say anything about this, since it ignores threading. So it depends on what your implementation guarantees. I think that most implementations do make no guarantees, however, so it is up to you to ensure external synchronization. As a general rule, static data in a multithreaded environment is an invitation to problems. There are different ways of mitigating the problem; in cases like the above, for example, using a char const[], rather than string, sidesteps all potential problems. Quote: It means that Meyers singleton cannot be used on concurrent environment without additional non-trivial synchronization as well. I'm not sure what you mean by "Meyers singleton"; in the last article I read by Scott Meyers concerning singletons, he explicitly treated this problem. The synchronization is fairly trivial from a programmer's point of view, although it has some runtime cost. You can reduce the cost significantly by using platform specific atomic access mechanisms, but at least in some cases (e.g. Sun CC under Solaris) they require inline assembler. Or you can just ensure that the singleton is instantiated before threading is started -- in my own code, I require as an absolute precondition that no additional threads are started before entering main, and I use something like: Singleton* Singleton::ourInstance = Singleton::instance() ; Singleton* Singleton::instance() { if ( ourInstance == NULL ) { ourInstance = new Singleton ; } return ourInstance ; } (Both ourInstance and instance() are static members of Singleton, obviously.) This ensures that Singleton::instance() is called at least once during static initialization, i.e. before entering main. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Axter Guest Posted: Sat Dec 31, 2005 7:24 pm    Post subject: Re: Using local static variables within concurrent environme Vladimir Frolov wrote: Quote: Hello, I have found problem using local static variable within concurrent environment. Undefined behavior can be caused not only by recursive control re-entering the declaration of local static variable (as it is specified in standard, see [stmt.dcl]) but also concurrence control re-entering. const std::string string_global_data("global_data"); const std::string& f2() { // Ok. f2() can be invoked concurrently return string_global_data; } One has to note that function f1() unlike f2() cannot be invoked concurrently f2() unlike f1() produces undefined behavior when called from multiple translation units before main() gets called. In other words, if another global object located in a different translation unit calls f2(), (IAW C++ Standard) there is no garantee that global object string_global_data has been initialize. There's no garanteed method for the order of initialization of global complex objects located in different translation units. It's far more likely that a singleton will run into this problem, then it would be likely to have concurrent problem. If you do have concurrent logic, then I would recommend removing the concurrent logic in order to safely create the singleton. But I would not recommend using a global variable. Using a local static variable is much safer, and gives you more defined behavior. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top ul Guest Posted: Fri Jan 06, 2006 12:37 pm    Post subject: Re: Using local static variables within concurrent environme Now I do not understand Here:http://groups.google.com/group/comp.lang.c++.moderated/browse_thread/thread/4cd24271dca8a9fa/6fd827bd8f383414?hl=en#6fd827bd8f383414 was discussed that global variables guaranteed to be initialized before the fist use. But you write that: There's no garanteed method for the order of initialization of global complex objects located in different translation units. Where is the truth? Regards [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Axter Guest Posted: Sat Jan 07, 2006 12:27 pm    Post subject: Re: Using local static variables within concurrent environme ul wrote: Quote: Now I do not understand Here:http://groups.google.com/group/comp.lang.c++.moderated/browse_thread/thread/4cd24271dca8a9fa/6fd827bd8f383414?hl=en#6fd827bd8f383414 was discussed that global variables guaranteed to be initialized before the fist use. But you write that: There's no garanteed method for the order of initialization of global complex objects located in different translation units. Where is the truth? The link you posted has 30 replies. Please state exactly which reply you're referring to. In any case, the C++ standard makes no garantee of the order of initialization of global complex objects that are in multiple translation units before main() is called. See following links: http://www.informit.com/guides/content.asp?g=cplusplus&seqNum=212&rl=1 And read SECOND paragraph in the following link: http://www.codeguru.com/cpp/tic/tic0114.shtml But for the final word, check the official C++ standards [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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